302 JOURNAL OF THE LESS-COMMON METALS THE SYSTEM THORIUM-GERMANIUM ALLAN BROWN* AND J. J. NORREYS The Gefzeral Electric Company Limited, Central Research Laboratories, Hirst Research Centre, Wembley (Great Britain) (Original manuscript received August rgth, 1962; revised manuscript December 14th. 1962) SUMMARY An investigation of the thorium-germanium system has established the formation of the phases ThaGe, ThsGe 2, ThGe, ThGer.5. ThGer.6, ThGer and Tho.sGez. Crystal structure data for all the phases except ThsGe are given. A tentative phase diagram has been constructed on the basis of metallographic and X-ray analyses and from melting point determinations. The phase equilibrium and structure results are discussed and compared with corre- sponding results for the thorium-silicon and uranium-silicon systems. INTRODUCTION On the basis of X-ray analyses of thorium-germanium alloys prepared by powder metallurgy in the temperature range ~000~-1500~C, THARP, SEARCY AND NOWOTNY~ have reported the formation of five phases; ThGeo.,*o.i, ThaGer, ThGei.s+o.s, or-ThGe2 and ThGes.o*o.4. During an examination of the reactions between thorium-germanium alloys and liquid bismuth it became apparent that the compositions and crystal structures reported for the last three of these phases were in error or incompletely reported, and that a further phase with a composition between those of “a-ThGez” and “ThGe3.~0.4” was formed below 6oo--650°C. Errors in composition in the earlier work were probably due to the poor quality of the thorium available to the authors. In view of these results, the thorium-germanium system was re-examined using both metallographic and X-ray diffraction techniques to determine the compositions of the phases and to construct a tentative phase diagram. EXPERIMENTAL The materials used for alloy preparation were thorium of 99.4% purity supplied by U.K. Atomic Energy Research Establishment, Harwell, and germanium of better than 99.999% purity. The major impurity in the thorium was oxygen, present as thoria, to the extent of 0.5 wt.%. The principal metallic impurities were calcium and aluminium both present to the extent of about 200 p.p.m. Allowance was made for the impurity content in the calculation of charges for melting, and in the subse- quent estimation of the compositions of the phases. * Now at the Institute of Chemistry, University of Uppsala, Sweden. J. Less-Common Metals, 5 (1963) 302-313
Transcript
302 JOURNAL OF THE LESS-COMMON METALS
THE SYSTEM THORIUM-GERMANIUM
ALLAN BROWN* AND J. J. NORREYS
The Gefzeral Electric Company Limited, Central Research Laboratories, Hirst Research Centre, Wembley (Great Britain)
(Original manuscript received August rgth, 1962; revised manuscript December 14th. 1962)
SUMMARY
An investigation of the thorium-germanium system has established the formation of the phases ThaGe, ThsGe 2, ThGe, ThGer.5. ThGer.6, ThGer and Tho.sGez. Crystal structure data for all the phases except ThsGe are given. A tentative phase diagram has been constructed on the basis of metallographic and X-ray analyses and from melting point determinations. The phase equilibrium and structure results are discussed and compared with corre- sponding results for the thorium-silicon and uranium-silicon systems.
INTRODUCTION
On the basis of X-ray analyses of thorium-germanium alloys prepared by powder metallurgy in the temperature range ~000~-1500~C, THARP, SEARCY AND NOWOTNY~ have reported the formation of five phases; ThGeo.,*o.i, ThaGer, ThGei.s+o.s, or-ThGe2 and ThGes.o*o.4.
During an examination of the reactions between thorium-germanium alloys and liquid bismuth it became apparent that the compositions and crystal structures reported for the last three of these phases were in error or incompletely reported, and that a further phase with a composition between those of “a-ThGez” and “ThGe3.~0.4” was formed below 6oo--650°C. Errors in composition in the earlier work were probably due to the poor quality of the thorium available to the authors.
In view of these results, the thorium-germanium system was re-examined using both metallographic and X-ray diffraction techniques to determine the compositions of the phases and to construct a tentative phase diagram.
EXPERIMENTAL
The materials used for alloy preparation were thorium of 99.4% purity supplied by U.K. Atomic Energy Research Establishment, Harwell, and germanium of better than 99.999% purity. The major impurity in the thorium was oxygen, present as thoria, to the extent of 0.5 wt.%. The principal metallic impurities were calcium and aluminium both present to the extent of about 200 p.p.m. Allowance was made for the impurity content in the calculation of charges for melting, and in the subse- quent estimation of the compositions of the phases.
* Now at the Institute of Chemistry, University of Uppsala, Sweden.
J. Less-Common Metals, 5 (1963) 302-313
THE SYSTEM THORIUM-GERMANIUM 303
The principal method of alloy preparation was arc-melting the elements together on a water-cooled copper hearth using a tungsten electrode. An atmosphere of argon at a pressure of 20 cm Hg was maintained. Each button was melted and turned several times to obtain a high degree of homogeneity. In view of the volatility of germanium under these conditions reliance was placed only on those preparations which showed very small changes of weight on melting.
The new thorium-germanium phase was obtained by heating alloys containing approximately 60 at.% germanium in liquid bismuth or tin at temperatures below 600°C. The most germanium-rich phase was also obtained using this method at temperatures above 650°C and also by heating thorium and germanium powders in liquid bismuth at 500%. The powders or crushed arc-melted button, together with the bismuth or tin were sealed under vacuum into either glass or silica capsules, depending on the temperature at which the experiment was to be done. The capsules were simultaneously heated and shaken for periods of up to IOO h. A sample of the new thorium-germanium phase, extracted from the bismuth matrix with I:J nitric acid, was examined by spectrographic analysis. Within the limits of the sensitivity of this technique no bismuth could be detected.
Arc-melted alloys and preparations obtained by reaction in liquid bismuth were heat-treated at various temperatures in the range 400 to 14oo’C. Up to IOOO’C, specimens were heated in alumina boats sealed into evacuated glass or silica capsules. At higher temperatures heat treatment was by high frequency induction heating, in a vacuum of better than 10-4 mm Hg.
The approximate melting-points of a number of alloys were measured to obtain an indication of the shape of the liquidus curve and to obtain eutectic temperatures. The measurements were made using a disappearing-filament optical pyrometer. The specimens were heated either in tantalum boats through which an electric current was passed or by high frequency induction. The brightness temperature at which each alloy began to flow was taken as its melting point. In all but eutectic alloy this was preceded by a stage at which the samples appeared to soften. No corrections were applied for the emissivity of the material.
The alloys were examined by conventional metallographic techniques. The etch- ants used were 0.5% hydrofluoric acid and a mixture of 66% methyl alcohol, 33% nitric acid and 1% sodium silicofluoride.
X-ray examination was made with an 11.46 cm diameter Debye-Scherrer camera and CuKol radiation. Values for the cell dimensions of the phases were subsequently obtained using a Guinier focusing camera and CrKori radiation (loci = 2.2896 A) with silicon (a, = 5.4306 A) as an internal standard.
RESULTS
Melting points and cell dimensions for the compounds in the system are listed in Table I. With one exception, variationsin the cell dimensions of the intermediate phases could not be detected and it is concluded that these thorium-germanides have only small homogeneity ranges.
Alloys containing up to 40 at.% germanium
Alloys containing 40 at.% germanium comprised a single phase, ThaGez. Between 15 and 40 at.76 germanium three phase mixtures were formed consisting of ThsGez,
J. Less-Common Metals, 5 (1963) 302-313
304 A. BROWN, J. J. NORREYS
THE SYSTEM THORIUM-GERMAiXIURI 305
thorium and a phase with a complex diffraction pattern. Alloys containing less than
15 at.% germanium comprised mixtures of this phase and thorium. Metallographic examination indicated that the thorium-rich phase was formed by a peritectoid reaction between thorium and Th&eg, Fig. I. At germanium contents above 15 at.%, the reaction does not go to completion unless the alloy is subsequently heated at about IOOO’C. Alloys containing 25 at.% germanium, treated in this way, yield the thorium-rich phase as a single product, Table II. This phase is therefore designatedTh&c.
Fig. I. Arc-melted thorium-33 at. yb germanium alloy. The structure consists of ThsGee (grey) rimmed with ThaGe in a matrix of ThsGc/thorium cutcctic. Some thoria (black) is prcwnt.
Etched Gh”l, (‘H&H, 33”,! HNOs, I 9:) NazSiFe. ( x 1000)
2.i 9.9
4.5 9oo
5 .i 9oo
60 C)OO
61 9oo
Th f ThsGc -1. ThsGes
Th~Ge~ i- ThGe
ThGe + ThGc, .,j
ThCVl.:,
(‘c -type solid solution
Th:~Gc
Th:sGcz + ThGc
ThGc -+ ThGc1.s
ThGel.j
ThGel.5 + ThGc,.,j
Gr 9oo soo
63 9oo
67 900 4.50
69 600
5oo 4.50
40 170
24
24 700
[PO
3.50 7oo
ThGc, .6 ThGel.6
Cc -type solid solution + Tho.sGee (trace)
ThGel.6 + Tho.sGcz ThGe,.a + Th,j.sGei
Tho.sGez The. sGca Th,,.gGca
ThGel.6 ThGc1.a
ThGe1.e + Tho.gGer
ThGc1.G + Tho.sGcr ThGc, .B + Tho. oGcz
Tho.gGcl The. ~Gce Tho.sGez
306 11. RROWN,J. J. NORREYS
The temperature of the zThsGe + ThsGe2+3Th peritectoidreaction is 13~0~--14oo~C. A eutectic mixture is formed between thorium and ThsGea at 12 at.% germanium and melts at about 1450°C.
Crystal structure data for ThsGez differ slightly from the earlier results’. So far it has not been possible to interpret the diffraction pattern of ThsGe. In a sample containing ThsGe and thorium annealed at 950°C the cubic cell dimension of thorium was 5.112 + 0.002 A. The value quoted for the pure element is 5.087 AZ, and the result indicates a small solubility of germanium in the thorium lattice. Metallographic examination gives no indication of any appreciable change of solid solubility of ger- manium in thorium up to 1300°C.
Alloys containing 40-50 at.% germanium
ThGe was obtained as a single phase at 50 at.% germanium. Below this germanium content mixtures of ThsGez and ThGe were obtained. A eutectic mixture of the two occurs at a composition of 48-49 at.% germanium.
Crystal structure data for ThGe are in agreement with the earlier resultsi.
Alloys containing 50-60 at.% germanium
The alloys comprised a single phase at 59-60 at.% germanium, and the phase with this composition is designated ThGei.5. Within the composition range, mixtures of ThGe and ThGei.5 were formed.
Powder diffraction data for ThGei.5 are comparable with those given by THARP et al. for ThGei.s*o.s. The structure of this phase has not been completely solved but interpretation of the powder data gives some indication of the probable atomic arrangement. The Debye-Scherrer patterns are closely similar to those of the defect C32 (AlBz)-type structure of ThSi 1.67 (Th&)3. Most of the reflections given by the hexagonal structure, however, show splitting and additional weak reflections are present which cannot be indexed on the basis of this simple structure type.
Guinier-type patterns show that the reflections indexed as IO~O and IOII on the basis of the hexagonal structure cell each generates two components which have an intensity ratio of z I. Considering the multiplicities of the hexagonal reflections, this result can be expressed in terms of interplanar spacings as follows
Consideration of Fig. 2 shows that this effect is most easily accounted for by a simple orthorhombic distortion. By comparison, a simple monoclinic distortion would lead to each of the above reflections splitting into three components of equal intensity.
The principalreflections in the pattern can be indexed on the basis of a simple ortho- rhombic cell with axes a, b and c corresponding to the original hexagonaldirections [IoIo], [OOOI] and [r210] respectively. The weak reflections can be indexed in terms of a larger orthorhombic cell with the dimensions a, zb and 2c containing eight thorium and twelve germanium atoms. Conditions limiting possible reflections are 1 = 2% for okl and ho1 reflections and the possible space groups are therefore Pcc2 (CiJ and Pccm (L$). In the most symmetrical case, the space group is Pccm and the metal
J. Less-Common Metals, 5 (1963) 302-313
THE SYSTEM THORIUM-GERMANIUM 307
Fig. L. Diagram showing relationship between hexagonal C32 lattice and orthorhombic cell of ThGel.5. Dashed line represents boundary of C3z cell projected on (ooo I). Heavy line shows 1)oundary of orthorhombic ccl1 of ThGel.5 projected on (010). Only positions of metal atoms in
(0001) plane are shown.
TABLE III
(;“INIER PoWD&R DATA FOR ORTHORHOMBIC ThGel.5
CrKal radiation; ,?.a~ = 2.2896 A; a0 = 6.989 & 0.002 A; DO = 8.432 & 0.003 A; CO = 8.136 i 0.003 A.
1.831 m.w. 1.823 m. 1.808 m. 1.805 m. I.794 w. 1.7jS m.w. 1.746 w.
* V.S. = very strong: s. = strong: m. = medium; m.w. = medium weak; w. = weak; V.W. = very weak.
J. L.ess-Common Metals, 5 (1963) 302-3 13
308 A. BROWN, J. J. NORREYS
atoms occupy the twofold special positions (a), (c), (f) and (h) which correspond to the metal atom positions in the hexagonal C3z structure. In this arrangement, the thorium atoms would contribute only to the principal reflections which obey the extra condition 1 = 2n for hkl. The remaining weak reflections would then be due to the germanium atoms, eight of which must occupy the general position (Y), or at least four the special position (4). Alternatively, the twelve germanium atoms might be distributed among sixteen possible positions in a disordered arrangement.
Diffraction data for ThGer.5 are given in Table III.
Alloys containing 60-63 at.% germanium
Nine alloys were prepared in this composition range. Between 60 and 61 at.‘I:;,
germanium, the preparations comprised mixtures of ThGer.5 and a phase with the C, (a-ThSia)-type structure. Between 61 and 63 at.% germanium, single-phase alloys with the C, -type structure were obtained by arc-melting. This phase appears to have a small range of composition, as indicated by progressive changes in 8, the Bragg angle of the X-ray reflections, with increasing germanium content.
Heat treatment of the single-phase arc-melted preparations at 900°C for 24 h resulted in precipitation of a second phase, except in the case of those containing N 62 at.% germanium. ThGer.5 precipitated from the thorium-rich solid solutions and Tho.gGez from germanium-rich solid solutions, Table II. Accompanying such precipitation the X-ray diffraction pattern of the C, phase was modified, so that it
Fig. 3. Arc-melted thorium-61 at.O/ ,0 germanium alloy, after 24 h at room temperature. The struc- ture consists of ThGel.5 and dark etching ThGel.0, which has begun to show fissures. Etched 66(‘(,
CHZOH, 33%, HNOs, 1% NazSiFs. (x 340)
J. I.rss-Conzmon Mt+als, 5 (1903) 302-3 T 3
THE SYSTEM THORIUM-GERMANIUM 309
became identical with that of the preparations at 62 at.% germanium. In view of the
stability of preparations at 62 at.% germanium it seems appropriate to describe the Cc-type phase as ThGel.6 (61.5 at.% germanium). At temperatures approaching the melting point the composition range extends from 61 to 63 at.% germanium. However, examination of an arc-melted alloy containing 61 at.% germanium which had been reheated but not remelted in the arc-furnace, Fig. 3, suggests that the extent of solid solubility decreases rapidly with temperature. C,-type solid solution alloys containing other than 62 at. o/O germanium showed evidence of ageing at room- temperature. This was indicated in the diffraction patterns by a progressive loss of definition with time. The effect, which was not associated with oxidation, occurred most rapidly in the germanium-rich solid solutions.
Whilst Debye-Scherrer patterns indicate that ThGer.6 has the tetragonal C,-type structure, patterns obtained with a Guinier camera show that there is a small struc- tural distortion and that the X-ray reflections aremore correctly indexed in terms of a face-centred orthorhombic cell in which the axes a, b, and c correspond tothetetra- gonal directions [IIO], [iTo] and [OOI] respectively as shown in Fig. 4. Conditions
Fig. 4. The relationship between the tetragonal Cc structure and the orthorhombic cell of ThGel.6. Dashed line represents boundary of Cc projected on (001). Heavy line shows boundary of or- thorhombic cell projected on (001). Only positions of X atoms are indicated. Height of ‘X atoms in terms of parameter z: 0, at i + z; 0, at + + z; 0, at $ - z; A, at z; I 1, at - z; 0,
at ) + z; x, at * - 2.
limiting the possible reflections are h + k and k + 1 = mz for hkl, k + 1 = qn for okl, h + 1 = 472 for hol, and h + k = 4n for hko. These correspond to the restric- tions for the space group Fddd (0::). I n view of the relationship between this and the tetragonal C,-type structure, the thorium atoms probably occupy the eightfold special positions (a) with the germanium atoms (from 12.5 to 13.5 per unit cell depending on composition) distributed among the sixteenfold (g) positions to give a defect lattice.
J. Less-Common Metals, 5 (1963) p-313
310 A. BROWN, J. J. NORREYS
The cell dimensions of this solid solution phase increase from ao = 5.885 & o.oozA, bo = 5.859 &- 0.002 A and CO = 14.219 f 0.006 A in a single-phase alloy containing 6r at.% germanium to a0 = 5.917 f 0.002 8, bo = 5.900 f 0.002 A and CO = 14.225 & 0.006 w in a two-phase alloy formed at 63-64 at.% germanium. The cell dimensions quoted in Table I and the diffraction data given in Table IV refer to alloys containing N 62% germanium.
TABLE IV
GUINIER POWDER DATA FOR ORTHORHOMBIC ThGel.6
CrKal radiation; lart = 2.2896 A;
a0 = 5,913 & o.002 A; br, = 5.889 & o.ooz A; co = 14.“‘g xt 0.006 A.
In the investigation by THARP, SEARCY AND NOWOTNY~, to which reference was made above, a phase with the Cc-type structure was designated ol-ThGe2. The cell dimensions reported for this phase, a0 = 4.106 f 0.003 A and CO = 14.193 & 0.005 A are similar to those obtained for ThGel.6, namely a0 -4.17 A and CO = 14.219 f 0.006 A if the structure is referred to a tetragonal C,-type cell for purposes of com- parison. Although the values given for “a-ThGez ” are smaller than those for ThGel. 6, the similarity is sufficiently close to suggest that they are the same phase. Discrepan- cies might be accounted for by the poor quality of thorium available for the earlier Work.
Alloys containing 63-68 at. o/o germanium
Alloys in this composition range comprised mixtures of ThGel.8 and a phase which contains 68-69 at.% germanium. Powder diffraction data for this phase show that it corresponds to the compound described by THARP et al. as ThGes.o&to.r. A detailed examination shows that the phase has an orthorhombic crystal structure related to the C4g (ZrSiz)-type structure and a crystallographic composition of ThGe#. 1n view of the germanium content of the single-phase alloys, it is evident that this phase is deficient in thorium and is best described as Tho.sGez.
No differences can be detected between diffraction patterns of Tho.gGez obtained by arc-melting and by reaction in liquid bismuth.
J. Less-Common Metals, 5 (1963) 302-313
THE SYSTEM THORIUM-GERMANIUM 311
Alloys containing more than 68 at.% germanium
These alloys comprised mixtures of Tho.QGeQ and germanium. A eutectic mixture melting at 900°C was obtained at a composition of 90 at.% germanium.
Stoichiometric ThGe2
No single phase preparations were obtained in arc-melted alloys with germanium contents corresponding to ThGeQ. However, as a result of heating alloys containing 60 at.% germanium in liquid bismuth or tin at temperatures below 650°C crystals of a new thorium-germanium phase were obtained. A single-crystal examination showed that this phase has the C49-type structure4 and is therefore isostructural with UGeQ5. Diffraction patterns obtained after crystals of this phase had been heated to 7oo°C, showed that only ThGe1.Q andTho.QGeQ were present. It is therefore conclud- ed that the new phase has a composition of, or close to, ThGeQ and that it decomposes peritectoidally at 6oo-650°C. So far it has not been possible to form ThGeQ in detec- table amounts by heat-treating two-phase, arc-melted alloys containing 66.7 at.% germanium at temperatures below 600°C.
DISCUSSION
The results of the metallographic and X-ray examination of thorium-germanium alloys in the as-cast and heat-treated conditions are given in the phase diagram shown in Fig. 5. It should be noted that it was not possible to determine whether ThGe has a higher melting-point than ThGe 1.5 from an examination of the micro- structure of alloys in the range 50-60 at.% germanium. Approximate melting point determinations, however, indicate that this is the case.
There are marked similarities between the phase diagram for the thorium- germanium system and those given for the thorium-silicon3 and uranium-silicon6 systems. Thus, in each there are eutectics of the type M/MQXz and X/X-rich phase. The closest resemblance lies, however, between the Th-Ge and U-Si systems. In these, a phase MQX is formed by a peritectoid reaction between M and MQXQ and there is, in addition, a eutectic of the type MQXQ/MX. Consequently, in thecomposi- tion range 0-50 at.% X, where X = Si or Ge, the microstructures of Th-Ge alloys bear a close similarity to those of corresponding U-Si alloys.
Of the phases containing up to 50 at.% Ge, ThQGeQ has the Dga-type structure and is isostructural with U&r and ThQSiQQ. ThGe, on the other hand, has the BI
(NaCl) arrangement and is isostructural with ThAsQ, whereas USi and ThSiQ both have the B27 (FeB) arrangement.
The germanium-rich portion of the system comprises four phases with crystal struc- tures based on the crystallographic formula ThGeQ. Departures from stoichiometry in three of these phases can be judged from the volume occupied by one formula unit as follows: ThQ.QGeQ, 69.6 AQ, ThGer.6, 6r.9 AQ, ThGer.6, 59.9 AQ, compared with 72.3 AQ for ThGeQ. It is noteworthy that the thermal stability in this series increases on either side of the ThGeQ composition, in parallel with the departure from stoichiometry.
Non-stoichiometric phases similar to ThGel.5 have been found in the thorium- silicon and uranium-silicon systems at the composition MSil.67 (MQSirJQ~lQ. In the as-cast condition, these phases have an undistorted C3z-type structure in which the axial ratio c/a is greater than unity and a disordered arrangement of defects and
J. Less-Common Metals, 5 (1963) ‘jO2-313
312 A. BROWN, J. J. NORREYS
silicon atoms appears probable. A series of rare-earth silicides with the same composi- tion is reported briefly by LUNDIN for M = Y, Gd, Dy, Er and Lull. Full details have not been published but the structures are hexagonal with c/a >I, and the cell dimensions are of the same order of magnitude as those of ThSii.67 and USii.67, so that they might also be supposed to have the defect-C32 structure. In view of these results, the structure distortion in ThGe 1.5 can be explained in terms of a greater departure from the MX2 composition coupled with the probable ordering of the ger- manium atoms.
LtThGe
600 -
400-
200
t
A 0
‘IgGe +Th3Ge 2
rhgGe:,
+ Th Ge
30 40 !
F-..,
Th Ge
rh+&,
ThGez + Tho,9Ge2
70 60 90 0
ATOMIC “1. GERMANIUM
Fig. 5. Phase diagram of the thorium-germanium system. n , Observed melting points.
Defect Cc-type phases have been found in the Th-Si and U-Si systems at composi- tions approximating to ThSir.8 and USir.ss 12. There is a greater departure from stoichiometry in ThGei.6 and the orthorhombic structure of this phase corresponds to a C,-type structure in which the interplanar spacing in the [IIO] direction is slightly larger than that in the [IIO] direction. The possible cause of this distortion is outlined below, with reference to Fig. 4. Here the tetragonal and orthorhombic cells are pro- jected on (001) the positions of the X-atoms (germanium or silicon) being denoted by symbols representative of their height above the (001) plane. It is seen that these atoms lie at the intersections of planes with indices 220 and 220. The fourfold symme-
J. Less-Common Metals, 5 (1963) 302-373
THE SYSTEM THORIUM-GERMANIUM 313
try of the structure in the LOOI] direction clearly depends on the isotropic packing of X-atoms about the metal-atom positions. Accordingly, removal of X-atoms from every fourth (220) plane would represent a departure from isotropic packing and lead
to a contraction of the lattice in the [IIO] direction relative to [IIO]. Such a distortion should be sensitive to changes of composition. Thus a decrease in the X-atom con- tent should lead to a greater departure from unity in the value of the a/b axial ratio of the orthorhombic unit cell. It is noteworthy that a change of this kind is observed in the ThGel.6 solid solution range, the value of a/b increasing from I.0029 in the germanium-rich alloys to 1.0044 in the thorium-rich alloys.
This result is of interest in connection with the orthorhombic distortion of the C,-type structure found by PERRI, BINDER AND POSTER in a series of rare-earth silicides with the nominal composition MSiz where M = Y, Pr, Nd, Sm and Dy. In particular these authors report a marked deficiency of silicon in the Gd-Si phase and although the structure distortion is not of the same type as that found in ThGel.6 it can be accounted for in terms of the same model. Thus, with reference to Fig. 4, the silicon atoms are located at the intersections of sets of planes with indices zoo and 020. Removal of some of the silicon atoms from alternate (020) planes should therefore lead to a contraction in the [OIO] direction relative to [IOO] as is found in the Gd-Si phase. Removal of silicon atoms from alternate (200) planes would result in a contraction in the [IOO] direction relative to [OIO] thus accounting for the distortions in the remainder of these orthorhombic phases.
Stoichiometric USi2, like ThGez, has so far only been obtained by crystallization in liquid bismuthl0.12. The compounds decompose peritectoidally at the relatively low temperatures of 450~ and -600°C respectively and the difficulty of forming them by heat-treating two-phase, arc-melted alloys of the appropriate composition can be explained by the slow diffusion of silicon or germanium through the coarse- grained reactants at temperatures below the peritectoid temperature.
ACKNOWLEDGEMENTS
This investigation formed part of a contract for the United Kingdom Atomic Energy Authority Research Group, Harwell, and the authors wish to thank the Director for permission to publish the results. We are also grateful to Professor GUNNAR H~~GG for permission to use the Guinier cameras at the Institute of Chemistry, University of Uppsala.
REFERENCES
1 A. G. THARP. A. W. SEARCY ANDH.NOWOTNY, J. Electrochem. Sot., 105 (1958) 473. 2 H.A. WILHELM,O.N.CARLSONAND H.E.LuNT, U.S. AtomicEnergyComm.Pub1.A.E.C.L).
3603 (1953). 3 A. BROWN AND J. J. NORREYS,~. Inst. Metals, 89 (1961) 238, 4 A. BROWN, Acta Cryst., 15 (1962) 652. 5 E. S. MAKAROV AND V. N. BYKOV, Kristallografiya, 4 (1959) 23. 6 A. R. KAUFMANN, B. D. CULLITY AND G. BITSIANES, Trans. ,4.I.M.E., log (1957) 23. 7 1%‘. H. ZACHARIASEN, Acta Cryst., 2 (1949) 94. * E. L. JACOBSON, R. D. FREEMAN, A. G. THARP AND A. IV. SEARCY, J. Am. Chem. Sot., 78
(1956) 4850. 9 R. FERRO, Acta Cryst., 8 (1955) 360.
10 A. BROWN AND J. J. NORREYS, Nature, 183 (1959) 673. 11 C. E. LUNDIN, The Rare Earths, John Wiley, New York, 1961, p. 224. I2 A. BROWN AND J. J. NORREYS, Nature, 191 (1961) 61. 13 J. A. PERRI, I. BINDER AND B. POST, J. Phys. Chem., 63 (1959) 616.
