Pyrochlores. V. Thermoanalytic, X-ray, neutron, infrared, and dielectric studies of A2Ti20, titanates'

OSVALD KXOP AND FRAN~OIS BRISSE' Depnrtnze~it of Chemistry, Dalhousie University, Halifax, Nova Scotia

AND

LOTTE CASTELLIZ Departnlent of Clzemical Engineering, Nova Scotin Technical College, Halifnx, Nova Scotia

Received August 14, 1968

A number of con~positions AZ3+TiZ4+ 0 7 and of related mixed phases were prepared and investigated. For the trivalent lanthanons from Sm to Lu, and for Y, the homogeneous A,Ti207 phases were of the cubic pyrochlore type. Their lattice parameters gave a very good linear relationship when plotted against the Templeton-Dauben radii of A3+. The room-temperature stability region of the pyrochlore titanates at ordinary pressures extends from an Ahrens radius ratio, r(A3+):r(Ti4+), of about 1.22 to about 1.50, taking into account homogeneous mixed phases. The compositions Bi2Ti207, In2Ti207, and Cr,Ti20, were not cubic pyrochlores; the results of an X-ray and neutron examination of Sc2Ti20, indicate that further work is needed to clarify its structural properties.

The structures of Lu, Y, Gd, and Snl titanates were refined from X-ray powder data and that of Y,(Ti,,6,Zro.36),07, from neutron powder data. The structure of Er2Ti207 was re-refined from pre- viously published data without significant improvement. The positional parameter of the majority oxygen atom, x(OZ), in A2B207 pyrochlores has been found to increase, and the distortion of the B06 octahedra to decrease, with the increasing r(A3+):y(B4+) ratio. This is in agreement with a similar result obtained earlier from Mossbauer spectra for A2FeSb0, pyrochlores.

The variation of the frequencies of the principal absorption bands in the infrared spectra of the titanates has been correlated with ?(A3+). No evidence of ferroelectricity was found in Sm and Er titanates between room temperature and 4.2 OK.

The Templeton-Dauben set of empirical crystal radii for trivalent lanthanons has been supplemented by values for Sc, In, TI, and Bi derived from the C-type sesquioxides and the pyrochlores. The self- consistency and applicability of the expanded set have been tested on several series of isostructural con~pounds.

Canadian Journal of Chemistry, 47, 971 (1969)

In a previous paper (1) it was established that Er2Ti20,, which is a typical representative of the cubic AZ3+ B24i07 phases, is isostructural with pyrochlore, (NaCa)(NbTa)06F. The crystal structures of another 4 members of the A2Tiz07 series (A = Y, Lu, Gd, and Sm) have now been refined from X-ray powder diffractometer data, and the systematic study (2) of the cubic pyro- chlore stannates, A2Sn207, has been extended to the titanate series. For reasons similar to those stated in ref. 2, a mixed phase, Y2(Tio,,,Zro~3,)2- O,, was also prepared and investigated by neutron powder diffraction.

The main points of interest, as with the stannates, have been the variation of the posi- tional parameter x(02) of the majority oxygen atom (and thus the distortion of the TiO, octa- hedra) with the size of A3', and the limits of

T o r Part IV, see ref. 37. Vresent address: Division of Pure Chemistry. National

Research Council of Canada, Ottawa, ~ a n a d a ;

existence of the cubic pyrochlore structure. Results of preliminary work on the latter problem were reported earlier (3).

Experimental The origin and purity of the starting materials h a ~ e

been described in ref. 2. In addition, MnOz (Fisher Certified Reagent), Gaz03 (4 N) and ZrO, (3 N) (Koch- Light Laboratories Limited), GeOz (K & K Laboratories, Inc.), and ZrO(N03)z (Tizon Chemical Corporation; stated to contain less than 100 o.o.m. Si) were used.

a .

Solutions of titanyl oxalate were prepared from TiC14 that had been purified by the method described in ref. 4.

Titanates of series D (dry-mixed) were prepared as in ref. 1. The individual firing schedules are listed in Table I. The progress of the solid-state reaction was followed by powder photography. In series C (coprecipitated, or totally precipitated) the hydroxides were precipitated with - - N H ~ O H frim mixed solu&ons of the A nitrate and titanyl oxalate of correct stoichiometries. The resulting slurry was dried at 120°, calcined at 900" for 1 h, ground to pass 400 mesh, and calcined at 1100" for 10 h. The calcine was pressed to pellets and fired at 1300" for 2 h, ground, pressed again, and fired at 1400" for 2 h. When the heat treatment differed from the standard procedure details are given in Table I (La, Y, Sc).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

972 CANADLAP\ JOURNAL OF CHEMISTRY. VOL 47, 1969

TABLE I Lattice parameters of cubic A,Ti,O, titanates and related phases

-

A (or intended composition) ao(P), A Preparation"

1000/24, 1400/16; not P Fusion in electric arc 155011, air-quenched

95016, 1000136, 1100/12, 1400148

155012, air-quenched

1700?

140012, 1400124

142511, air-quenched

1700?

1000/2, 135018, 140018, 145014

140012, slow cooling in furnace

140012, slow cooling in furnace

95016, 1000/36, 1100/12, 1400148

155012, air-quenched

Fusion in electric arc 1200 1525/f, air-quenched 600111, 1200/12$3

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. V 973

TABLE I (concluded) - -- - - ---- - -- - - - - - -- -- - - - -.

A (or intended composition) no(P), A Preparation" Referencef-

Sm,-,La, (x = 0.1, 0.2)

Lu, -,Sc, x = 0.2 0.4 0.6 0.8

--

Mixed compositions (this work)

1350112, 1350112, 1400112; "La,Ti,O,"

9.9762+12 1400124, 1500118 9.9351 + 18 1400124. 1500118

HoY 10.0961i. 3 140013, 140018

YbNd 10.1695?7 135013, 140012, 160011

Y O 9 G a o . ~ 10.0888i.11 1350136, 1400124; Pf ? D Y I -,In, (.u = 0.2 to 0.8) 95011 week, 1000/24, 1100124, 1250112; not P

10.224oI 20 950)6; 1000)12; ioooji2 10.2522i-23 9.5016, 1000/12, 1000/12 10.2957f 15 950/6, 1000/12, 1000/12; P f ? (or distorted P )

Er2Ge207 1000/12, 120014, 130014, 140018; not P

Y2Ge207 1000/12 1100/24, 1350124, 1350124; not P ScZGeZ07 950112, 120018, 1400/12; not P

*lo00124 stands for 1000"124 h. Reference 3: 1000116, 1000-1400/5, 140012, cooled in the furnace; the temperature dropped from 1400 to 10600 during the first hour. Reference 17: 1200-1350110-14.

TC, this work, coprecipitated; D, this work, dry-mixed. $Converted to present h (Cu Kc0 (cf. ref. 2). §YzTizOi fired at 1420120 1s reported to be weberite f ? (34).

Not listed in Table I are the following firing schedules: LnzO3 + 2TiOZ (Ln = Pr, Nd); BizOa + 2TiO2 (95016, 1000112, 1000/12)3; Cr203 + 2Ti0, (950112, 120013, 1300/8, 140012); YzO3 + 2Ge0, (1000124, 1100/24, 1350124, 1350124); and Er,03 + 2GeOz (1000/12, 1200/4, 1300/4,1400/8). Preparative firing was not carried out for MnO, + Ti02 (only thermogravirnetric analysis (t.g.a.) up to 1310° at 6"/rnir1)~ and Inz03 + 2Ti02 (only differential thermal analysis (d.t.a.) up to 1600'at 2O0/min).

Terbium presented a complication in that the com- mercial b r o ~ n oxide is not Tb203 but a phase of un- certain composition usually referred to as Tb407. To determine the actual Tb content a sample of the oxide --

3950/6 stands for 950" for 6 h, etc. 4Mn02 goes over to Mn203 between 880 and 960". The

sesquioxide, which exists above 1085', is stable on further heating in stagnant air (6"/min) to at least 1320".

was heated in a thermobalance in stagnant air (for details see ref. 1). The t.g.a. curve showed an intermediate plateau between 600 and 940". When the final plateau (above 1130") is taken to correspond to Tb,03, the composition of the intermediateoxide would beTbOl,,,,, i.e. almost Tb7OI2, and the initial composition would be TbOl 790, i.e. almost Tb,Og. The endothermic peaks in the d.t.a. curve corresponded with the plateaus of the t.g.a. curve. The h a 1 product, Tb203, could be re- oxidized almost reversibly. When the heating was carried out in a stream of oxygen, the positions of the t.g.a. plateaus shifted toward higher temperatures (Fig. 1, Table 11).

A similar investigation was carried out on the com- mercial black Pr6011. The t.g.a. and d.t.a. curves were more complicated than for TbO, (Fig. 1, Table 11). Pr203 appears to be stable in atmospheric air above 1360".

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

TABLE I1

Thermogravimetric analysis of TbO, and Pro, (2"/min) --

Plateau (heating) x Plateau (cooling) Y

TbO, (stagnant air) Room temp.-520" 1.790 1280-1190' 1 .500

600-940" 1.692 700-530" 1.695 1130-1280" 1 ,500 390c-room temp. 1.798

TbO, (flowing oxygen) Room temp.-575" 1 .790 1400-1280" 1 .500

660-101 5" 1.687 760-585" 1.689 1340-1400" 1.500 480"-room temp. 1.791

Pro, (stagnant air) Room temp.-460" 1.833 1400-1180" 1 ,500

480-720" 1.805-1.768 ca. 990" 1 -674 770-1035" 1.702-1.698 920-730" 1.7i211.715

1070-1260" 1.654-1 ,604 500"-room temp. 1.833 1360-1400" 1 .500

- DTA IO0C/mln

I

P r 6 O l l

TGA Z0C/min

DTA

0 n W

P

TEMPERATURE. 'C

FIG. 1. Differential thermal analysis and thermo- gravimetric analysis curves of TbO, and Pro, in stagnant air. The formulae Tb7Oi2 and Pr701Z are only approx- imations (see text).

The information from t.g.a. and d.t.a. mas used merely to ensure that correct amounts of Tb and Pr \+ere introduced to yield products of the intended cor~lposi- tions. However, the analyses were done with care, and the results concerning the intermediate stability levels can be used to verify the findings on the systems Tb-O and Pr-O reported by other authors (5-7).

The process of formation of some of the titanates was followed by thermogravimetry as well as by d.t.a. There was no loss of weight on heating intimate mixtures of Ln203 + 2Ti02 (Ln = La, Nd, Sm, Er) to 1400" in air. For TbO, + TiO, and Pr01.833 + TiOZ the total weight decreased with increasing temperature till the sesqui- oxides formed, but there was no further loss on continued heating and no gain on cooling. This was confirmed for TbO, + Ti02 by d.t.a.

Diferential Thermal Analysis A Deltatherm Model 2000-16 differential thermal

analyzer (Technical Equipment Corporation, Denver, Colorado) was capable of reaching 1600" in non-reducing atmospheres. The heating rate could be varied in steps from 2 to 2O0/min. The Pt-Rh sample block was designed to accommodate 4 samples plus 4 alumina references. Some of the d.t.a. experiments were carried out with a home-made differential thermal analyzer of the Mineral Sciences Division, Department of Energy, Mines and Resources, in Ottawa, Canada.

Infrared Spectra Infrared spectra were recorded from 4000 to 250 cm-I

on a Perkin-Elmer model 521 spectrophotometer using KBr pellets (1 mg sample/200 mg KBr) with a KBr reference.

Permittivity Measurements These were performed by Y. S. Lim of the Physics

Department, Dalhousie University, Halifax, Nova Scotia. For details see ref. 2.

X-ray and Neutron Drffraction Details will be found in refs. 1, 2, and 8. No foreign

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. V 975

lines were visible in well-exposed powder photographs of ,,,,,,- the pyrochlore titanates. Films of Gd, Sm, Nd, and Eu titanates taken with Ni-filtered CuK radiation had excep- tionally dark backgrounds. A Rigaku-Denki high- I temperature X-ray camcra was used in the examination ! of ia,Ti,O,.

For details of the least-squares refinement, ref. 2 may 302:

be consulted. Neutral and ionic scattering factors and dispersion corrections were taken from the following I sources : I

Y, Sm, Gd, Er. Lu (neutral and 3 +), and Ti4+ :f, ref. 9; ' disueision corrections, ref. 10. lool

0, Ti, and Ti3+ : f and dispersion corrections, ref. 11. 1 0 2 - : f , ref. 12. I

Results and Discussion I I n agreement with previous reports (3, 13-20)

the Eanthanons from Lu to Sm as well as Y were found to form cubic A2Ti20, pyrochlores (Table I), whereas Nd, Pr, and La gave X-ray powder patterns of a different type. A notable e~ceptioil is the finding of Padurow and Schusterius (21), who stated that La2Ti207 had a cubic pyrochlore structure with a = 10.60 A. We have been unable to confirm this result. All attempts to prepare a cubic La2Ti207 pyrochlore failed, be it by solid-state reaction or by firing the copreclpitated hydroxides. The possibil~ty of a phase transformation was examined with a high- temperature X-ray camera. There was no signif- icant change in the powder pattern of La2Ti207 (coprecipitated and fired at 1400") as the tem- perature was continuously raised from room temperature to 1490". (Sm, _,La,)2Ti207 com- positions gave patterns of the same type as La2Ti,0,. Moreover, compared with the a, values obtained for La2Ti,07 by extrapolating the lattice parameters of the cubic pyrochlores (Fig. 2), the value of 10.60 is about 0.19 A too high and thus improbable, quite apart from the impurities in Padurow and Schusterius's La203 which would tend to lower the a, of the cubic phase.

A!I the reflections observed in the powder pattern of Sc2Ti207 could be indexed on a fluorite unit cell. Because of the similarity of the atom~c scattering factors of Sc3+ and Ti4+, it was not possible to decide whether the product had the pyrochlore or the disordered fluorite-type structure? With b(Sc) = 1.18, b(Ti) = -0.34,

-'Brlxner (17), who had prepared Sc2Ti20, earlier, listed ~t with the cubic pyrochlore titanates and quoted an o, that corresponds to a double fluorite cell (Table I).

FIG. 2. Dependence of a0(A2Ti2O7) on the radius of A3 +. Full circles, (Y1 -,Bi,)2Ti207 ; double circles, ( L U ~ - , S C , ) ~ T ~ ~ O ~ ; half-full circle, YbNdTi20,. The value for Sc is 2no of the fluorite pattern.

and b(0) = 0.577 (all in 10-l2 cm (22)), neutron diffraction ought to make the distinction possible.

To this end a large batch of Sc,Ti,O, was prepared by coprecipitation (1000/24, 1200124, 1400124, 1400124, 95012 days, cooled with the furnace). The neutron diffraction pattern of the product was neither that of pyrochlore nor that of fluorite, but X-ray powder photographs taken before and after the neutron diffraction exper- iment contained only lines of a fluorite-type pattern. The films were of very good quality, with sharp diffraction lines.

These results are at variance with what would be expected from the investigation of the system Sc203-TiO, by Komissarova et al. (23). These authors concluded that there is no phase of the composition Sc203 .2Ti02. The phase closest to Ti02 is the S phase 2Sc203.3Ti02, which is rhombohedra1 (a,, = 7.742 k 5 A, E,, = 71.94 rt 0.10") below 1150 1 50°, and of the defect fluorite type above this temperature. The com- position Sc20, .2Ti02 should thus correspond to a mixture of G(Sc203-TiO,) and rutile if cooled slowly or annealed below 1 150°, or to a single phase of the fluorite type if quenched from temperatures above 1 150".

The conflicting evidence from the X-ray and neutron diffraction patterns of our sample fitted neither case. A possible explanation might be that the high-temperature disordered fluorite phase did not transform, under the above annealing conditions, into a mixture of 6 and TiO, but had

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

FIG. 3. Lattice parameter ao(P) vs. composition. Left, A2(Ti,-,Ge,),07 (A = Sm, Y, Er); center., (Lul_,Sc,),- TiZ07; ~ i g h t , (Y1 -IBix)ZTiZ07

nevertheless attained a considerable degree of short-range order. This, as well as vacancy ordering in the anion sublattice, might give rise to extra neutron diffraction maxima but not necessarily to additional X-ray reflections. A situation of this kind has recently been described for the phase Y 2 0 3 .2Ce02 by Barker and Wilson (25). Further work to clarify the discrepancy is underway, but even on the evidence available at present it is clear that the Sc,Ti,O, sample was not a cubic pyrochlore.

Chromium and indium titanates apparently had formed (cf. Fig. 5) but they did not have the pyrochlore structure6. The In compound was greyish, the Cr compound was dark brown. No compound appeared to form on firing InAlO, (C-type, a, = 10.1070 + 9 A) + 2Ti0, at 1400" for 12 h. MnTiO, with the ilmenite structure rather than Mn,Ti,O, was obtained by firing MnO, + TiO, or Mn,O, + 2Ti0, in air; this result was confirmed by t.g.a.

The structure of Bi titanate appeared to be related to that of pyrochlore (cf. Bi2Sn207, refs. 15 and 2). The extra lines present in the X-ray powder pattern could not be accounted for on a cubic pyrochlore cell. However, mixed phases

6Roth (15) did not obtain a compound on firing I n Z 0 3 + 2Ti02 at 1550" for 3 h. Hamelin (24) stated Cr2TiZ07 to form by firing the dry-mixed oxides at 1440" in air for 10 min as well as under other firing conditions. She reported the d spacings of the phase but made no attempt at indexing.

(Y,-,Bi,)2Ti207 that contained up to x = 0.75 bismuth were prepared as cubic pyrochlores (Table I, Fig. 3).

The (Lu, - ,SC, )~T~~O, system was investi- gated to determine the lower limit of existence of the pyrochlore phase. The limit is approximately at x = 0.55 (Table I, Fig. 3), which corresponds to an average Ahrens ionic radius of about 0.83 A.

Although Cr3+ is too small to form a pyro- chlore phase in its own right, the slight lowering of a,(P) of Y2Ti20, on introducing chromium (Table I) would seem to indicate that a small amount of Cr had entered the pyrochlore struc- ture. A similar observation was made with gallium. In spite of the similarity of the Ahrens ionic radii of In and Sc, a mixed Y-In pyrochlore titanate did not appear to form.

The two mixed titanates HoYTi,O, and YbNdTi,O, were pyrochlores, as expected.

Mixed Phases A, (Ti, -,M,),07 (M = Ge, Zr) No pyrochlore pattern was obtained for pure

Y2Ge207, Er2Ge207, or SC,G~,O, ,~ but it was possible to estimate the extent to which Ge could ---

'Recently Shannon and Sleight (26) prepared high- pressure cubic A,GeZO, pyrochlores (A = Gd, Dy, Ho, TI, Er, Tm, Yb, Lu, Y, In, and Sc); a corresponding Sm phase did not form. The lattice parameters of Y,Ge,O, (9.897 ? 2 A) and Er,Ge,O, (9.864 k 2 A) would fall 0.03-0.05 A below the points obtained for the corre- sponding A,(Ti, -,Ge,)207 phases by extrapolating the ao(P) vs. x straight lines of Fig. 3 to x = 1.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. V 977

be accommodated in Er, Y, and Sm titanates as host structures (Table I, Fig. 3):

Very little Ge was present in the homogeneous Sm phase at the saturation limit. Assuming that the stability of the pyrochlore structure in A2Ti20, is determined primarily by the radius ratio r(A3+):r(B4+), the upper limit of the ratio can be estimated from Sm2(Tio ,,,Ge, ,,,),07 as 1.50 (Ahrens). The largest value of r(A3+) in combination with r(Ge4') = 0.53 A would then be 1.50 x r(Ge4+) = 0.80 A, which is smaller than r(Lu3+). The lanthanons and scandium would thus not be expected to form pyrochlores with Ge. In a similar manner one can estimate the average maximum values of r ( ~ ~ + ) ~ ~ ~ ~ ~ = r(A3 +)/ 1.50 appropriate to Y and Er in the mixed germanates. As Table 111 shows, the estimates,

TABLE I11 --

F(B4+), A s r.(A3 +) :r.(B4+)

A (limit) li~nit calcd. (limit)

Ahrens radii Sm 0.08 0.668 0.668 1.50 Y 0.36 0.626 0.613 1.45

"Imoroved" radii

though lower, are reasonably close to the average radii F ( B ~ + ) , ~ , ~ , calculated from the experimental saturation limits.

When "improved" radii are used, the upper limit of the radius ratio becomes 1.56 and the largest value of r(A3'), 0.89 As. This value falls between r(Er3+) and r(Y3+) (cf. ref. 21, which would suggest that a cubic Er2Ge207 ought to exist. Since neither Y nor Er form pure ger- manate pyrochlores at ordinary pressures, the implication is that the effective size of the oxygen ion exerts a controlling influence on the upper stability limit of the pyrochlore structure even when the radius ratio is favorable, similar to the way in which it controls the lower limit (81, and

prevents the germanates from existing as pyro- chlores. In other words, the value oftheupper limit of the radius ratio decreases with decreas- ing r(A3') and r(B4+), which is the reason for I:(B~+),,,,, being increasingly lower than ?(B4+),,,,, (cf. Table 111). The same is of course true of the Ahrens radii, even disregarding the fact that the Ahrens radius of Sc3+, 0.81 A, is much too large (cf. next section). The upper limit can, however, be extended to include the pure germanates by application of high pressure (cf. footnote 7).

The three Y2(Ti,-,Zr,),07 phases (x = 0.36, 0.5, and 0.64) were pyrochlores (Table I).

Other Compositions An attempt was made to prepare, by firing the

dry-mixed oxides, mixed titanate pyrochlores of the type M2' N4+Ti,07. The combinations tried were CaGe (1200°), SrGe (12007, CaSn (1 500°), CaZr (1400°), SrZr (1400°), CaCe (140OC), and SrCe (1400"). The average ionic size of the com- bination CaSn is about the same as r(Lu3'), and CaZr would correspond to Er3+. SrZr, CaCe, and SrCe are still within the size range of the trivalent lanthanons (Ahrens radii), and compounds are known in which Ca2+Sn4+ or Ca2+Zr4+ sub- stitute for a trivalent ion of similar size. Examples are CaSn(B03), (nordenskioldite) and La(Ca,,,- Zr, ,,)03, though CaZrTi20, has previously been reported to have a symmetry lower than cubic (28), in fact tetragonal (calzirtite, ref. 29).

None of the products gave a pyrochlore pattern.

Lattice Parameters The agreement with the literature values was on

the whole good (Table I). The differences between corresponding a,(P) values in series C and D were within 30, with the exception of Er2Ti20, and Lu,Ti207. The final a, is probably affected by the method of preparation and the firing cycle.

The overall agreement of the lattice parameters reported in refs. 15,2, and 17 with those of series D was compared by regressing the a, sets on each other. Very high correlation coefficients were obtained, the highest one being that for series I9 and the an values of ref. 3 (Table IV). Repression

- of series 6 on ~,(A,s~,o;) (2) also gave a very 8A set of empirical r(B4-) for six-coordination has been high correlation coefficient, the slopes of the

derived from the lattice parameters and stability limits of regression and reverse regression lines being very cubic A23+B,4i07 pyrochlores and BO, rutiles that leads to a more sailsfactory description of the crystal chemistry nearly unity. As with the stannates (21, the rela- of such compounds (27). Provisional values for Ge and tionship betweell the lattice paralneters of the Ti are 0.57 and 0 62 b. respectively. These values together with the r ( ~ 3 + ) ' ~ f Templeton and Dauben (cf, ref, 2) titanates and r(A3 +) linear IV, Fig. 2)* constitute the above "improved" radii. For the D series least squares gave the equations

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

ANADIAU JOURNAL OF CHEMISTRY. VOL. 47, 1969

TABLE IV Standard deviations and correlation coefficients of a, and r(A3 +) regressions*

Correlation lo40, A Regression of ao(P) of 1040, A coefficient (reverse regression)

- Series D on ao(P) (15) Series D on ao(P) (17). Series D on ao(P) (serles C) Series D on no(P) (3)f- Series D on no(AzSn,O,) (2) Ref. 17 on r(A3 +) Ref. 15 on r(A3 +) Series D on r(A3 +)

*Templeton-Dauben radii (cf. ref. 2). ?Converted to present h(Cu K a ) (cf. ref. 2).

with standard deviations of the same order as were obtained for the stannates (2).

When the lattice parameters of the homo- geneous (Lu, -,SC,)~T~,O, phases (x = 0.2, 0.4) are substituted in the last equation, effective crystal radii of the "(L~,-,SC,)~'" ion are obtained, from which an average value of r(Sc3' ), 0.742 + 2 A, can be calculated. Extrapolating the correlation of ao(C) and the Templeton- Dauben radii (2) to Sc,03 (a, = 9.8455 A (30)) gave r(Sc3-) = 0.733 A.

The accuracy of these linear extrapolations may be questioned and the radius obtained from the mixed Lu-Sc titanates may be too high. Lu, as the larger ion of the two, may tend todominatethe lattice parameters in a situation which is marginal with respect to the stability of the pyrochlore phase. But there can be little doubt that the effective radius of Sc3+ in six or near-six co- ordination is much smaller than the value of 0.81 Pi ascribed to it by Ahrens. We have adopted the average of the above two values, 0.737 2 7 A, as a provisional working value to be used in the pyrochlore chemistry.

In a similar manner a value can be obtained for the radius of Bi(II1) from homogeneous (Y, -,- Bi,),Ti,O, phases (x = 0.2, 0.4, 0.5, 0.6; Table I). The average for the four compositions, 1.033 i 3 A, is again quite different from the "'ionic" radius listed for trivalent bismuth by Ahrens, 0.93 A, but the straight-line fit of Fig. 3 justifies accepting this figure as a provisional working value for r(Bi(II1)) in pyrochlores. The extrapolated lattice parameter of Bi,Ti,O,

would be 10.354 A. Similarly, extrapolating for Bi,Sn20, (cf. ref. 2) would yield 10.642 A, which is not too far from the dimension of the cubic pseudocell quoted for the stamate by Roth (1 5), 10.68 A.

It should be noted that these radii are for six- coordination, since they have been arrived at from the linear relationship of a,(P) and the r(A3+) of Templeton and Dauben, but the actual coordination of A in the pyrochlore structure is 2 + 6.

Mechanism of Formation of tlze Pyrochlore Phase Differential thermal analysis experiments with

dry-mixed A203 + 2Ti0, at heating rates of 10 and 2O0/min in stagnant air revealed two exo- thermic peaks of roughly equal intensity (Fig. 4, curve 1). While the position of the first peak varied little with A, the temperature corresponding to the second peak maximum depended markedly on A and passed through a minimum at Gd or Sm (Fig. 5).

The presence of two exothermic peaks indicates that the formation of the final product, A,Ti20,, proceeds in two or more steps. A likely possibility is the formation of A,TiO, as the precursory phase. To test this hypothesis d.t.a. curves of dry-mixed Gd203 4 TiO, (Fig. 4, curve 2) and Gd,TiO, + TiO, (Fig. 4, curve 3) were recorded under similar conditions9. The curve for Gd,03 + TiO, contained a single broad exothermic peak with a maximum at ca. 1152". Similarly, only one broad exothermic peak was observed in the curve for Gd,TiO, + TiO,, with a maximum at ca.

9Gd2Ti05 was prepared by firing Gdz03 + TiO, a t 1350" for 24 h. The powder pattern of the product was different from those of rutile, Gd203 and GdzTizO, (cf. refs. 19, 31, and 32).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. V

FIG. 4. Differential thermal analysis curves of dry-mixed Gdz03 + 2Ti02 (I), Gdz03 f Ti02 (2) and Gd2TiOS + TiO, (3) in stagnant air. Rate of heating, 2O0/min.

FIG. 5. Temperatarcs of the d.t.a. maxima of curvc 1 (Fig. 4) plottcd against the radius of A3+. Hcating rates in stagnant air: open circles, 1Oo/min; full circles, 2O0/min.

1230". When allowance is made for the difficulties corresponds to the 1099" peak of curve 1 and thus in attempting to reproduce d.t.a. conditions with to the formation of the 1 :1 compound, Gd,TiOS, sufficient accur'cy from one sample to another, while the 1230" peak corresponds to the 1201" and for the different solid state reactivities of the peak of curve 1 and to the formation of the 1 :2 three mixturca (different reacting species, particle compound, Gd,Ti,O,. The latter in turn is of size, extent of surface contact, thermal conduc- course the 1 :1 product ofthe reaction Gd,TiO, + tivity), it may be concluded that the 1152" peak TiO,.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

TABLE V Results of least squares structure refinements of A23+Ti24+072- (CU KY; full matrix)

-- -- -- - ---- - -- -- - -- - -- --

Lu Er Y Gd S m - -- --

Overall temperature factor After cvcle 10 9 10 10 9

Individual isotropic temperature factors

The appreciable overlap of the two maxima of curve 1 indicates that the two stages were not clearly separated and that some Gd2Ti207 had formed already at temperatures within the first peak. That the two-stage process of formation of Ln,Ti,O, in d.t.a. experiments is essentially dynamic in character was demonstrated from an X-ray photograph of the product obtained by firing a mixture of Er,03 + 2Ti0, at 1100' for 10 h. This temperature corresponds to the minimum between the two peaks in the d.t.a. curve of Er203 + 2Ti0,; the firing was an iso- thermal subsolidus process. There was no evidecce of Er2Ti0,. All the lines of the com- posite pattern could be accounted for by Er,Ti,O,, Er203, and TiO,.

The appearance of the two exothermic peaks in the d.t.a. curve of In203 + 2Ti0, is additional evidence of compound formation in this system. The dry-mixed oxides in Roth's experiment (15) may have been brought to too high a temperature too quickly for a reaction to begin before the In,03 volatilized.

The d.t.a. curve of TbO, ,,, + TiO, showed that Tb2Ti20, formed after the higher oxide had decomposed to Tb203.

Least-Squares Refinement The structures of Lu, Gd, Sm, and Y titanates

were refirled from X-ray ponder diffractometer data (CLI KY) using (I) individual isotropic tem- perature factors and a block-diagonal approxi-

mation F-program written for IBM 1620/40K (cf. ref. 1); (11) individual isotropic temperature factors and an ad lzoc full-matrix F-program written for IBM 360150; and (111) an overall temperature factor and program 11. The weight- ing scheme was W3 of ref. 35. One set of initial parameter values was employed in all the refine- ments : K = 1.000, ~ ( 0 , ) = 0.4200, B(A) = 0.15 A2, ,(Ti) = 0.60A2, B(0,) = B(0,) = 1.50A2, and B(overal1) = 0.50 A'. In addition the struc- ture of Er,Ti207 (1) was re-refined to see the effect of a different weighting scheme and a different set of scattering factors.

The refinement was relatively insensitive to the choice of weighting scheme and, for each of the three programs, to the assumed degree of ionicity of the constituent atoms. On Hamilton's &? criterion (36) there was no significant difference between the results obtained with f's for neutral atoms, for (A3 +, Ti3 + , 02 - ) , and for (A3 +, Ti4 +, 0,-). However, program I gave consistently lower x(0,) values than I1 or 111 (Fig. 6), with large e.s.d.'s of B(0,) and B(O,), while some of the B(0,) obtained with 11 were (positive or negative) indefinite (Table V). The overall tem- perature factors obtained with 111 were always physically reasonable and the x(0,) values were closely similar to those obtained with 11, in fact always within l o (Table V and Fig. 6). On the whole, the ionic model with Ti4+ yielded the lowest-correlated parameters.

As for Er2Ti207, there were small differences

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL,: PYROCHLORES. v 981

FIG. 6. Variation of the positional parameter x ( 0 , ) with r(A3+). BDA, block-diagonal approximation (refinement I); F M I1 and FM 111, full-matrix refinements I1 and 111. The points are slightly displaced to facilitate plotting.

in the final values of the temperature factors, but the x(0,) values obtained with j ( ~ r ~ ', ~ i ~ ' , 0' -) from all three programs were the same within lo of the individual values. The refinement results quoted in ref. 1 were not different within 20, and mostly within lo, from those obtained with re- finement I in the present work.

The similarity of the x(0,) values obtained from 11 and 111 indicated that the success of refining with program 11, which is physically more reasonable than 111, was not prejudiced by the relatively small number of IFo/ used in the refine- ment. The final R(I1) values were slightly lower than R(IJI), and the slopes of the best x(0,) vs. r(A3+) lines were practically the same for I1 and 111, and similar to the corresponding lines for the stannates (2) (Fig. 6)''. The final x(0,) values

1°For comparison, the refinements of the three stannates (2) were repeated nith I1 and 111. For I1 the final s(O,), R(A), B(Ti), B(OI), B(02), R, and e.s.d.(w) values respectively were: Y, 0.4093 i 52, 0.85 1 31, 0.12 F 18, -0.68 t 129, 2.75 + 123, 0.85%, 2.215; Sm, 0.4140 + 9 2 , 0.63 ? 73, 0.80 1 91, 0.63 + 213, 3.25 + 227, 0.81°:, 3.536; La, 0.4216 i 5, 1.32 1 32, -0.22 + 31, 4.17 z 71, -0.21 1 21, 0.71 %, 0.170. For

from I1 (Table V) have therefore been adopted as the best set. The observed and calculated structure factors based on I1 are compared in Table VI.

When the x(0,) values are plotted against r(A3') there is an appreciable scatter, but the increase of x(0,) with the size of A3 + is reason- ably evident. While it is true that the trend dis- appears when the uncertainty intervals are increased to 3o(x), corresponding plots for the stannates (Fig. 6) and for two hafnates (27) show similar behavior. One may conclude that x(0,) increases with r(A3+) in both the titanate and the stannate series, even though the accuracy of the refinements is perhaps not quite high enough to demonstrate this unambiguously. Thus the larger A3' the smaller the distortion of the BO, octa- hedron for a particular B4+, and the larger the B4+ ion, for a particular A3 +, the greater the distortion. In other words, the greater the r(A3'):r(B4') ratio: the less distorted is the BO,

I11 the final x(02), B(overall), R and e.s.d.(w) values respectively mere: Y, 0.4154 + 13, 0.40 3, 1.03 %, 2.465: Sni, 0.4203 i 18, 0.71 t 3, 1.17%, 3.661; La, 0.4204 i 10, 0.77 i 22, 1.92%, 0.391.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

TABLE V I

Observed and calculated structure factors (Cu Ka, Ti4+, refinement 11)

*Not used in the refinement. ?Not measured.

octahedron. An independent and striking con- firmation of the decrease of the distortion of BO, with increasing r(A3+) in the A,FeSbO, pyro- chlore series has been obtained from the Moss- bauer spectra of these phases at room temperature (37). The quadrupole splitting at the Fe5, nucleus decreases with increasing r(A3+), thus indicating a progressively smaller distortion of the FeO, octahedron.

Compared with the other titanates, the values of x(0,) for Gd,Ti,O, and Sm2Ti20, are less reliable because of the exceptionally high back- grounds and low peak-to-background ratios of most reflections in the diffractometer patterns of these two phases, and also because of the large and probably uncertain dispersion corrections for f(Gd) for Cu Ka radiation.

The powder intensity data for the five titanates have been submitted to the American Society for Testing and Materials (ASTM) powder diffraction file.

Re$/zemeizt of Y , (Ti, ,64z~a0 , 3 ,) 207 front Neutron Data

Twelve IF,I were extracted from the neutron powder pattern (Fig. 7) by Gaussian analysis

(cf. ref. 2) and utilized in refinements I, 11, and 111. The initial parameter values were the same as for the refinements from X-ray data except for B(overall), which was 0.70 W2. The neutron scattering amplitudes were taken as b(Ti) = -0.34, b(Zr) = 0.62, b(0) = 0.577, and h(Y ) = 0.80 (all in 10-l2 cm) (22).

None of the refinements was completely satis- factorv. Some of the temllerature factors were (positive or negative) indefinite or even negative. No significant difference resulted on using b(Y) = 0.788 x lo-'' cm (38).

The best x(02) value was that from 211, 0.4177 + 21. It was identical with that from 11, while the value obtained in refinement I was con- siderably lower. The final R values were in the range 6-9%.

Interatomic Distances and Bond A~~gles The apparent interatomic distances and bond

angles (not corrected for thermal motion) are listed in Table VII. They have been calculated from the final 4 0 , ) (11). The uncertainty limits are based on 3o(x), and on 4o(a,) for distances independent of ~ ( 0 , ) .

The shortest Ti-0 distances in the five titanates

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. V

FIG. 7 . Raw (top) and reduced (bottom) neutron diffraction patterns of Y,(Tio.e~Zro.,,),O, (monochromator, Al(111); h = 1.10 A; counting time per point, 10 inin).

all fall between the two Ti-0 distances reported only on a,. It is thus shorter in the titanate than by Baur for rutile, 1.944 f 4 and 1.988 f 6 in the stannate pyrochlores. For example, the (cf. ref. I), and they are all within the range known Y-0, distance in the stannate is 2.2457 2 10 A from TiO, octahedra in other compounds (2), while in the titanate it is 2.1856 f 6 A, which (Table VIII and ref. 1). is one of the shortest Y-0 distances reported

Comparison of the A-0 distances is more (Table VIII, and Table V of ref. 2). The A-0, difficult to make. The A-0, distance depends distances are on the whole rather short, as might

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

TABLE VII

Apparent interatomic distances and bond angles in A,Ti,O, pyrochlorese

Distance or angle Lu Er Y Gd Sm

Distances, A

Bond angles, (degrees)?

- . . - . - .

*For atomic positions see Table IV of ref. 1; 05(2) is now 0z(5) etc. (the same applies to Table IV of ref. 2). S[02(4)-Ti(l)-02(5)1 + [02(5)-Ti(l)-0,(6)1 = 2R: [01(l)-A(1)-0~(3)1 + [O,(l)-A(I)-0,(2)1 = 2R; A(1)-01(1)-

A(2) = 109.47' (tetrahedral angle); Ti-Ti, A-A, TI-A = a,, 4214.

TABLE VIII

M-0 distances in some compounds of Gd, Er, Lu, Y, and Ti(IV)*

Cornoound Coordination Distance, A Reference

Gd3Fe,0,, (garnet)

Gd-0 distances 3 0 + 6 0 2.30; 2.50 1 0 + 1 0 + 2 0 + 2 0 + 2 0 2:26+5; 2.3614; 2.3812; 2.3912; 2 .82 i2

(12-coordinated; shortest d~stances only) 4 0 f 4 0 + 4 0 + 4 0 2.38; 2.47; 2.67; 2.87 (all 1 8)

(shortest distances only)

Er-0 distances 6 H Z 0 + 3H20 2.37; 2.52

Lu-0 distances

Y-0 distances

Ti(IV)-0 distances 1 0 + 1 0 + 2 0 + 1 0 + 1 0 1.83+5; 1.8414; 1.9417; 2.0614; 2.2017 1 0 + 2 0 + 2 0 + 1 0 1.843116; 1.952+18; 1.973117; 2.167119

*For Sm-0 distances see ref. 2. For additional values of Er-0, Y-0, and Ti(1V)-0 distances see refs. 1 and 2.

be expected from the tetrahedral coordination of the 0, atom, and hence not typical. Subtracting u(A3+) for six-coordination from the A-0, distances gives values from 1.321 A (Lu) to 1.252 A (Sm) for the radius of the oxygen atom; in view of the quite small uncertainties in the A-Dl distances these differences must be con- sidered as meaningful. By contrast, subtracting r(A3+) from the A-0, distances leaves residues

that do not significantly differ within the 30, and in fact within the 20 limits, the average value of the "oxygen radius" being about 1.59 A.

The A-0, distances increase progressively from Lu to Sm. As in the stannates, they are longer than the corresponding distances reported for six-coordinated A: but within the range for higher coordination. In the two cases where they are known for both series, they are not signif-

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES v 985

L'lAVE NUMBER, cm-'

FIG. 8. Infrared transmission spectra of representative pyrochlore titanates and related compounds.

icantly different within 30 (Sm) or even within 20 (Y). This seems to indicate that the 0, atom, for which the A-0 distance varies with B4+$ is under constraint in a "forced" coordination, and the A-0, distance is not free to adjust in the same way as the A-0, distance does through change in 4 0 , ) . As a consequence the 0, atom will be polarized by the same nearest neighbor A to an extent that will depend on the size of B4+.

The difference between A-0, and A-0, introduces an ambiguity into attempts to assign an "effective radius" to A3+ in the 2 + 6 co- ordination. The standard radii for six-coordina- tion leave 0, undersized and 0, too large, thus leadlng to the conclusion that the effective 6' shape"" of the A atom in this coordination is an ol2late splieroid.

Infrared Spectra Compared with the spectra of the pyrochlore

stannates in the 1600-250 cm-I region (2), the features of the corresponding titanate spectra were not nearly as well defined. This applies also to the spectrum of BaTiO, (1) compared with BaSnO, (2), and to rutile compared with SnO,. The main feature of the titanate spectra in this region is a very strong band with a wide, flat maximum exhibiting shallow fine structure peaks at 540-575 (A), 440-460 (B), 390-440 (C), and about 390 cm-' (D) (Fig. 8). Another band was sometimes observed at 280-295 cm-l, but its position could not be determined accurately because it was too close to the liinit of the instrument range.

When the trivalent cation in the titanate

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

986 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

changes, the frequency of the B band does not appear to change, but the frequencies of A and C are shifted (Fig. 9). The A and B frequencies of Y2Ti,0, are noticeably above the corresponding frequencies of the lanthanon titanates. This is in contrast with thestannateseries (2), where the peak frequencies of the yttrium member were indis- tinguishable from those of Ho and Er stannates. The assumption that was made for the stannates does not seem to hold for the titanates. That is, the Ti atom is not sufficiently heavy to provide, together with Y, a quasi-stationary framework I'or the oxygen atoms. Consequently the effect of approximately halving the mass of the lanthanon atoms makes itself felt in the increased frequencies of Y2Ti20,, even though the increase may not be as much as would appear from Fig. 9 because of the spread of the points about the least-squares straight lines.

Changes of frequency of the absorption bands in the expected direction were observed on replacing Ti in Er2Ti20, partially with Ge, and Y in Y,Ti,O, with Bi. The mass average of

3er i 8 , , I ,I , , , , , LI. Y3 Tm Er Y H o Dy Tb Gd Eu Sm

r ( ~ " ) ,

FIG. 9. Variation of the frequencies of the A, B, and C absorption maxima with u(A3+). Full circles, Y2TiZ07.

0 HEATING

10 COOLING

100 200 300

TEMPERATURE, 'K

FIG. 10. Permittivity vs, temperature relationships for Sm2Ti207 and Er2Ti207.

Y,,,Bi,,, is very close to the mass of Sm, but the general features of the spectrum of the mixed phase resemble more those of Bi,Ti20, (which is not a cubic pyrochlore) than those of Sm2Ti,0,. The spectrum of La,Ti20, may be viewed as derived from the spectrum of Sm2Ti20, by splitting of the main absorption bands and an appreciable increase in the intensity of the shoulder between 700 and 800 cm-l : a detailed correlation cannot be offered at present.

The spectra of the In and Cr phases, which are believed to be homogeneous phases, are also shown in Fig. 8. The Sc2Ti,0, phase which gave a fluorite-type pattern had a spectrum with no distinct features but resembling, in a general way, the spectrum of Sc,Sn,O,, (2).

Dielectric Properties Permittivity measurements on two pyrochlore

titanates, Sm2Ti,0, and Er2Ti,0,, from room temperature to 4.2 OK gave no indication of ferro- electric transitions (Fig. 10).

Conclusions ( I ) The linear relationship, which exists be-

tween a, of the pyrochlore titanates and the Templeton-Dauben radii of A3': makes it possible to derive provisional values of the crystal radii of Sc3+ and Bi(II1) for six-coordination. Similarly, linear extrapolation for C-type sesqui- oxides (2) provides a check on the compatibility,

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KNOP ET AL.: PYROCHLORES. v 987

TABLE IX

Ionic (crystal) radii of A3 + in six-coordination, in Ah - -

This work$ A3 + Goldschmidt Pauling Ahrens Zachariasen T and Dt Others (provisional)

*Values in italics constitute the best set (adopted in the present work). pTernpleton and Dauben (cf. ref. 2). :The lattice parameters of the C-type oxides used in the calculation were: TI, 10.543 (50). In, 10.1 17+ 1 (51); P-Fe20,,9.4? (52): Am, 11.03 i 1

(53). Pu, 11.04i2 (54); Cm, 11.01 + 2 (55); Pm, 10.99 (56); and Bk, 10.8865 (49) (all in'&,). For Cf the B-type sesquloxlde was used (57); its exac't stoichiometry was, however, not known.

with the Templeton-Dauben set, of the values quoted for the crystal radii of some other six- coordinated trivalent ions. Comparing the radii derived in this way for TI3+, In3+, and Fe3+ (Table IX) with the corresponding Ahrens values reveals a major discrepancy for r(T13 '), but only a small difference for In3+; the values for Fe3+ are identical1 I.

Linear extrapolation using the same relation- ship leads to crystal radii for six-coordinated trivalent Pu, Am, Cm, and Bk which differ only insignificantly, or not at all, from the values listed recently by Peterson and Cunningham (49). It also shows that Espinosa's a, of Pr203 (C-type), 11.152 $- 3 A (66), though higher than the 11.140 + 2 A quoted by Eyring and Baenziger

l shannon (71) has suggested that the effective radius of in the 3-3 perovskites is considerably smaller than the Ahrens value, and probably less than 0.85 A; for Sc3+ he uses a value of 0.73 A and for In3+, 0.77 A (estimated from his Fig. 2). Shannon's values thus corroborate the validity of the effective crystal radii in six-coordination deduced for the three ions in the present work.

(69), is still below the calculated value, 11.165 + 5 A. The lattice parameter reported by Sastry et al. (70), 11.160 + 6 A, is however in close agree- ment.

Whatever the precise significance and appli- cability of empirical crystal radii, if a consistent set for a particular coordination is available, it can serve for correlation, verification, and predic- tion of various metric properties of crystal struc- tures. The usefulness and self-consistency of the Templeton-Dauben set, augmented by Geller's value for Y3 +, has been demonstrated earlier (2). The enlarged set of Table IX has been tested on several isostructural series of compounds (Fig. 1 1)12. The straight-line fits are seen to be very satisfactory for coordination numbers from 6 to 8 and for structure types as different as CrB and the

12The unit-cell dimensions of the compounds shown in Fig. 11 were taken mostly from ref. 58. In addition the following references were used: AzO3 (B-type), 56, 57, 61, 62; A z 0 3 (high-pressure corundum), 63; AUO,, 64; AzGe207 (high-pressure pyrochlore), 26; Fe and Ga garnets, 65, 66; Ca3Al2Si3OI2, 59, 60; Sr hydrogarnets, 59; AGa, 67, 68.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

KKOP ET AL.: PYKOCHLOKES. v 989

garnets. I t is probably true, however, that these radii will in general give best linear correlations for oxygen compounds with structures of high symmetries, in which variations of the positional parameters from one member of an isostructural series to next are relatively insignificant compared with the variations of the unit-cell volume. Where appreciable covalency and pronounced asyni- metric orbital occupancies are involved, the radii lose their universality. An example is Bi. When occupying sites of highly regular coordination at random, such as in mixed phases of the pyrochlore or fluorite type (Fig. 1 I), Bi(II1) falls in the r(A3 +) sequence just below Ce3+. In other structures, where the coordination is less regular and the degree of covalency higher, the apparent size of the Bi atom decreases, approaching perhaps that of Pr3+ or even Nd3 '. This is evident in Fig. 11. Although there may be some doubt about the exact co~ilposition of at least one of the two forms of BiPO, referred to in the drawing, BiOCl illustrates this point adequately. A similar tendency may exist for TI.

Successful correlation of the Templeton- Dauben radii with the empirical crystal radii of the smaller trivalent ions (Al, Ga, Ti, V, Cr, Mn, Co, Rh) is handicapped by the paucity of suitable isostructural series that would include com- pounds of Sc and In and at least a few of the smaller lanthanons ; the spin configurations of the d electrons may also have to be taken into consideration in some cases.

(2) The stability region of the pyrochlore titanates extends from an Ahrens radius ratio, r(A3 +) :r(B4+ ), of about 1.22 (in mixed Lu-Sc phases) to about 1.47 (Sm,Ti,O,); or, using the "improved" radii, from about 1.27 to about 1.55. The upper limit is slightly extended in Sm titanate- germanate phases to 1.50 (Ahrens) or 1.56 ("improved").

Although Bi,Ti207 is not a cubic pyrochlore, in mixed Y-Bi titanates the pyrochlore phase extends to about Y,.,Bi, ,,Ti,O,, i.e. to a radius ratio of ca. 1.36 (Ahrens) or 1.61 ("improved"). The corresponding values for a pure Bi pyro- chlore would be 1.37 and 1.67.

If the present value proposed for r(Bi(lII)), 1.033 A, is accepted, the upper limit in the Y-Bi titanates is higher than in the Ln compo~inds, though pure Bi titanate should be, on the radius- ratio criterion, outside the pyrochlore field, as is in fact the case. However, Bi2Sn20,, which is not a pyrochlore either, has a radius ratio of 1.31

(Ahrens) or 1.44 ("improved"), and is thus inside the upper ratio limit for the pyrochlore stannates, 1.61 (Ahrens) or 1.48 ("improved") (in La,Sn,- 0,). These observations again demonstrate the exceptional behavior of bismuth as a potential pyrochlore former.

(3) The positional parameter x(0,) in the titanates increases with r(A3') and has larger values than in the corresponding stannates. Consequently the distortion of the BO, octahedra decreases with the increasing r(A3 ') :r(B4 +) ratio.

(4) No evidence of ferroelectric transitions in Sm and Er titanates was found by permittivity measurements between room temperature and 4 2 OK.

(5) Unlike in Y,Sn,O,, in Y,Ti,O, the atoms of the two metals are not sufficiently heavy for their sublattices to behave like a stationary frame- work with respect to the oxygen atoms. Some of the absorption maxima of Y2Ti,0, in the 600- 300 cm-I region have therefore frequencies significantly higher than those of the correspond- ing maxima of Er and Ho titanates.

Acknowledgments We should like to express our gratitude to the

following individuals and agencies: The staff of the Neutron Physics Branch,

Atomic Energy of Canada Limited, for permis- sion to use their neutron diffraction facilities; Dr. F. R. Ahmed, for making available to us the least-squares program of ref. 1 ; Dr. M. Falk, for placing at our disposal the infrared spectrometer; Mr. Y. S. Lim, for carrying out the dielectric measurements; Dr. M. H. Jericho, for making possible the dielectric study at low temperatures; Dr. N. F. H. Bright and Mr. R. H. Lake, for the differential thermal analysis of the Gd203 i- TiO, mixtures; Drs. R. D. Shannon and W. W. Barker, for seeing the manuscripts of their papers (refs. 26 and 25) before publication; Dr. V. Seidl and Mr. C. Ayasse, for assistance with some of the computations; and Drs. B. I. Pokrovskii and F. M. Spiridonov of the Moscow State University, for encouraging us to investigate Sc,Ti20, in detail and for a generous gift of high-purity scandium oxide.

Financial support of this investigation by the Defence Research Board and the National Research Council of Canada through grants to the first-named author is hereby acknowledged. Some of the preparative work and a part of the

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

990 CANADIAK JOURNAL OF CHEMISTRY. VOL. 47, 1969

X-ray study were carried out under Defence Research Board contract No. 800107/200024 and are reported in the present form with the permission of the Board.

Last but not least we should like to thank the staff of the Dalhousie Computing Centre for their unfailing assistance with machine computation.

1. 0. KNOP, F. BRISSE, L. CASTELLIZ, and SUTARNO. Can. J. Cheni. 43, 2812 (1965).

2. F. BRISSE and 0. KNOP. Can. J. Chem. 46, 859 (1968).

3. L. CASTELLIZ and 0. KNOP. 13th Progress Report, December 31st, 1963; and 14th Progress Report, June 30th, 1964. Defense Research Board Contract No. 200024. Nova Scotia Technical College, Halifax, Nova Scotia.

4. W. S. CLABAUGH, R. T. LESLIE, and R. GILCHRIST. J. Res. Natl. Bur. Std. 55. 261 (1955).

5. N. C. BAENZIGER, H. A. EICK, H. S. SCHULDT, and L. EYRING. J. Am. Chem. Soc. 83, 2219 (1961).

6. R. E. FERGUSON, E. D. GUTH, and L. E Y R I ~ G . J. Am. Chem. Soc. 76, 3890 (1954).

7. E. D. GUTH, J. R. HOLDEN, N. C. BAENZIGER, and L. EYRIUG. J. Am. Chem. Soc. 76, 5239 (1954).

8. F. BRISSE and 0. KNOP. Can. J. Chem. 45, 609 (1967).

9. D. T. CROMER and J. T. WABER. Acta Cryst. 18,104 (1 965) \ - - - - 3 -

10. D. T. CROMER. Acta Cryst. 18, 17 (1965). 11. International tables for X-ray crystallography. Vol.

111. Kynoch Press, Birmingham. 1962. 12. M. TOKONAMI. Acta Cryst. 19, 486 (1965). 13. D. G~uscX and I. POPESCU. Bull. soc. roumaine

phys. 40, 13 (1939). 14. F. JOKA, G. SHIRA~E, and R. PEPINSKY. Phys. Rev.

98, 903 (1955). 15. R. S. ROTH. J. Res. Natl. Bur. Std. 56. 17 (1956). 16. J. B. MACCHESNEY and H. A. SAUER. J. Am.

Ceram. Soc. 45, 416 (1962). 17. L. H. BRIXNER. Inorg. Chern. 3, 1065 (1964). 18. J. L. WARING and S. J. SCH~EIDER. Am. Ceram.

Soc. Bull. 43, 4, 263 (1964). 19. F. QUEYROUX. Con~pt. Rend. 259, 1527 (1964). 20. J. D. AULT and A. J. E. WELCH. Acta Cryst. 20,410

(1966). 21. N. N. PADUROW and C. SCHUSTERIUS. Ber. Deut.

Keram. Ges. 31, 391 (1954). 22. G. E. BACOB. Neutron d~ffractlon. 2nd ed. Oxford

Un~verslty Press, Oxford. 1962. 23. L. N. KOMISSARO~A, B. I. POKROVSK~~, and V. V.

NECHAEVA. Dokl. Akad. Nauk SSSR, 168, 1076 (1966).

24. M. HAMELIU. Bull. Soc. Chim. France, [5] 24, 1421 (1 957).

25. w.-W. BARKER and A. F. WILSON. J. Inorg. Nucl. Chem. 30,1415 (1968).

26. R. D. SHAKNON and A. W. SLEIGHT. Inorg. Chem. 7, 1649 (1968).

27. 0. K N ~ P and F. BRISSE. TO be vublished. 28. L. W. COUGHANOUR, R. S. ROTH; S. MARZULLO, and

F. E. SENNETT. J. Res. Natl. Bur. Std. 54, 191 11 955) \ . - - - , .

29. Yu. A. PYATEKKO and Z. V. PUDOVKI~A. Kristallo- grafiya, 9, 98 (1964); Soviet Phys.-Cryst. English Transl. 9, 76 (1964).

30. 0. Khop and J. M. HARTLEY. Can. J. Chem. 46, 1446 (1968).

31. J. L. W A R I ~ G and S. J. SCHNEIDER. J. Res. Natl. Bur. Std. A. 69. 255 (1965).

32. F. Q~EYROUX, A. H A R ~ I , and R. COLLO\GUE~. Bull. Soc. Franc. Ceram. [72], 37 (1966).

33. E. ALESHI~ and R. ROY. J. Am. Ceram. Soc. 45. 18 (1962).

34. E. ALESHIN. M.Sc. Thesis. The Pennsylvania State University, State College, Pennsylvania. 1959.

35. SUTARNO, 0. KYOP, and K. I. 6. REID. Can. J. Chen~. 45, 1391 (1967).

36. W. C. HAMILTON. Acta Cryst. 18, 502 (1965). 37. 0. KNOP, F. BRISSE, R. E. MEADS, and J. BAINBRIDGE.

Can. J. Chem. 46, 3829 (1968). 38. M. G. PATOK and E. N. MASLEPU. Acta Cryst. 19,

679 (1965). 39. A. PABST. J. Chen~. Phys. 11, 145 (1943). 40. P. COPPENS and M. EIBSCHUTZ. Acta Cryst. 19, 524

(1965). 41. J. WEIDEKBORNER. Acta Cryst. 14, 1051 (1961). 42. D. R. FITZWATER and R. E. RUNDLE. Z. Krist. 112,

362 (1959). 43. H. L. YAKEL, W. C. KOHLER, E. F. BERTAUT, and

E. F. FORRAT. Acta Cryst. 16, 957 (1963). 44. S. F. BARTRAM. Inorg. Chem. 5, 749 (1966). 45. L. A. HARRIS and H. L. YAKEL. Acta Cryst. 22,354

11 9671 46. R: RGTH and A. D. WADSLEY. Acta Cryst. 21, 974

(1966). 47. W. G. MUMME and A. D. WADSLEY. Acta Cryst. 23,

754 (1967). S. GELLER. Acta Cryst. 10, 248 (1957). J. R. PETERSON and B. B. CUNNI~GHAM. Inorg. Nucl. Chem. Letters, 3, 327 (1967). H. E. SWAMON and R. K. FUYAT. Natl. Bur. Std Clrc. No. 539. 2, 28 (1953). M. MAREZIO. Acta Cryst. 20, 723 (1966). M. BONNEVIE SVEVDSEN. Naturwiss. 45, 542 (1958). D. H. TEMPLE TO^ and C. H. DAUBEU. J. Am. Chem. Soc. 75, 4560 (1953). L. B. ASPREY, F. H. ELLINGER, S. FRIED, and W. H. ZACHARIASEK. J. Am. Chem. Soc. 77, 1707 (1955). J. C. WALLMA~. J. Inorg. Nucl. Chem. 26, 2053 (1 964). F. WEIGEL and V. SCHERER. Radloch~m. Acta, 4, 197 (1 965) J. L: G R ~ E N and B. B. C U ~ ~ I N G H A W . Inorg. Nucl. Chem. Letters, 3, 343 (1967). R. W. G. WYCKOFF. Crystal structures. 2nd ed. Vols. 1-111. Interscience Publ~shers. Inc.. New York. , , 1963-1965.

59. JUN ITO. Private communication. 1967. 60. B. J. SKINNER. Am. Mineralogist, 41, 428 (1956). 61. R. S. ROTH and S. J. SCHKEIDER. J. Res. Natl. Bur.

Std. A, 64, 309 (1960). 62. H. R . HOEKSTRA. Inorg. Chern. 5, 754 (1966). 63. R. D. SHANNON. Solid State Commun. 4, 629

(1966). 64. W. RUDORFF, H. ERFURTH, and S. KEMMLER-SACK.

Z. Anorg. Allgem. Chen~. 354, 273 (1967). 65. F. BERTAUT and F. FORRAT. Colnpt. Rend. 244, 96

(1957) 66. G: D('ESPI~OSA. J. Chem. Phys. 37, 2344 (1962). 67. A. E. DWIGHT, J. W. DOWNEY, and R. A. COAAER,

JR. Acta Cryst. 23, 860 (1967). 68. 0. SCHOB and E. PARTHP. Acta Cryst. 19, 214

(1 9651. 69. L. E < R I ~ G and N. C. BAE~ZIGER. J. Appl. Phys. 33,

428 (1962). 70. R. L. N. SASTRY, P. N. MEHROTRA, and C. N. R. RAO.

Trans. Faraday Soc. 62, 1632 (1966). 71. R. D. SHANKOA. Inorg. Chem. 6, 1474 (1967).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

This article has been cited by:

1. R. N. P. Choudhury, Biswajit Pati, Piyush R. Das, R. R. Dash, Ankita Paul. 2013. Development of Electronic and ElectricalMaterials from Indian Ilmenite. Journal of Electronic Materials 42:4, 769-782. [CrossRef]

2. X. J. Wang, H. Y. Xiao, X. T. Zu, Y. Zhang, W. J. Weber. 2013. Ab initio molecular dynamics simulations of ion–solid interactionsin Gd2Zr2O7 and Gd2Ti2O7. Journal of Materials Chemistry C 1:8, 1665. [CrossRef]

3. I.V. Piir, N.A. Sekushin, V.E. Grass, Y.I. Ryabkov, N.V. Chezhina, S.V. Nekipelov, V.N. Sivkov, D.V. Vyalikh. 2012. Bismuthmanganese titanate: Crystal structure and properties. Solid State Ionics 225, 464-470. [CrossRef]

4. Oliver Merka, Olga Raisch, Frank Steinbach, Detlef W. Bahnemann, Michael Wark, J. Nino. 2012. Effects of Nonstoichiometryand Cocatalyst Loading on the Photocatalytic Hydrogen Production with (Y 1.5 Bi 0.5 ) 1− x Ti 2 O 7−3 x and (YBi) 1− x Ti2 O 7−3 x Pyrochlores. Journal of the American Ceramic Society n/a-n/a. [CrossRef]

5. Mei Yang, Yang Yu-Guang, Zheng Wen-Chen. 2012. Theoretical calculations of the optical band positions and spin-Hamiltonianparameters for Cr3+ ions in Y2Ti2O7 crystal. Physica B: Condensed Matter 407:18, 3881-3883. [CrossRef]

6. Craig Bryan, Catherine A. Whitman, Michel B. Johnson, John F. Niven, Patrick Murray, Alex Bourque, Hanna A. Dąbkowska,Bruce D. Gaulin, Mary Anne White. 2012. Thermal conductivity of (Er_{1−x}Y_{x})_{2}Ti_{2}O_{7} pyrochlore oxide solidsolutions. Physical Review B 86:5. . [CrossRef]

7. Surajit Saha, Pallavi Ghalsasi, D. V. S. Muthu, Surjeet Singh, R. Suryanarayanan, A. Revcolevschi, A. K. Sood. 2012. Phononanomalies and structural transition in spin ice Dy2Ti2O7: a simultaneous pressure-dependent and temperature-dependent Ramanstudy. Journal of Raman Spectroscopy 43:8, 1157-1165. [CrossRef]

8. A. Hardy, S. Van Elshocht, C. De Dobbelaere, J. Hadermann, G. Pourtois, S. De Gendt, V.V. Afanas’ev, M.K. Van Bael. 2012.Properties and thermal stability of solution processed ultrathin, high-k bismuth titanate (Bi2Ti2O7) films. Materials ResearchBulletin 47:3, 511-517. [CrossRef]

9. Shahid P. Shafi, Bradley C. Hernden, Lachlan M. D. Cranswick, Thomas C. Hansen, Mario Bieringer. 2012. Topotactic OxidationPathway of ScTiO 3 and High-Temperature Structure Evolution of ScTiO 3.5 and Sc 4 Ti 3 O 12 -Type Phases. InorganicChemistry 51:3, 1269-1277. [CrossRef]

10. David S. D. Gunn, Neil L. Allan, Henry Foxhall, John H. Harding, John A. Purton, W. Smith, Michael J. Stein, Ilian T. Todorov,Karl P. Travis. 2012. Novel potentials for modelling defect formation and oxygen vacancy migration in Gd2Ti2O7 and Gd2Zr2O7pyrochlores. Journal of Materials Chemistry 22:11, 4675. [CrossRef]

11. J. Roberto Esquivel-Elizondo, Beverly Brooks Hinojosa, Juan C. Nino. 2011. Bi 2 Ti 2 O 7 : It Is Not What You Have Read.Chemistry of Materials 23:22, 4965-4974. [CrossRef]

12. J A Hodges, P Dalmas de Réotier, A Yaouanc, P C M Gubbens, P J C King, C Baines. 2011. Magnetic frustration in the disorderedpyrochlore Yb 2 GaSbO 7. Journal of Physics: Condensed Matter 23:16, 164217. [CrossRef]

13. T. Hartmann, A. Alaniz, F. Poineau, P.F. Weck, J.A. Valdez, M. Tang, G.D. Jarvinen, K.R. Czerwinski, K.E. Sickafus. 2011.Structure studies on lanthanide technetium pyrochlores as prospective host phases to immobilize 99technetium and fissionlanthanides from effluents of reprocessed used nuclear fuels. Journal of Nuclear Materials 411:1-3, 60-71. [CrossRef]

14. Lu Cai, Alex L. Arias, Juan C. Nino. 2011. The tolerance factors of the pyrochlore crystal structure. Journal of Materials Chemistry21:11, 3611. [CrossRef]

15. Xiaoyang LiuHigh Pressure Synthesis and Preparation of Inorganic Materials 97-128. [CrossRef]16. A.R. Heredia, M. Quintana García, J.L. Pérez Mazariego, R. Escamilla. 2010. X-ray diffraction and Raman spectroscopy on

Gd2(Ti2−yTey)O7 prepared at high pressure and high temperature. Journal of Alloys and Compounds 504:2, 446-451. [CrossRef]17. C. Stanek, C. Jiang, B. Uberuaga, K. Sickafus, A. Cleave, R. Grimes. 2009. Predicted structure and stability of A4B3O12 δ-phase

compositions. Physical Review B 80:17. . [CrossRef]18. Oliver Janka, Thomas Schleid. 2009. Two fluoride-containing oxotantalates(V) of the light lanthanides: NdFTa2O6 and

La3Cl3TaO5F. Solid State Sciences 11:11, 1992-1997. [CrossRef]19. K. Krishnankutty, K. R. Dayas. 2008. Synthesis and characterization of monoclinic rare earth titanates, RE2Ti2O7 (RE = La, Pr,

Nd), by a modified SHS method using inorganic activator. Bulletin of Materials Science 31:6, 907-918. [CrossRef]20. Yuping Tong, Zongxue Yu, Lude Lu, Xujie Yang, Xin Wang. 2008. Rapid preparation and characterization of Dy2Zr2O7

nanocrystals. Materials Research Bulletin 43:10, 2736-2741. [CrossRef]21. A. Garbout, A. Rubbens, R. N. Vannier, S. Bouattour, A. W. Kolsi. 2008. Raman scattering and X-ray diffraction on YBiTi 2 O

7 prepared at low temperature. Journal of Raman Spectroscopy 39:10, 1469-1474. [CrossRef]

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

22. B.J. Sun, Q.L. Liu, J.K. Liang, J.B. Li, L.N. Ji, J.Y. Zhang, Y.H. Liu, G.H. Rao. 2008. Subsolidus phase relations in the ternarysystem SnO2–TiO2–Y2O3. Journal of Alloys and Compounds 455:1-2, 265-268. [CrossRef]

23. A. V. Shlyakhtina, S. N. Savvin, A. V. Levchenko, I. V. Kolbanev, O. K. Karyagina, L. G. Shcherbakova. 2008. Synthesis and high-temperature electrical conductivity of Ln2Ti2O7 and LnYTi2O7 (Ln = Dy, Ho). Inorganic Materials 44:3, 299-304. [CrossRef]

24. Yuping Tong, Yanping Wang, Zongxue Yu, Xin Wang, Xujie Yang, Lude Lu. 2008. Preparation and characterization of pyrochloreLa2Zr2O7 nanocrystals by stearic acid method. Materials Letters 62:6-7, 889-891. [CrossRef]

25. H. Y. Xiao, X. T. Zu, Fei Gao, W. J. Weber. 2008. First-principles study of energetic and electronic properties of A[sub 2]Ti[sub2]O[sub 7] (A=Sm, Gd, Er) pyrochlore. Journal of Applied Physics 104:7, 073503. [CrossRef]

26. Masanobu Higashi, Ryu Abe, Hideki Sugihara, Kazunari Domen. 2008. Photocatalytic Water Splitting into H2 and O2 overTitanate Pyrochlores Ln2Ti2O7 (Ln = Lanthanoid: Eu–Lu). Bulletin of the Chemical Society of Japan 81:10, 1315-1321.[CrossRef]

27. Lihong Zhao, Zhidong Tian, Zheng Tang, Ziyu Wang, Benpeng Zhu, Lili Wang, Rui Xiong, Jing Shi. 2007. Synthesis andmagnetic properties of spin-liquid Tb2Ti2O7. Wuhan University Journal of Natural Sciences 12:6, 1039-1042. [CrossRef]

28. F. Matteucci, G. Cruciani, M. Dondi, G. Baldi, A. Barzanti. 2007. Crystal structural and optical properties of Cr-doped Y2Ti2O7and Y2Sn2O7 pyrochlores. Acta Materialia 55:7, 2229-2238. [CrossRef]

29. Leszek Kępiński, Mirosław Mączka, Jerzy Hanuza. 2006. Anti-Stokes Yb3+ emission—Valuable structure information in spectraof rare earth compounds measured with FT-Raman spectrometers. Spectrochimica Acta Part A: Molecular and BiomolecularSpectroscopy 65:5, 1025-1029. [CrossRef]

30. A. Garbout, S. Bouattour, M. Ellouze, A.W. Kolsi. 2006. Synthesis, FT-IR and X-ray diffraction investigations of gadolinium-substituted pyrochlore oxide Gd1.82Cs0.18Ti2O6.82 via a sol–gel process. Journal of Alloys and Compounds 425:1-2, 88-95.[CrossRef]

31. Yu. F. Shepelev, M. A. Petrova. 2006. Structures of two high-temperature Dy2TiO5 modifications. Russian Journal of InorganicChemistry 51:10, 1636-1640. [CrossRef]

32. Victor V. Atuchin, Valery G. Kesler, Natalia V. Pervukhina, Zhaoming Zhang. 2006. Ti 2p and O 1s core levels and chemicalbonding in titanium-bearing oxides. Journal of Electron Spectroscopy and Related Phenomena 152:1-2, 18-24. [CrossRef]

33. Christopher S. Wright, Janet Fisher, David Thompsett, Richard I. Walton. 2006. Hydrothermal Synthesis of a Cerium(IV)Pyrochlore with Low-Temperature Redox Properties. Angewandte Chemie International Edition 45:15, 2442-2446. [CrossRef]

34. Christopher S. Wright, Janet Fisher, David Thompsett, Richard I. Walton. 2006. Hydrothermal Synthesis of a Cerium(IV)Pyrochlore with Low-Temperature Redox Properties. Angewandte Chemie 118:15, 2502-2506. [CrossRef]

35. Antonio F. Fuentes, Khalid Boulahya, Miroslaw Maczka, Jerzy Hanuza, Ulises Amador. 2005. Synthesis of disordered pyrochlores,Ti2O7 (, Gd and Dy), by mechanical milling of constituent oxides. Solid State Sciences 7:4, 343-353. [CrossRef]

36. K.B. Helean, S.V. Ushakov, C.E. Brown, A. Navrotsky, J. Lian, R.C. Ewing, J.M. Farmer, L.A. Boatner. 2004. Formationenthalpies of rare earth titanate pyrochlores. Journal of Solid State Chemistry 177:6, 1858-1866. [CrossRef]

37. Ryu Abe, Masanobu Higashi, Zhigang Zou, Kazuhiro Sayama, Yoshimoto Abe. 2004. Photocatalytic Water Splitting into H2 andO2 over R2Ti2O7 (R = Y, Rare Earth) with Pyrochlore Structure. Chemistry Letters 33:8, 954-955. [CrossRef]

38. P Bonville, J A Hodges, M Ocio, J P Sanchez, P Vulliet, S Sosin, D Braithwaite. 2003. Low temperature magnetic properties ofgeometrically frustrated Gd 2 Sn 2 O 7 and Gd 2 Ti 2 O 7. Journal of Physics: Condensed Matter 15:45, 7777-7787. [CrossRef]

39. Z. Wang, C.H. Yang, D.L. Sun, J.F. Hu, H. Wang, H.C. Chen, C.S. Fang. 2003. Atomic force microscopic imaging and wetetching of Bi2Ti2O7 thin films. Materials Science and Engineering: B 102:1-3, 335-338. [CrossRef]

40. Erin E Erickson, Danielle Gray, Kathryn Taylor, Robin T Macaluso, Lee Ann LeTard, G.S Lee, Julia Y Chan. 2002. Synthesis,structure and dielectric characterization of Ln2Ti2−2xM2xO7(Ln=Gd, Er; M=Zr, Sn, Si). Materials Research Bulletin 37:13,2077-2083. [CrossRef]

41. Ming-Wen Chu, Mayté Caldes, Olivier Joubert, Marcel Ganne, Yves Piffard, Luc Brohan. 2002. Synthesis and structuralcharacterization of the new defect pyrochlore Nd1.68Ti2O6.52. Solid State Sciences 4:2, 167-173. [CrossRef]

42. Licia Minervini, Robin W. Grimes, Yasunori Tabira, Ray L. Withers, Kurt E. Sickafus. 2002. The oxygen positional parameterin pyrochlores and its dependence on disorder. Philosophical Magazine A 82:1, 123-135. [CrossRef]

43. J A Hodges, P Bonville, A Forget, M Rams, K Królas, G Dhalenne. 2001. The crystal field and exchange interactions in Yb 2Ti 2 O 7. Journal of Physics: Condensed Matter 13:41, 9301-9310. [CrossRef]

44. J. Lian, L. Wang, S. Wang, J. Chen, L. Boatner, R. Ewing. 2001. Nanoscale Manipulation of Pyrochlore: New NanocompositeIonic Conductors. Physical Review Letters 87:14. . [CrossRef]

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

45. M. Gingras, B. den Hertog, M. Faucher, J. Gardner, S. Dunsiger, L. Chang, B. Gaulin, N. Raju, J. Greedan. 2000. Thermodynamicand single-ion properties of Tb3+ within the collective paramagnetic-spin liquid state of the frustrated pyrochlore antiferromagnetTb2Ti2O7. Physical Review B 62:10, 6496-6511. [CrossRef]

46. Yasunori Tabira, Ray L. Withers, Licia Minervini, Robin W. Grimes. 2000. Systematic Structural Change in Selected Rare EarthOxide Pyrochlores as Determined by Wide-Angle CBED and a Comparison with the Results of Atomistic Computer Simulation.Journal of Solid State Chemistry 153:1, 16-25. [CrossRef]

47. Licia Minervini, Robin W. Grimes, Kurt E. Sickafus. 2000. Disorder in Pyrochlore Oxides. Journal of the American Ceramic Society83:8, 1873-1878. [CrossRef]

48. Y. Tabira, R. L. Withers. 1999. The determination of an unknown oxygen atom position in rare-earth zirconate pyrochlores bya 111 systematic-row convergent-beam electron diffraction technique. Philosophical Magazine A 79:6, 1335-1346. [CrossRef]

49. D Chen, R Xu. 1998. Hydrothermal Synthesis and Characterization of La2M2O7 (M = Ti, Zr) Powders. Materials ResearchBulletin 33:3, 409-417. [CrossRef]

50. I. Radosavljevic, J.S.O. Evans, A.W. Sleight. 1998. Synthesis and Structure of Pyrochlore-Type Bismuth Titanate. Journal of SolidState Chemistry 136:1, 63-66. [CrossRef]

51. Maria M. Milanova, Masato Kakihana, Momoko Arima, Masatomo Yashima, Masahiro Yoshimura. 1996. A simple solutionroute to the synthesis of pure La2Ti2O7 and Nd2Ti2O7 at 700–800°C by polymerized complex method. Journal of Alloys andCompounds 242:1-2, 6-10. [CrossRef]

52. Masato Kakihana, Maria M. Milanova, Momoko Arima, Toru Okubo, Masatomo Yashima, Masahiro Yoshimura. 1996.Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750oC Using Yttrium-Titanium Mixed-Metal Citric Acid Complex.Journal of the American Ceramic Society 79:6, 1673-1676. [CrossRef]

53. Kevin Eberman, Per Önnerud, Tae-Hwan Yu, Harry L. Tuller, Bernhardt J. Wuensch. 1996. Rietveld X-ray Powder ProfileAnalysis and Electrical Conductivity of Fastion Conducting Gd2(Til-ySny)2O7 Solid Solutions. MRS Proceedings 453. .[CrossRef]

54. C. De Mello Donegá, A. Meijerink, G. Blasse. 1995. Non-radiative relaxation processes of the Pr3+ ion in solids. Journal of Physicsand Chemistry of Solids 56:5, 673-685. [CrossRef]

55. V. Kahlenberg, Prof.Dr.H. Böhm. 1995. On the existence of a pyrochlore-type phase in the system Bi2O3–TiO2. Crystal Researchand Technology 30:2, 237-241. [CrossRef]

56. S. Möhr, Hk. Müller-Buschbaum. 1994. Eine neue phase mit defekt-pyrochlor-struktur: TiYO2,1. Journal of Alloys andCompounds 210:1-2, 115-118. [CrossRef]

57. M.A. Subramanian, A.W. SleightChapter 107 Rare earth pyrochlores 16, 225-248. [CrossRef]58. Clare P Grey, Christopher M Dobson, Anthony K Cheetham. 1992. Susceptibility matching in MAS NMR. The determination

of hyperfine tensors from paramagnetic stannates. Journal of Magnetic Resonance (1969) 98:2, 414-420. [CrossRef]59. A.V. Prasadarao, Ulagaraj Selvaraj, Sridhar Komarneni, Amar.S. Bhalla. 1991. Grain orientation in sol-gel derived Ln2Ti2O7

ceramics (Ln = La, Nd). Materials Letters 12:5, 306-310. [CrossRef]60. Junichi Takahashi, Takashi Ikegami, Keisuke Kageyama. 1991. Occurrence of Dielectric 1:1:4 Compound in the Ternary System

BaO-Ln2O3-TiO2 (Ln = La, Nd, and Sm): II, Reexamination of Formation of Isostructural Ternary Compounds in IdenticalSystems. Journal of the American Ceramic Society 74:8, 1873-1879. [CrossRef]

61. Naoki Wakiya, Nobuo Kieda, Atsushi Saiki, Kazuo Shinozaki, Nobuyasu Mizutani. 1991. Synthesis of a new compound,Ca0.8Ce1.2Sn2O7, with pyrochlore structure. Journal of Solid State Chemistry 92:2, 320-326. [CrossRef]

62. Junichi Takahashi, Toshiaki Ohtsuka. 1989. Vibrational Spectroscopic Study of Structural Evolution in the CoprecipitatedPrecursors to La2Sn2O7 and La2Ti2O7. Journal of the American Ceramic Society 72:3, 426-431. [CrossRef]

63. P. E. Fielding, T. J. White. 1987. Crystal chemical incorporation of high level waste species in aluminotitanate-based ceramics:Valence, location, radiation damage, and hydrothermal durability. Journal of Materials Research 2:03, 387-414. [CrossRef]

64. P.A.M. Berdowski, G. Blasse. 1986. Luminescence and energy transfer in a highly symmetrical system: Eu2Ti2O7. Journal ofSolid State Chemistry 62:3, 317-327. [CrossRef]

65. W.J. Weber, J.W. Wald, Hj. Matzke. 1986. Effects of self-radiation damage in Cm-doped Gd2Ti2O7 and CaZrTi2O7. Journalof Nuclear Materials 138:2-3, 196-209. [CrossRef]

66. C.E. Bamberger, H.W. Dunn, G.M. Begun, S.A. Landry. 1985. Substitutional solid solutions from heterotypic lanthanidedititanates Ln2-xLn′xTi2O7. Journal of the Less Common Metals 109:2, 209-217. [CrossRef]

67. P. Tiedemann, Hk. M�ller-Buschbaum. 1985. Ein neuer Strukturtyp an Gd2Ti0,78W0,56O6,22 (= Gd18Ti7W5O56). Zeitschriftf�r anorganische und allgemeine Chemie 526:7, 67-72. [CrossRef]

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.

68. C.E. Bamberger, H.W. Dunn, G.M. Begun, S.A. Landry. 1985. Substitutional solid solutions of bismuth-containing lanthanidedititanates, Ln2−xBixTi2O7. Journal of Solid State Chemistry 58:1, 114-118. [CrossRef]

69. W.J. Weber, J.W. Wald, Hj. Matzke. 1985. Self-radiation damage in Gd2Ti2O7. Materials Letters 3:4, 173-180. [CrossRef]70. J.H.H. ter Maat, M.P. van Dijk, G. Roelofs, H. Bosch, G.M.H. van de Velde, P.J. Gellings, A.J. Burggraaf. 1984. Electrical

and catalytic properties of some oxides with the fluorite or pyrochlore structure. Materials Research Bulletin 19:10, 1271-1279.[CrossRef]

71. M.P. van Dijk, J.H.H. ter Maat, G. Roelofs, H. Bosch, G.M.H. van de Velde, P.J. Gellings, A.J. Burggraaf. 1984. Electricaland catalytic properties of some oxides with the fluorite or pyrochlore structure. Materials Research Bulletin 19:9, 1149-1156.[CrossRef]

72. L.H. Brixner. 1984. Structural and luminescent properties of the Ln2Hf2O7-type rare earth hafnates. Materials Research Bulletin19:2, 143-149. [CrossRef]

73. M. P. van Dijk, J. H. H. ter Maat, G. Roelofs, H. Bosch, G. M. H. van de Velde, P. J. Gellings, A. J. Burggraaf. 1984. Electricaland catalytic properties of some mixed conductors with the fluorite or pyrochlore structure. Recueil des Travaux Chimiques desPays-Bas 103:1, 38-39. [CrossRef]

74. M. T. Vandenborre, E. Husson, J. P. Chatry, D. Michel. 1983. Rare-earth titanates and stannates of pyrochlore structure;vibrational spectra and force fields. Journal of Raman Spectroscopy 14:2, 63-71. [CrossRef]

75. M.A. Subramanian, G. Aravamudan, G.V. Subba Rao. 1983. Oxide pyrochlores — A review. Progress in Solid State Chemistry15:2, 55-143. [CrossRef]

76. Lynne Soderholm, J.E. Greedan. 1982. Relationship between crystal structure and magnetic properties of (RE)2V2O7; RE=Lu,Yb, Tm. Materials Research Bulletin 17:6, 707-713. [CrossRef]

77. N.T. Vandenborre, E. Husson, H. Brusset. 1981. Analyse en coordonnées normales des composés A2IIIB2IVO7 (A = La, Nd; B= Zr, Hf ) de structure pyrochlore. Spectrochimica Acta Part A: Molecular Spectroscopy 37:2, 113-118. [CrossRef]

78. T. Van Dijk, R. B. Helmholdt, A. J. Burggraaf. 1980. Neutron Powder Diffraction Studies of Fluorite and PyrochloreNdxZr1−xO2−x/2 Solid Solutions with 0.25<x<0.55. physica status solidi (b) 101:2, 765-774. [CrossRef]

79. R.A. McCauley, F.A. Hummel. 1980. New pyrochlores of the charge-coupled type. Journal of Solid State Chemistry 33:1, 99-105.[CrossRef]

80. Ronald A. McCauley. 1980. Structural characteristics of pyrochlore formation. Journal of Applied Physics 51:1, 290. [CrossRef]81. J.E. Greedan. 1979. Preparation and crystal chemistry of some rare earth vanadium (IV) pyrochlores (RE)2V2O7; RE = Lu, Yb,

Tm, Lu1−xYx, Lu1−xScx. Materials Research Bulletin 14:1, 13-19. [CrossRef]82. D.J.M. Bevan, E. SummervilleChapter 28 Mixed rare earth oxides 3, 401-524. [CrossRef]83. C. Chien, A. Sleight. 1978. Mössbauer effect studies of europium pyrochlores. Physical Review B 18:5, 2031-2038. [CrossRef]84. Geneviève Vetter, Francine Queyroux, Jean-Claude Gilles. 1978. Preparation, stabilite et etude cristallographique preliminaire du

compose Bi2Sn2O7. Materials Research Bulletin 13:3, 211-216. [CrossRef]85. T. Shin-ike, G. Adachi, J. Shiokawa. 1977. On the pyrochlore type Ln2V2O7 (Ln: Rare-earth elements). Materials Research

Bulletin 12:12, 1149-1154. [CrossRef]86. V. A. Antonov, P. A. Arsenev, D. S. Petrova. 1977. Spectroscopic properties of the Nd3± ion in Y2Ti2O7 and Gd2Ti2O7

monocrystals. Physica Status Solidi (a) 41:2, K127-K131. [CrossRef]87. Y. Calage, J. Pannetier. 1977. Electric field gradients in pyrochlore compounds. Journal of Physics and Chemistry of Solids 38:7,

711-718. [CrossRef]88. G. Desgardin, C. Robert, B. Raveau. 1977. Nouveaux oxydes LnNaSb2O7 de type wébérite. Journal of Inorganic and Nuclear

Chemistry 39:5, 907-908. [CrossRef]89. Gerald Jeanne, Gilbert Desgardin, Gerard Allais, Bernard Raveau. 1975. Etude des propriétés ferroélectriques de quelques oxydes

ternaires de type pyrochlore. Journal of Solid State Chemistry 15:2, 193-202. [CrossRef]90. M. Faucher, P. Caro. 1975. Ordre et desordre dans certains composés du type pyrochlore. Journal of Solid State Chemistry 12:1-2,

1-11. [CrossRef]91. G. Jeanne, G. Desgardin, B. Raveau. 1974. Synthese et evolution structurale de nouveaux pyrochlores au bismuth. Materials

Research Bulletin 9:10, 1321-1331. [CrossRef]

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

HE

NR

Y M

AD

DE

N L

IBR

AR

Y T

EC

H S

ER

V o

n 04

/28/

13Fo

r pe

rson

al u

se o

nly.