THECRYSTAL STRIJCTIJRES O F RhTc
May 5 , 19.53
.ZND
RhTe2
2641
sure equation for the solid given by Stul15 and the entropies of solution calculated, the heats of solution are derived. The fact that the measured heats of solution are so close to those calculated may be taken as confirming the vapor pressure measurements. The results may be represented by the equation 17,070 - A i l 136 60 - AS log P,,, = - 4576T
+
4576
8 052 log T
I n selecting values of A S from Fig. 3 for use with this equation, a correction should be applied for compositions between 50 and 60 mole yo. I t will be 50 60 70 80 90 1 0 0 ; i O 60 7 0 SO 90 100 noted that the two points in this region, although Mole ?$ aluminum chloride. only slightly displaced from the theoretical curve, (and that within the experimental error) are verti- Fig. 3.--Differential heats and entropies of solution of AIzCle cally as much as 0.5 cal./’C. from the line. Values in sodium chloride-aluminum chloride mixtures in this range should therefore be corrected propor(1) Gas was passed over the liquid instead of tionately for the discrepancy if correct vapor presbubbled through it-sodium was still transferred sures are to be calculated. The phase diagram4 shows a compound NaAlC14 in large quantity. (2) Some barium chloride was put in the meltto exist in the solid; the entropy measurements the ratio sodium:barium in the distillate was 10 above suggest that it exists in the liquid also. The most reasonable assumption concerning the trans- times higher than in the melt. (3) From pure liquid sodium chloride very little fer of sodium in the gas phase is that the same compound is responsible. (Since this work was com- sodium was distilled. (4) Distillation from one limb to the other of an pleted, Howard6 has shown that the analogous fluoride, NaAlF4,is stable in the gas phase). Owing inverted U-tube showed that transfer was faster i n to the lack of precision in the results given in Fig. 2 an evacuated tube than in one filled with argon a t and uncertainties regarding the activities of sodium atmospheric pressure. ( 5 ) It was shown that condensation of a liquid chloride in the liquid mixtures and the entropy of phase could occur a t a temperature only a little NaillCL(gas) it is not possible to make precise calculations, but a AH of the order of -50 kcal. is in- below that of the liquid being distilled; this implies that the vapor was saturated. dicated for the reaction All these results point plainly to the conclusion iSaCl(gas) AlCI?(gas) = NaAlCll(gas) that distillation of sodium is by means of a volatile This is unexpectedly high. In order to confirm compound and, although there is no evidence as to that the transfer of sodium was in fact due to a vola- its composition, the assumption that it is NaAlC14 tile compound and not to spray or liquid films creep- is by far the most reasonable that can be made. The author is deeply indebted to Dr. N. W. F. ing over the inner surfaces of the apparatus a few Phillips and Dr. J. P. McGeer for invaluable discusadditional experiments were made. sion, criticism and encouragement. ( 5 ) D R Stull Znd Enn C h e m , 39, 540 (1947)
+
ARVIDA,QUEBEC,CANADA
(fi) E H Howard, T H IT~o u n N k L , 76, 2041 (1954)
[COSTRIRUT’IOSFROM
THE
BELLTELEPHOSE LABORATORIES, ISC.]
The Crystal Structures of RhTe and RhTe, BY S. GELLER RECEIVEDDECEMBER 22, 1954 The crystal structures of three rhodium tellurium phases have been determined from X-ray powder diffraction data. RhTe has the NiAs(B8) type structure. The high temperature form of RhTet has the Cd(OH)t(CG) type structure and the low temperature form has the pyrite (C2) type structure. The latter which has the lower volume per RhTet is a superconductor a t 1.51OK. discovered by Matthias, et al.1
Introduction Recently, Matthias reported’ that the compound RhTe2 is superconducting with a superconducting transition temperature of 1.51OK. Although not pointed out by Matthias a t the time, there exist two RhTe2 phases, only one of which is (1) B T. M a t t h i a s , E Corenzwit and C E Miller, P h y s Rcu , 93, 1415 (1954)
superconducting2 above 1.OG’K. Also there is a phase of composition RhTe which is not superconducting above 1.06°K.2 The purpose of this paper is to report the crystal structures of the aforementioned rhodium-tellurium phases. Apparently, Wohler, Ewald and Kral13 were the (2) B. T .Matthias, private communication. (3) 1, Wohler, K. Ewald and H. G.Krall, B e r . , 66, 1638 (1933).
TABLE I CRYSTALLOGRAPHIC DATAON RHODIUM-TELLURIUM COMPOUNDS Compound
RhTe
Systematic absences
Most probable
Lattice constants, A.
Hexagonal
(hh.Z),1 odd
Dsh(4)-P&/mmc
a = 3.99 =I= 0.01
78.0
9.81
72.0
8.28
267.2
8.90
space group
Vol./ unit cell,
RhTe2 (high temp. form)
Trigonal
None
D3d(3)-P8m
c = 5.66 i 0 . 0 1 a = 3 . 9 2 rt 0.01 c = 5 . 4 1 == ! 0.01
RhTel (low temp. form)
Cubic
(OM),k odd
Th(6)-Pa3
~1 =
first to prepare a RhTez. The essential part of their method was the heating of RhCls with excess Te in a COP stream a t about 750'. Wohler stated that he found it impossible to obtain a more Terich compound than RhTez. The compound made by Wohler, when examined under a microscope, showed fourfold symmetry. Biltz4 prepared a rhodium-tellurium compound in a manner similar to that of Wohler but claimed that the true formula was Rh2Te6 and that this compound had a "pseudopyrite" structure. The results of our work indicate that there are a t least three phases in the rhodium-tellurium system. These are RhTe with the NiAs (B8) type6 structure and two phases with the composition RhTee. One has the Cd(OH),(C6) types; the other has the pyrite (C2) type.' TABI E I1 COMPARISOS O F CALCULATED WITH OBSERVED IXTENSITIES OF RhTe' (hk.1)
Obsd.
10.0 10.1 10.2 11.0
3.45 2.94 2.19 1.99 1.65
20.1 20.2 00.4
1.47 1.41 1.27
21 * 1 21.3 11.4 7 20.0 10.5 21.3 22.0 31.1 10.6 31.2 30.4 40.1 31.3 21.5 j 20.6
1 1
}
1.19 1.15 1.07 0.998 ,944 ,910 ,908 ,893
d,
A.
X-Ray detd. density,
Crystal symmetry
Rel. ZcP b
Ret.
2.19
w-m vs vs
1.99
S
27 175 183 128
m-s
61
Calcd.
3.46 2.95
1.65 1.47 1.41
m-s
50
W
13
m
37
m
48
m-s
62
w-in
23
1.27 1.18
1.07 1.07 0.998 .945 ,909 .908 ,893
1
IO
w-m m-rn
16 14
w-m m m
34 34
m-s
,854
,855 j .e27 ,825
17
43
24 ,828 m 2;i 40.2 rn .e26 in-s .io 22.4 ,815 ,815 18 10.7 m ,787 .787 38 m-s 32.1 ,785 .785 Reflections with intensity below minimum observable are omitted. w = weak, m = medium, s = strong, vs = very strong. (4) W. Biltz, 2. anorg. Chem., 233, 282 (1937)
(5) Sfrukluvbcrichf,1, 84 (1931). (6) I b i d . , 1, 161 (1931). (7) Zbid., 1, 150 (1931).
6.411
0.002
?c
R.1
g./cc.
Experimental The compounds investigated were obtained from Matthias. The RhTe and the pyrite-type RhTe2 were prepared by mixing the proper proportions of rhodium and tellurium, sealing the mixture in an evacuated fused silica tube and heating to 900' for one to three days. The C d ( 0 H ) d y p e RhTez was made in the same manner but a t 1200' and quenched to room temperature. X-Ray powder photographs were taken using CuK radiation and a Straumanis type Xorelco camera with 114.6 mm. diameter. Intensities of the lines were estimated visually. Structure Determination.-Because all of the structures reported here are well-known types, it is unnecessary to go into great detail. Pertinent crystallographic data on the compounds are given in Table I . Relative intensities were computed using the relation
I = p 'F,
12
ikl
+
1 cos2 26 sin2 0 cos 0
x
10-6
where p is the multiplicity and Fhkl, the structure amplitude. The values of the atomic scattering factors were obtained from Vol. I1 of the Internationale Tabellen zur Bestimmung von Kristallstrukturen. S o corrections mere made for absorption. In all cases the comparison of calculated with observed intensities verified the correctness of the structures. These data are shown in Tables 11, 111, IV.
TABLE I11 COMPARISON O F CALCULATED WITH OBSERVED INTENSITIES OF C6-TYPE RhTer" (hk.1)
Obsd.
00.1 10.1 00.2 10.2 11.0 11.1 20 * 1 11.2 ') 10.3 20.2 00.4 21 * 1 20.3 21.2 30.0 11.4 21.3 \ 30.2 10.5 31.1 20 . e 5 31.2 30.4 10.G
5.37 2.87 2.70 2.11 1.96 1.84 1.62 1.59 1.437 1.352 1.251 1.235 1.159 1.133 1.114
a, A.
Calcd.
5.41 2 57 2.70 3 12 1.96
1.81 1 .62 1.59 1.,59 1.4:i0 1 ,353 1.24s 1.236
I
1,150
1.133 1.114
1
3 1 . :i
1 032 0 929 913
892
,870
1 031 0 928
913 690 e69 " ,872 )
21 . .i 40.2 a Reflections with intensity below minimum observable are omitted.
THECRYSTAL STRUCTURES OF RhTe
May 5 , 1955
AND
2613
RhTe,
Discussion TABLE IV COMPARISON OF CALCULATED WITH OBSERVED INTENSITIES The interatomic distances and numbers of equivalent neighbors are shown in Tables V, VI, 1'11. OF CB-TYPE RhTez hkl
111 200 210 21 1 220 22 1 311 222 320 321 400
322 411 33 1 420 42 1 332 422 430 43 1
520 432 52 1 440 44 1 433 53 1
Re1
Rel.
IO
IS
vw
11 16 195 130 28 5 138
'ir
vs S
w-m
.. !?J
532 620 621
22
S
(30
VW
8 4 2
Ill
27 37
JV
13
w
9 1 3
551 640
11 -in
.. ..
0
m
34
m
29
w-m m
17 33
..
3
..
1 3
vvw
64 1 721 633 552 642
Rel.
IO
O I
..
1
443 54 1 533 622 542 63 1 444 632 543
Rel. W
442 610
11
w-in
.. ..
hkl
27
vvw
3
vvw
5
w-m
1 14 6
m
25
w-m
13 2 2 2
.. .. .. ..
i
73 1 553
8 7
m
, .
.. ..
0
w-m
14
m
32
w-m
17
..
3
m-s
84
1i +
(8) S. Geller and B. B. Cetlin, t o be published.
Atom
No. of equiv. neighbors
Neighboring atom
Rh Rh Rh Te Te Te
6 2 6 6 6 6
Te Rh Rh Rh Te 'Ye
Distance,
A.
2.70 2.83 3.99 2.70 3.99 3.65
TABLE VI INTERATOMIC DISTANCES I N RhTet ( C&TYPE) Atom
1
In the case of RhTe, all atoms are in special positions of space group Dsh(4)-PGa/mrnc 2Rh in 2(a) : 000; 00 1 / ~ 2Te in 2(c) : '/a e/: I/,; '/: '/a a/, In the case of the high temperature modification of RhTez (Cd(0H)a type), a single crystal was isolated and Weissenberg and Buerger precession photographs taken. The diffraction symmetry of these photographs is D&m. As there are no systematic absences, the most probable space group was easily established to be Dad(3)-Ph. In the RhTet structure the atoms are in the following special positions 1Rh in l(a): 000 2Te in 2(d) : I/: z/: z; I/: 3 The single parameter .ZT~was taken to be 0.25 in the calculation of intensities. This value is probably correct t o within =t0.005. Finally, in the case of the low temperature modification of RhTet, the Rh atoms occup 4(a), (000; 0 '/z I/*; 3 )and x, ,- x , f ; 3 )of space the Te atoms 8(c), f (rxz; group Th(6)-Pa(3). The value of the single Te parameter was deduced from a comparison with the RhSe28 structure in which the parameter was determined very accurately from quantitative intensity measurements. In this structure, the distance of closest approach between two Se atoms, 2.50 A . , is soTewhat larger than the elementary Se-Se distance, 2.32 A . The Te parameter in RhTez was therefore deduced by multiplying the elementary Te-Te distance 2.86 A. by the obvious ratio leading t o a probable Te-Te distance in the crystal of 3.08 A. and to me = 0.362. This value leads to very good agreement (see Table I V ) between calculated and observed intensities and is felt to be accurate within h0.005. f/,
TABLE V INTERATOMIC DISTANCES IN RhTe
a
No. of equiv. neighborsa
Rh 6 Rh 6 Rh 6 Te 3 Te 6 Te 6 Te 3 Under the assumption that
A'eighboring atom
Distance,
.k.
Te 2.64 Te 4.65 Fh 3.92 Rh 2 64 Te 3.92 Te 3.52 Rh 4.65 IT? is exactly equal to I / < .
TABLE VI1 INTERATOMIC DISTANCES IN RhTet ( G-TPPE) Atom
Rh Rh Rh Rh Te Te Te Te Te Te
No. of equiv. neighbors
Neighboring atom
Distance, ?,
6
Te Te Te Rh Te Rh Te Te Rh Rh
2.65 4.30 4.04 4.55 3.07 2.65 3.53 3.95 4.30 4.04
6
2 12 1 3 3 3 3 1
No melts with compositions between RhTe and RhTez were photographed so that i t is not known whether the transformation from the BS to the C6 structure is similar to the continuous transformation of CoTe to COT^^.^ It is quite probable that i t is. But the transition of RhTez from the high temperature to the low temperature form is unlike that of CoTe2. I n the latter case, the low temperature phase has the marcasite (C18) type structure. The transition from the C6 to the C2 type probably does not affect significantly the valence of the Rh atom. In both structures the six nearest neighbors to an Rh atom are T e atoms a t the distance 2.64 A. The other atoms surrounding the Rh are in both cases so far from the Rh atom as to have no real effect on its valence. The TeRh-Te angles are 96' in the C6-type and 84' in the C2 type. The surroundings of a T e atom in the two structures are quite different, however. In particular, the Te-Te atoms occur in pairs, which may be said to be Te-Te molecules, the distance between atoms in these pairs, 3.07 .k., being somewhat larger than the elemental T e diameter, (9) S. Tengner, 2.amrg. ollgcm. Chcm., 288, 126 (1938) (10) Slrukturbericht. 1, 495 (1931).
NOTES
"44
Vol. 77
2.80 A. There does appear to be n valence change iri Te involved in the transition.
13. Ceballe who found a value of O.OOX.? ohtn-cni. a t room temperature 0.0030 ohm-cm. a t %OK. and The transition from CB to C2 type is not a simple O.O0Iqj-0.0020 ohm-crn. a t 2O0K., indicating that one. Examination of the structures leads one to this phase is either metallic or a degenerate seniithe conclusion t h a t the transition probably in- conductor. Hardy and Hulm'? have reported the only case volves a displacement in the (11.1) planes as well as motion perpendicular to these planes. The in which two intermetallic phases with the same transition also involves an appreciable contraction. composition are both superconducting. These The phase with the higher density (C2) is the are the thorium silicides (ThSi)?. However, no crystallographic data are as yet available on one of one which is superconducting a t 1.,jl"I
