Accepted Manuscript

Title: Synthesis, properties and phase transitions ofpyrochlore- and fluorite-like Ln2RMO7 (Ln=Sm, Ho; R=Lu,Sc; M= Nb, Ta)

Author: A.V. Shlyakhtina D.A. Belov K.S. Pigalskiy A.N.Shchegolikhin I.V. Kolbanev O.K. Karyagina

PII: S0025-5408(13)00833-7DOI: http://dx.doi.org/doi:10.1016/j.materresbull.2013.10.004Reference: MRB 7084

To appear in: MRB

Received date: 26-3-2013Revised date: 12-9-2013Accepted date: 1-10-2013

Please cite this article as: A.V. Shlyakhtina, D.A. Belov, K.S. Pigalskiy,A.N. Shchegolikhin, I.V. Kolbanev, O.K. Karyagina, Synthesis, propertiesand phase transitions of pyrochlore- and fluorite-like Ln2RMO7 (Ln=Sm,Ho; R=Lu, Sc; M= Nb, Ta), Materials Research Bulletin (2013),http://dx.doi.org/10.1016/j.materresbull.2013.10.004

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

Page 1 of 35

Accep

ted

Man

uscr

ipt

1

Synthesis, properties and phase transitions of pyrochlore- and fluorite-

like Ln2RMO7 (Ln=Sm, Ho; R=Lu, Sc; M= Nb, Ta)

A.V. Shlyakhtina1*, D.A. Belov2, K.S. Pigalskiy1, A.N. Shchegolikhin3, I.V. Kolbanev1,

O.K. Karyagina3

1Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4,

Moscow, 119991 Russia

2Faculty of Chemistry, Moscow State University, Leninskie gory 1, Moscow, 119991

Russia

3Emmanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul.

Kosygina 4, Moscow, 119991 Russia

* Corresponding author. Tel.: +7 495 9397950; fax: +7 499 2420253.

E-mail addresses: annashl@inbox.ru; annash@chph.ras.ru (A.V.

Shlyakhtina).

ABSTRACT

We have studied the new compounds with fluorite-like (Ho2RNbO7 (R=Lu, Sc)) and

pyrochlore-like (Sm2ScTaO7) structure as potential oxide ion conductors. The phase

formation process (from 1200 to 1600°C) and physical properties (electrical, thermo

mechanical, and magnetic) for these compounds were investigated. Among the niobate

materials the highest bulk conductivity is offered by the fluorite-like Ho2ScNbO7

synthesized at 1600°C: 3.8 × 10–5 S/cm at 750°C, whereas in Sm system the highest bulk

conductivity, 7.3 × 10–6 S/cm at 750°C, is offered by the pyrochlore Sm2ScTaO7

synthesized at 1400°C. In Sm2ScTaO7 pyrochlore we have observed the first-order phase

Page 2 of 35

Accep

ted

Man

uscr

ipt

2

transformation at ~650-700°C is related to rearrangement process in the oxygen

sublattice of the pyrochlore structure containing B-site cations in different valence state

and actually is absent in the defect fluorites.

The two holmium niobates show Curie–Weiss paramagnetic behavior, with the

prevalence of antiferromagnetic coupling. The magnetic susceptibility of Sm2ScTaO7 is a

weak function of temperature, corresponding to Van Vleck paramagnetism.

Keywords: Pyrochlore; Fluorite; Phase transition; Ionic conductivity; Thermo

mechanical analysis; Dielectric permittivity; Loss tangent; Magnetic susceptibility

1. Introduction

It is known that the R3+2M4+

2O7 (R= Ln, Sc, Y; M=Ti, Zr, Hf, Sn) pyrochlore

family can be extended by substituting a combination of M3+ and M5+ cations on the M4+

site to give R3+2(M3+M5+)O7 compounds. Such pyrochlores include the compounds with

R3+ = Ln and Sc, M3+ = Ln, Sc, Ga and In, and M5+ = Sb, Nb and Ta [1–5]. The synthesis

of the Ln2(RSb)O7 (Ln= Sm, Tb, Dy, Yb; R=Lu, Sc) rare-earth antimonates was

described in [2, 4, 5]. The Ln2(RSb)O7 (Ln=Nd, Sm, Ho, Yb; R=Lu, Sc) compounds

were prepared by Cherner through solid-state reactions [2, 3]. He mapped out the

stability region of the pyrochlore structure for Ln2RMO7 (Ln=Sm-Lu; R=Sm-Lu, Sc; M=

Sb, Nb, Ta) and proposed an approach to predicting the formation of mixed pyrochlore

oxides for a given chemical composition. Relatively recently, the Ln2ScNbO7 (Ln= Pr,

Nd, Eu, Gd, Dy) niobates have been synthesized by Zouari et al. [4], and the La2RNbO7

(R=Dy, Er, Yb) niobates, by Cai and Nino [5]. The dysprosium in Dy2ScNbO7 [6] and

Page 3 of 35

Accep

ted

Man

uscr

ipt

3

Dy2Sn2-xSbxO7+x/2 [7] was reported to exhibit spin ice behavior. The Ho2RNbO7 (R=Lu,

Sc) niobates were first described by Rossel [1]. The Arrhenius plots of conductivity were

obtained for Ln2LnNbO7 (Ln= La or Nd) with weberite structure [8]. In accordance with

[8] the total conductivity value (~10-6 S/cm at 730ºС) is weak function of the Ln radii.

The tantalate pyrochlores synthesized to date include Ho2LuTaO7, Sm2LuTaO7 and

Sm2ScTaO7 [1, 3], but their physical properties have not yet been investigated.

In a series of studies concerned with the oxygen ion conductivity of the

A3+2B4+

2O7 (A=Ln= Sm-Lu; B=M=Ti, Zr, Hf) pyrochlores, an assumption was made

that, in oxygen ion conductors with a high M4+–O (M = Ti, Hf) bond covalence, the

geometric factor (size mismatch between the A and B cations) plays an important role. In

this work, three compounds with the smallest size mismatch between the A and B cations

among the known Ln2RMO7 (А=Ln=Sm-Lu; В=R=Sm-Lu and В=M= Nb, Ta)

pyrochlore niobates and tantalates, containing B cations in different valence states, have

been chosen as potential oxygen ion conductors: Ho2ScNbO7, Ho2LuNbO7 and

Sm2ScTaO7 (ΔR = 0.3225, 0.2645 and 0.3865 Å, respectively).

This paper describes the synthesis, physical properties and phase transitions of

the Ln2RMO7 (Ln=Sm, Ho; R=Lu, Sc; M=Nb, Ta) compounds.

2. Experimental

Ho2ScNbO7 and Ho2LuNbO7 were prepared via reverse precipitation (pH = 10.2),

by adding titrated rare-earth chloride solutions and an ethanolic NbCl5 solution to

aqueous ammonia. The starting chemicals used were Ho2O3, Sc2O3, Lu2O3 and NbCl5

powders dissolved in hydrochloric acid. The niobium chloride solution proved unstable

and was stabilized by C2H5OH. The precipitate was recovered by centrifugation, washed

Page 4 of 35

Accep

ted

Man

uscr

ipt

4

with water several times until free of chloride ion and dried at 120°C for 24 h. Next, the

precursors were decomposed at 650°C over a period of 2 h. The resultant powders were

pressed at 200 MPa into pellets, which were then fired in the range 1200–1600°C in

order to follow the phase formation process.

Sm2ScTaO7 was synthesized from mechanically activated mixture of Sm2O3,

Sc2O3, and Ta2O5, which were pressed at 200 MPa and reacted in the same temperature

range as in the case of the niobates: 1200–1600°C.

The density was measured from samples dimensions and weights and is 71.9-

79.7% of theoretical value.

X-ray diffraction (XRD) patterns were collected on a DRON-3M automatic

diffractometer (Cu Kα radiation, 35 kV, 28 mA) in the 2θ range 13° to 65° (scan step of

0.1°(2θ), counting time per data point of 3 s).

Porous electrodes were made on both faces of the pellets by firing Pt paste

(ChemPur, Germany) at 1000°C for 0.5 h. The conductivity of the samples was

determined by impedance spectroscopy in the range 300–1000°C using two-probe

measurements in a NorECs ProboStat cell. The signal was monitored with a Beta-N

impedance analyzer at frequencies from 10 mHz to 3 MHz and an applied voltage of 0.5

V peak. The uncertainty in the conductivity values obtained was within 5%. In data

processing, we used ZView software [9]. The pyrochlore Sm2ScTaO7 and fluorite-like

Ho2LuNbO7 synthesized at 1400°C, 20 h and 1600°C, 14 h, respectively, were studied

by impedance spectroscopy measurements using heating–cooling cycles (heating rate of

2°C/min). The dielectric permittivity and loss tangent were determined for the

Page 5 of 35

Accep

ted

Man

uscr

ipt

5

Sm2ScTaO7 pyrochlore sample (Tsyn = 1400°C, 20 h) and fluorite-like Ho2LuNbO7

(1600°C, 14 h) sample at frequencies 0.3 Hz-300 kHz.

Thermomechanical analysis (TMA) curves were obtained in air between 30 and

900°C at a heating rate of 10°C/min and a load of 100 mN using a PerkinElmer TMA 7

analyzer and flat tip probe. The Sm2ScTaO7 synthesized at 1400°C, 10 h was studied

using heating–cooling–heating cycles.

Magnetic susceptibility was measured from 70 to 290 K at an ac field strength of

10 Oe and frequency of 980 Hz by the two-coil method using a home-built system. The

measurement accuracy was ~5 × 10–7 emu/cm3.

3. Results and discussion

3.1. Sm2ScTaO7 and Ho2ScNbO7, Ho2LuNbO7 phase formation

The phase formation data for Ho2ScNbO7, Ho2LuNbO7 and Sm2ScTaO7 in the

range 1200–1600°C are presented in Figs. 1, 2, 3. The XRD patterns in Fig. 1 illustrate

the Ho2ScNbO7 phase formation process. After heat treatment in the range 1200 ≤ T ≤

1400°C, the XRD pattern indicated coexistence of two compounds: fluorite Ho2ScNbO7

(a = 5.161(6) Å) and cubic holmium oxide, C-Ho2O3 [10] (Fig. 1, scans 1, 2; the lines of

C-Ho2O3 are marked by asterisks). Further annealing at 1500 or 1600°C resulted in the

formation of a fluorite phase with a = 5.162 (6) Å (Fig. 1, scans 3, 4). The phase

formation process for Ho2LuNbO7 synthesized in the temperature interval 1200–1600°C

from coprecipitated precursor is presented in Fig. 2. The unit-cell parameter of the

Ho2LuNbO7 fluorite samples annealed at 1500°C or 1600°C is 5.220 (4) or 5.224 (1) Å,

respectively (Fig. 2, scans 3, 4).

Page 6 of 35

Accep

ted

Man

uscr

ipt

6

The phase formation data for Sm2ScTaO7 prepared in the range 1200–1600°C

from mechanically activated oxides are presented in Fig. 3. The XRD patterns of the

samples prepared in the range 1200–1400°C show the 111, 311, 331 and 511 pyrochlore

superstructure peaks (Fig. 3, scans 1, 2 and inset). After annealing at 1500°C, the 111,

311 and 511 superstructure peaks are weaker (Fig. 3, scan 3). The XRD pattern of the

material prepared at 1600°C contains additional lines (marked by asterisks), attributable

to Sm3TaO7, a weberite-structure (W2) phase (ICDD–JCPDS PDF-2 No 38-1412) (Fig.

3, scan 4). It seems likely that, above 1500°C, there are two phases in equilibrium:

Sm2ScTaO7 (pyrochlore structure, P) and Sm3TaO7 (weberite structure, W2). The

pyrochlore cell parameter of the Sm2ScTaO7 sample prepared at 1400°C, 10 h and

1400°C, 20 h is 10.458(3) and 10.467(3) Å, respectively, in good agreement with ICDD–

JCPDS PDF-2 data (No 73-0019, a = 10.466(2) Å).

Thus, the pyrochlore phase formation processes takes place in tantalate system

only. Note that, on the whole, the tantalate Sm2ScTaO7 has rather weak pyrochlore

superstructure peaks, which is probably due to the small size of pyrochlore-structure

microdomains in a fluorite-like matrix. Similar XRD patterns, with a markedly reduced

intensity of the pyrochlore superstructure peaks, were reported earlier for the Ln2M2O7

(Ln=Sm-Gd; M= Zr, Hf) oxygen ion conductors [11, 12] and also for Gd2InSbO7 and

Nd2GaSbO7 [6]. The reduced intensity of the superstructure peaks suggests that the

material consists predominantly of a fluorite matrix, which contains short-range

pyrochlore order domains. Authors [12] explained this effect in Ln2Zr2O7 (Ln= La, Nd)

pyrochlores by considering that the X-ray contrast between La3+(Z = 57) and Zr4+ (Z =

40) is modest and that the anion-vacancy ordering only involves the O´ on the 8b sites

Page 7 of 35

Accep

ted

Man

uscr

ipt

7

and not the six O anions on the 48f sites. The X-ray contrast between A and B cations for

Ho2ScNbO7 is 16%, for Ho2LuNbO7 is 20% and for Sm2ScTaO7 is 32%. A and B sites

in Sm2ScTaO7 have better X-ray contrast then that of A and B sites in Ho2ScNbO7 or

Ho2LuNbO7. One can see the pyrochlore superstructure peaks in Sm2ScTaO7 tantalate. It

is possible the pyrochlore superstructure peaks are very weak in niobates because of the

bad X-ray contrast.

3.2. Electrical conductivity of Ho2ScNbO7, Ho2LuNbO7 and Sm2ScTaO7

Figs. 4, 5, 6 shows the impedance spectra of the Ho2ScNbO7 (1600°C, 4 h),

Ho2LuNbO7 (1500°C, 4 h) and Sm2ScTaO7 (1400°C, 10 h) samples. In those spectra, we

were able to identify the bulk and grain-boundary contributions (Table 1). From the

impedance spectroscopy data, we obtained Arrhenius plots of bulk conductivity for the

three compounds (Figs. 7, 8).

Fig. 7 shows the temperature dependences of bulk conductivity for the

Sm2ScTaO7 samples prepared at 1400, 1500 and 1600°C.

The 750°C conductivity of Sm2ScTaO7 increases with decreasing synthesis

temperature (Table 2): 1.3 × 10–6 S/cm in the two-phase sample synthesized at 1600°C

(Fig. 7, curve 1), 2.9 × 10–6 S/cm in the sample synthesized at 1500°C (Fig. 7, curve 2)

and 7.3 × 10–6 S/cm in the sample fired at 1400°C (Fig. 7, curve 3). These results

correlate with the XRD data on pyrochlore phase formation in the Sm–Sc–Ta–O system

(Fig. 3): the conductivity is higher when the pyrochlore superstructure peaks are

stronger. For example, the conductivity of the Sm2ScTaO7 prepared at 1400°C exceeds

that of the two-phase sample synthesized at 1600°C (Fig. 7, curves 3, 1).

Page 8 of 35

Accep

ted

Man

uscr

ipt

8

Note the rather sharp change in conductivity near 650°C, which seems to result

from a structural phase transition.

Given the extremely slow equilibrium in the Sm – Ta - O systems, the

Sm2ScTaO7 was sintered at 1200 or 1400ºС for the long time 40 or 20 h to provide

equilibrium conditions for the pyrochlore material formation in the tantalate system. Fig

7, insert shows the temperature dependence of bulk conductivity for the Sm2ScTaO7

samples prepared at 1400ºС, 20h (curve 1) and 1200ºС, 40h (curve 2). One can see the

unusual effect on the conductivity curve for these samples near 650ºС. Energy activation

(Ea) of bulk conductivity for Sm2ScTaO7 (Tsyn.=1400 ºС, 20h) is 1.41 eV before phase

transition and 0.7 eV after its. In accordance with energy activation value (0.7 eV at T≥

750°C) [13] the bulk conductivity has oxygen ion type at T≥ 750°C.

The results of the further investigation of this phase transformation in

Sm2ScTaO7 will discussed later (in paragraph 3.3.).

Fig. 8 and Table 2 present the bulk conductivity data for the niobates. Note that

raising the firing temperature from 1500 to 1600°C increases the conductivity of

Ho2ScNbO7 by a factor of 3 (from 1.2 × 10–5 to 3.8 × 10–5 S/cm at 750°C) (Fig. 8, curves

1, 2). The Ho2ScNbO7 and Ho2LuNbO7 niobates synthesized at 1500°C are almost

identical in conductivity (Table 2): ~ 1.2 × 10–5 S/cm at 750°C (Fig. 8, curves 1, 3).

Ho2LuNbO7, prepared at 1600ºС (Fig. 8, curve 4), has the higher conductivity (~ 2 × 10–

5 S/cm at 750°C), than Ho2LuNbO7 synthesized at 1500ºС (Fig. 8, curve 3) (Table 2).

Ho2ScNbO7 synthesized at 1600°C (Fig. 8, curve 2) has higher conductivity, then

Ho2LuNbO7, prepared at the same temperature (Fig. 8, curve 4) (Table 2). So, the

conductivity of niobates Ho2RNbO7 (R= Lu, Sc) increases when R3+ radii decreases. For

Page 9 of 35

Accep

ted

Man

uscr

ipt

9

Ho2LuNbO7 and especially for Ho2ScNbO7 prepared at 1600ºС, one can see the small

deviations from the Arrhenius behaviour near 650°C, which seems to result from a

structural phase transition as well as it was observed for Sm2ScTaO7 pyrochlore.

3.3. Phase transition of Sm2ScTaO7, Ho2ScNbO7 and Ho2LuNbO7

The Arrhenius plots of conductivity for the pyrochlore-like Sm2ScTaO7

synthesized in the temperature range 1200-1500°C (Fig. 7 and Fig. 7, insert) show an

abnormal changes in conductivity at ~650°C. A first-order structural phase transition

presumably related to oxygen rearrengement in pyrochlores can be identified by TMA.

The temperature variation of the size (thickness) of a Sm2ScTaO7 specimen in a heating–

cooling–heating cycle is illustrated in Figs. 9, 10, 11. Also shown are the corresponding

linear thermal expansion coefficient (TEC) curves, 1 ( )Tα , obtained from the TMA

curves as

1

0

1( ) = LTL T

ρα ∂⎛ ⎞⎜ ⎟∂⎝ ⎠

,

where L0 is the specimen thickness at 30°C. These curves represent the temperature

derivative of dL/L0 as a function of temperature , where dL is the change in specimen

thickness. In addition, Figs. 9, 10, 11 presents temperature dependences of the linear

TEC averaged over the temperature interval from Tref to T1:

1 .0 0

1,1 .

( ) ( )( )

ref

refref

dL dLT TL LT T

T Tα

−=

−,

Page 10 of 35

Accep

ted

Man

uscr

ipt

10

where T1 is the upper (variable) boundary of the temperature interval, Tref is its lower

boundary (reference point). The TMA curve for the first heating has a pronounced

sigmoidal shape, which may be due to residual internal stress relaxation in the specimen,

with an inflection near 622°C (Fig. 9). During the second heating after slow cooling from

900°C to room temperature at 10 K/min, the Sm2ScTaO7 specimen showed "classical"

behavior (Fig. 11), typical of materials that undergo a thermally induced phase transition,

which usually results in an increase in free volume in the material and/or activates the

molecular mobility of its constituents. Even though the transition is very broad (roughly

from 350 to 750°C), drawing tangents to two portions of the TMA curve demonstrates

that there is an inflection near 640°C (presumably, the phase transition temperature).

Despite the arbitrariness and low accuracy of transition temperature determination by

drawing tangents to portions of a TMA curve, we think it reasonable to assume that the

phase transition of Sm2ScTaO7 occurs at 610–650°C (Figs. 9, 11). This is also evidenced

by the local minimum or inflection in the TEC curves (Figs. 9, 11, curves 2). Note that,

in both the first and second heating cycles, the linear TEC of Sm2ScTaO7 was ~ 8.0 ×

10–6 K–1 below the phase transition (300°C) and ~ (1.3–1.6) × 10–5 K–1 above the

transition. An important point is that the phase transition temperature inferred from the

TMA data agrees rather well with the temperature of the break in the conductivity as a

function of temperature: ~650°C. It is also worth noting that there is pronounced thermal

hysteresis in the thermomechanical behaviour of the material: the transition temperature

evaluated from the cooling curve as described above is about 424°C. First-order phase

transitions are as a rule accompanied by thermal hysteresis, which is well seen in the

present TMA curves.

Page 11 of 35

Accep

ted

Man

uscr

ipt

11

The conductivity of Sm2ScTaO7 (Tsyn= 1400 ºC, 20 h) was investigated using

the heating and cooling rate 2ºС/min. Fig. 12 shows the heating-cooling conductivity

data for Sm2ScTaO7, which are really demonstrated the hysteresis of conductivity in the

temperature range 600-900ºС. However, the kinetic effect can take place also. Present

data confirmed the first order phase transition in Sm2ScTaO7.

We have performed the investigation of the first-order phase transition in

Sm2ScTaO7 pyrochlore (Тsyn. = 1400, 20 h) by impedance spectroscopy at 300-1000ºC

and frequencies (0.3 Hz-300 kHz). Figs. 13, 14 present the temperature dependences of

the permittivity and loss tangent, respectively. The permittivity of this sample has one

maximum ~ 700ºС, which position is independent of frequency in the range 0.3 Hz-300

kHz. This maximum is accompanied by a minimum in the temperature dependence of the

loss tangent at 0.3 and 3 Hz (Fig. 14, curves 1, 2) and the intensity of the peak is inverse

proportional to the frequency. These results indicate that Sm2ScTaO7 pyrochlore

undergoes a first-order structural phase transition at 650-700ºС.

Most likely, the structural changes in question are due to the oxygen re-

arrangement in pyrochlore containing B-site cations in different valence states. Similar

transitions were observed earlier in Ln3+2Ti4+

2O7 pyrochlores (for example in Er2Ti2O7

[14]). The second-order phase transition in (Dy1-xCax)2Ti2O7-δ (x=0, 0.1) pyrochlore

ceramics is related to the re-arrangement in the oxygen sublattice near 700-800ºС [15].

The results of [14-16] confirmed the idea about oxygen mobility in Ln23+Ti2

4+O7

pyrochlores in the 700-800ºС temperature interval.

The Arrhenius plots of bulk conductivity for the fluorite-like Ho2ScNbO7 and

Ho2LuNbO7 synthesized at 1500 and 1600°C presented at Fig. 8, curves 1, 2

(Ho2ScNbO7) and 3, 4 (Ho2LuNbO7), respectively). The rise of synthesis temperature

Page 12 of 35

Accep

ted

Man

uscr

ipt

12

from 1500 to 1600°C led to the increase of bulk conductivity. One can see the small

deviations from the Arrhenius behaviour near 650°C, which seems to result from a

structural phase transition for 1600°C samples (Fig. 7, curves 2, 4).

The conductivity of fluorite-like Ho2LuNbO7 (1600 ºС, 14 h) was investigated

using the heating and cooling rate 2ºС/min. Fig. 15 shows the heating-cooling

conductivity data for fluorite-like Ho2LuNbO7, which are demonstrated the hysteresis

of conductivity in the temperature range 350-750ºС, which is much less than that of the

hysteresis in the Sm2ScTaO7 pyrochlore.

We have investigated the temperature dependence of the dielectric permittivity

and the loss tangent for Ho2LuNbO7 (1600, 14 h) at low frequencies from 0.3 Hz to 300

kHz. Figs. 16, 17 present the temperature dependences of the permittivity and loss

tangent for Ho2LuNbO7 (1600ºС, 14 h), respectively. The permittivity of this sample has

two maxima (Fig. 16), one of which shifts to higher temperatures, from 550 to 700°C,

with increasing frequency, while the position of the other (very weak) (~700-730°C) is

essentially independent of frequency in the range 0.3 to 30 Hz (Fig. 16, curves 1, 2). The

shift of the former maximum in permittivity to higher temperatures with increasing

frequency suggests that it originates from point-defect relaxation. One can see that the

second maximum is accompanied by a poorly defined minimum in the temperature

dependence of the loss tangent at 0.3 and 3 Hz (Fig. 17, curves 1, 2). These results

indicate that first-order structural phase transition just starts at 350-750ºС in Ho2LuNbO7

(Tsyn= 1600ºС, 14h). So, this transformation at 350-750ºС is a characteristic of the

pyrochlore structure than the defect fluorite structure. Ho2LuNbO7 after long thermal

annealing at 1600ºС, 14 h probably consists predominantly of a fluorite-matrix, which

contains short-range pyrochlore order domains.

Page 13 of 35

Accep

ted

Man

uscr

ipt

13

So, the niobates Ho2RNbO7 (R= Lu, Sc) synthesized at 1600°C,4-14 h probably

also start undergo a first order structural phase transition at ~650-700°C, but effects

characterizing that phase transition are expressed weakly. It is possible that phase

transition associated with the re-arrangement process in the oxygen sublattice of the

pyrochlore structure and is not typical for defect fluorites. The starts of the pyrochlore

structure formation in Ho2LuNbO7 niobate led to the small deviation from the Arrhenius

behaviour near 650 -700°C, the hysteresis of conductivity in the temperature range 350-

750°C and appearance of dielectric permittivity maximum at 650-700°C, which

accompanied by a minimum in the temperature dependence of the loss tangent at very

low frequencies. The conductivity, dielectric permittivity and loss tangent are more

sensitive to the appearing of short-range pyrochlore order domains in Ho2ScNbO7 and

Ho2LuNbO7 then X-ray technique.

3.4. Magnetic properties of fluorite-like Ho2ScNbO7, Ho2LuNbO7 and Sm2ScTaO7

pyrochlore

Magnetic susceptibility was measured as a function of temperature for the

samples that had a pyrochlore superstructure as determined by XRD: Ho2ScNbO7

synthesized at 1400°C and Ho2LuNbO7 and Sm2ScTaO7 synthesized at 1500°C (Figs. 1,

2, 3). Fig. 18 presents the data for Ho2ScNbO7, which is seen to exhibit typical

paramagnetic behaviour. Its temperature-dependent susceptibility follows a Curie–Weiss

law. In the temperature range 120–290 K, the best fit effective magnetic moment (μeff)

and Weiss constant (θW) are µeff = 10.36(4)µB and θW = –8(1) K (Table 2). The

susceptibility of Ho2LuNbO7 shows similar behavior, with µeff = 10.14(4)µB and θW = –

Page 14 of 35

Accep

ted

Man

uscr

ipt

14

8(1) K (Fig. 19 and Table 2). The µeff values obtained are close to the 10.6µB expected

for a free Ho3+ ion. The slight decrease is due to an admixture of nearby excited levels,

whose energy is governed by the symmetry of the oxygen coordination of the Ho3+ site.

That the Weiss constant is negative indicates that antiferromagnetic coupling prevails, in

contrast to the typical spin-ice compound Ho2Ti2O7, which exhibits weak ferromagnetic

coupling [17].

The magnetic susceptibility of Sm2ScTaO7 shows qualitatively different

behavior: a gradual, almost linear increase with decreasing temperature (Fig. 20). This

behavior is characteristic of Van Vleck paramagnetism and has been observed previously

in other samarium oxide compounds, both pyrochlores [18] and structurally similar,

fluorite-like phases [19].

4. Conclusions

In this work we have investigated the phase formation process, structure and

different physical properties of new compounds - Ho2RNbO7 (R=Lu, Sc) and

Sm2ScTaO7 – potential oxide ion conductors. Fluorite-like Ho2ScNbO7 was obtained at

1500 and 1600°C, fluorite Ho2LuNbO7 is stable in the temperature range 1200 - 1600°C,

whereas pyrochlore Sm2ScTaO7 exists in the range 1200–1400°C. At higher

temperatures, it coexists with the weberite-structure (W2) phase Sm3TaO7.

The three compounds under investigation have bulk conductivity: 7.3 × 10–6

S/cm at 750°C in Sm2ScTaO7 and 2.0 × 10–5 or 3.8 × 10–5 S/cm at 750°C in Ho2RNbO7

(R=Lu, Sc), respectively. The bulk conductivity of niobates Ho2RNbO7 (R= Lu, Sc)

increases when R3+ radii decreases.

Page 15 of 35

Accep

ted

Man

uscr

ipt

15

Sm2ScTaO7 pyrochlore undergoes a first-order structural phase transition at

~650-700°C, as evidenced by a sharp change in conductivity and supported by TMA data

and low frequency measurements of the dielectric permittivity and the loss tangent. The

thermal expansion coefficient below and above the transition is ~8 × 10–6 and ~ (13–16)

× 10–6 K–1, respectively.

The conductivity, dielectric permittivity and loss tangent are more sensitive to the

appearing of short-range pyrochlore order domains in Ho2ScNbO7 and Ho2LuNbO7 then

X-ray technique.

Ho2LuNbO7 and Ho2ScNbO7 show Curie–Weiss paramagnetic behavior, with the

prevalence of antiferromagnetic coupling, whereas Sm2ScTaO7 is a Van Vleck

paramagnet.

Acknowledgments

This work was supported by the Presidium of the Russian Academy of Sciences

(program Synthesis of Inorganic Substances with Controlled Properties and Fabrication

of Related Functional Materials, grant no. 8/2013) and the Russian Foundation for Basic

Research (grant no. 13-03-00680) and by the Department of Materials Sciences of the

Russian Academy of Sciences (program of the Basic Investigations of New Metal,

Ceramic, Glass- and Composite- Materials, grant no. 2/2013).

Page 16 of 35

Accep

ted

Man

uscr

ipt

16

References

[1] H.J. Rossel. J. Solid State Chem. 27 (1979) 115.

[2] Ya.E. Cherner, Structure, Polymorphism and Dielectric Properties of Pyrochlore and

Other A2B2O7 Compounds, Cand. Sci. (Phys.–Math.) Dissertation, Rostov-on-Don, 1981.

[3] V.S. Filipjev, Ya. E. Cherner, O.A. Bunina, V.F. Seregin. Krystallografiya 27

(1982) 601.

[4] S.Zouari, R. Ballou, A. Cheikh-Rouhou, P. Strobel. Mater. Lett. 62 (2008) 3767.

[5] L. Cai, J.C. Nino. J. Eur. Ceram. Soc. 30 (2010) 307.

[6] P. Strobel, S. Zouari, R. Ballou, A. Cheikh-Rouhou, J.-C. Jumas, J. Olivier-

Fourcade. Solid State Sci. 12 (2010) 570.

[7] X. Ke, B.G. Ueland, D.V. West, M.L. Dahberg, R.J. Cava, P. Schiffer. Phys. Rev. B

76 (2007) 214413.

[8] G.G. Каsimov, Е.М. Еlovskich, М.G. Zuev. Isv. AN SSSR Inorg. Mater. 12 (1976)

1600.

[9] ZView for Windows, Impedance/Gain Phase Analysis Software, Version 2.3f,

Scribner Associates Inc.

[10] Y. Yokogawa, M. Youshimura. J. Am. Ceram. Soc. 80 (1997) 965

[11] A.V. Shlyakhtina, S.N. Savvin, A.V. Levchenko, A.V. Knotko, Petra Fedtke,

Andreas Busch, Torsten Barfels, Marion Wienecke, L.G. Shcherbakova. J. Electroceram.

24 (2010) 300.

[12] P.E.R. Blanchard, R. Clements, B.J. Kennedy, C.D. Ling, E. Reynolds, M. Avdeev,

A.P.J. Stampfl, Z. Zhang, L.-Y. Jang. Inorg. Chem. 51 (2012) 13237.

Page 17 of 35

Accep

ted

Man

uscr

ipt

17

[13] J.C.C. Abrantes, A. Levchenko, A.V. Shlyakhtina, L.G. Shcherbakova, A.L.

Horovistiz, D.P. Fagg, J.R. Frade. Solid State Ionics. 177 (2006) 1785.

[14] M. Martos, B. Julian-Lopez, E. Cordoncillo, P. Escribano J. Phys. Chem. B 112

(2008) 2319.

[15] D.A. Belov, A.V. Shlyakhtina, S. Yu. Stefanovich, A.N. Shchegolikhin, A.V.

Knotko, O.K. Karyagina, L.G. S hcherbakova. Solid State Ionics 192 (2011) 188.

[16] A.F. Fuentes, K. Boulahya, M. Maszka, J. Hanuza, U. Amador. Solid Stat. Sci. 7

(2005) 343.

[17] S.T. Bramwell, M.N. Field, M.J. Harris, I.P. Parkin, J. Phys.: Condens. Matter

12 (2000) 483.

[18] Y. Kobayashi, T. Miyashita, T. Fukamachi, M. Sato, J. Phys. Chem. Solids 62

(2001) 347.

[19 17] Y. Doi, Y. Harada, Y. Hinatsu, J. Solid State Chem. 182 (2009) 709.

Page 18 of 35

Accep

ted

Man

uscr

ipt

18

Tables Table 1

The 550ºC capacitance values and conductivities.

Sample

number Composition

σbulk,

×10-7

S/cm

Cbulk,

×10-12

F

σgbapp,

×10-6

S/cm

Cgb,

×10-8

F

σgbsp,

×10-10

S/cm

1 Sm2ScTaO7 2.3 10.2 n/d n/d n/d

2 Ho2LuNbO7 4.6 7.4 3.1 2.2 10.4

3 Ho2ScNbO7 37.8 12.4 21.5 32.4 8.2

Table 2

Bulk conductivity data (σ) at 750ºC, effective magnetic moment (μeff) and Weiss

constant (ΘW) for the compounds under investigation

Composition Tsynth, ºC σ, S/cm at 750 ºC μeff, μB θW, K Sm2ScTaO7 1400

1500 1600

7.3×10-6 2.9×10-6 1.3×10-6

–

–

Ho2ScNbO7 1400 1500 1600

9.9 ×10-6 1.2×10-5 3.8×10-5

10.36(4) – 8(1)

Ho2LuNbO7 1500 1600

1.2×10-5 2.0×10-5

10.14(4) – 8(1)

Page 19 of 35

Accep

ted

Man

uscr

ipt

19

Figures captions

Fig. 1. XRD patterns of Ho2ScNbO7 prepared by heat-treating coprecipitation products:

(1) 1200°C, 48 h; (2) 1400°C, 10 h; (3) 1500°C, 4 h; (4) 1600°C, 4 h.

Fig. 2. XRD patterns of Ho2LuNbO7 prepared by heat-treating coprecipitation products:

(1) 1200°C, 24 h; (2) 1400°C, 10 h; (3) 1500°C, 4 h; (4) 1600°C, 14 h.

Fig. 3. XRD patterns of Sm2ScTaO7 prepared by heat-treating mechanically activated

oxides: (1) 1300°C, 4 h; (2) 1400°C, 10 h; (3) 1500°C, 4 h; (4) 1600°C, 4 h and inset:

1200°C, 40 h.

Fig. 4. Impedance spectra of Ho2ScNbO7 (1600°C, 4 h).

Fig. 5. Impedance spectra of Ho2LuNbO7 (1500°C, 4 h)

Fig. 6. Impedance spectra of Sm2ScTaO7 (1400°C, 10 h).

Fig. 7. Temperature dependences of bulk conductivity for Sm2ScTaO7 prepared by heat-

treating mechanically activated oxides at (1) 1600°C, 4 h; (2) 1500°C, 4 h; (3) 1400°C,

10 h; and inset: Temperature dependences of bulk conductivity for Sm2ScTaO7 prepared

at (1) 1400°C, 20 h; and (2) 1200°C, 40 h; and

Fig. 8. Temperature dependences of bulk conductivity for Ho2ScNbO7 prepared via heat

treatment at (1) 1500°C, 4 h; at (2) 1600°C, 4 h; Ho2LuNbO7 prepared via heat treatment

at (3) 1500°C, 4 h; (4) 1600ºC, 14 h.

Fig. 9. Thermomechanical analysis data for Sm2ScTaO7 for the first heating: (1) TMA

curve, (2) linear TEC, (3) average TEC.

Fig. 10. Thermomechanical analysis data for Sm2ScTaO7 for the cooling from 900°C: (1)

TMA curve, (2) linear TEC, (3) average TEC.

Fig. 11. Thermomechanical analysis data for Sm2ScTaO7 for the second heating at a rate

of 10 K/min: (1) TMA curve, (2) linear TEC, (3) average TEC.

Page 20 of 35

Accep

ted

Man

uscr

ipt

20

Fig. 12. Heating (1) and cooling (2) conductivity data (the rate of heating and cooling is

2°/min) for the Sm2ScTaO7 pyrochlore, sintered at 1400°C, 20 h.

Fig. 13. The temperature dependence of the dielectric permittivity at low frequencies

from 0.3 Hz to 300 kHz for the Sm2ScTaO7 sintered at 1400°C, 20 h ((1) 0.3 Hz, (2) 3

Hz, (3) 30 Hz, (4) 300 Hz, (5) 3 kHz, (6) 30 kHz, (7) 300 kHz).

Fig. 14. The temperature dependence of the loss tangent at low frequencies ((1) 0.3 Hz,

(2) 3 Hz, (3) 30 Hz).

Fig. 15. Heating (1) and cooling (2) conductivity data (the rate of heating and cooling is

2°/min) for the fluorite-like Ho2LuNbO7 , sintered at 1600°C, 14 h.

Fig. 16. The temperature dependence of the dielectric permittivity at low frequencies

from 0.3 Hz to 300 kHz for the Ho2LuNbO7 , sintered at 1600°C, 14 h ((1) 0.3 Hz, (2) 3

Hz, (3) 30 Hz, (4) 300 Hz, (5) 3 kHz, (6) 30 kHz, (7) 300 kHz).

Fig. 17. The temperature dependence of the loss tangent at low frequencies ((1) 0.3 Hz,

(2) 3 Hz, (3) 30 Hz).

Fig. 18. Temperature dependences of ac magnetic susceptibility for Ho2ScNbO7. The

solid line represents calculation results for a free Ho3+ ion and the dashed line is a least-

squares fit to the Curie–Weiss formula.

Fig. 19. Temperature dependences of ac magnetic susceptibility for Ho2LuNbO7. The

solid line represents calculation results for a free Ho3+ ion and the dashed line is a least-

squares fit to the Curie–Weiss formula.

Fig. 20. Temperature dependences of ac magnetic susceptibility for Sm2ScTaO7.

Page 21 of 35

Accep

ted

Man

uscr

ipt

21

Figures

10 20 30 40 50 60 700

4500

Inte

nsity

(a.u

.)

2θ,ο

* *

111

200220

311

1234

**

Ho2ScNbO7

222

Fig. 1

Page 22 of 35

Accep

ted

Man

uscr

ipt

22

10 20 30 40 50 60 700

5500

Inte

nsity

(a.u

.)

2θ,o

111

200 220311

222

1

2

3

4

Ho2LuNbO7

Fig. 2

Page 23 of 35

Accep

ted

Man

uscr

ipt

23

10 20 30 40 50 60 70

10 20 30 40 50 60 700

2700In

tens

ity (a

.u.)

2θ, ο

111 311

222

400 331 511 440 622 444

1

2

3

4* * * *

Sm2ScNbO7 2θ, o

Inte

nsity

(a.u

.)

111 311

222

400

331 511

622440

444

Fig. 3

Page 24 of 35

Accep

ted

Man

uscr

ipt

24

0 20 40 60 80 100 120 140

0

10

20

30

40

50

550 oC 600 oC 650 oC 700 oC

-Z"/k

Ω

Z'/kΩ

560 kHz

145 kHz

10 mHz16 Hz

285 kHz

38 kHz780 kHz

Fig. 4

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.1

0.2

0.3

0.4

200 kHz

550 oC 600 oC 650 oC 700 oC

-Z"/M

Ω

Z'/MΩ

27 kHz

100 kHz

74 kHz

200 kHz9.8 kHz

10 mHz

32 Hz

Fig. 5

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.00

0.25

0.50

0.75

145 kHz

550 oC 600 oC 650 oC 700 oC

-Z"/M

Ω

Z'/MΩ

9.8 kHz

340 Hz

10 mHz

145 kHz

27 kHz

3.5 kHz100 kHz

Fig. 6

Page 25 of 35

Accep

ted

Man

uscr

ipt

25

-8

-7

-6

-5

-4

-3

-2

-1

0

0.7 0.9 1.1 1.3 1.5 1.7 1.9

log(σT

), SK

/cm

1000/T, K-1

1

2

3

-8

-6

-4

-2

00.7 1 1.3 1.6

log(σT

), SK

/cm

1000/T, K-1

12

Fig. 7

Page 26 of 35

Accep

ted

Man

uscr

ipt

26

-7

-6

-5

-4

-3

-2

-1

00.7 0.9 1.1 1.3 1.5 1.7 1.9

log(σT

), SK

/cm

1000/T, K-1

1

2

3

4

Fig. 8

Page 27 of 35

Accep

ted

Man

uscr

ipt

27

Fig. 9

Page 28 of 35

Accep

ted

Man

uscr

ipt

28

Fig. 10

Page 29 of 35

Accep

ted

Man

uscr

ipt

29

Fig. 11

0.8 1.0 1.2 1.4 1.6 1.8

-7

-6

-5

-4

-3

-2

-1

21

lgσ*

T, S

*K/c

m

1000/T, K-1

heating cooling1

2

Fig. 12

Page 30 of 35

Accep

ted

Man

uscr

ipt

30

300 400 500 600 700 800 900 1000101

102

103

104

105

106

107

ε'

T, oC

7 6 5 4 3 2 1

1

2

3

4

56

7

Fig. 13

300 400 500 600 700 800 9000

50

100

150

200

250

tgδ

T, oC

3 2 1

3

2

1

Fig. 14

Page 31 of 35

Accep

ted

Man

uscr

ipt

31

0.8 1.0 1.2 1.4 1.6 1.8

-6

-5

-4

-3

-2

-1

0

lgσ*

T, S

*K/c

m

1000/T, K-1

heating cooling

1

2

12

Fig. 15

300 400 500 600 700 800 900 1000101

102

103

104

105

ε'

t, oC

7 6 5 4 3 2 1

1

2

3

4

5

6

7

Fig. 16

Page 32 of 35

Accep

ted

Man

uscr

ipt

32

300 400 500 600 700 800 900 10001

10

100

1000

10000tg

δ

t, oC

3 2 1

1

2

3

Fig. 17

Page 33 of 35

Accep

ted

Man

uscr

ipt

33

Fig. 18

Fig. 19

Page 34 of 35

Accep

ted

Man

uscr

ipt

34

Fig. 20

Page 35 of 35

Accep

ted

Man

uscr

ipt

35

Research Highlights

< The phase formation of Ln2RMO7 (Ln=Sm, Ho; R=Lu, Sc; M= Nb, Ta) at 1200-

1600ºC < The bulk conductivity and magnetic susceptibility were measured < The bulk

conductivity of Sm2ScTaO7 has oxygen ion type at T≥ 750°C < The first-order structural

phase transition was observed in Sm2ScTaO7 at ~650-700°C < This phase transformation

is not typical for defect fluorites <