Nov 29, 1978 - 57,. 39-51. TAYLOR, J. B., CALVERT, L. D. & WANG, Y. (1977). J. Appl. Cryst. 10, 492-494. WANG, Y., CALVERT, L. D., GABE, E. J. & TAYLOR, ...
CRYSTAL DATA MnsSi3-type phases. A discussion of some aspects of the crystal chemistry of these phases has already been given by Wang, Gabe, Calvert & Taylor (1976). Data on the pnigogen-rich phases is less complete. Eu2Sb3 is indexed as monoclinic, pseudo-orthorhombic, in conformity with Ono et al. (1971) for Eu2As3. Eu2Sb3 is isostructural with both Eu2As3 and Ca2As3 as can be demonstrated by powder calculations using the coordinates of Deller & Eisenmann (1976). The latter use a cell which has fl = 131.6 °, and with respect to it, the cell of Ono et al. is 101/010/001 and h a s ' t = 9 0 . 7 °. In the system Eu-Bi there is a phase richer in Bi than Eu 11Bi~o which has a powder pattern similar to YbBi2 (unpublished data). The structure type could not be identified and the only crystals available were malleable and of too low quality for further study.
References DELLER, K. & EISENMANN, B. (1976). Z. Naturforsch. Teil B, 31, 1023-1027.
251
GAMB~O, R. J. (1967). J. Less-Common Met. 12, 344-352. MURRAY, J. J., MARX, P. L. & TAYLOR, J. B. (1979). J. LessCommon Met. In the press. ONO, S., HuI, F. L., DESPAULT, J. G., CALVERT, L. D. & TAYLOR, J. B. (1971). J. Less-Common Met. 25, 287-294. TAYLOR, J. B., CALVERT, L. O., UTSUNOMIYA, T., WANG, Y. & DESPAULT, J. G. (1978). J. Less-Common Met. 57, 39-51. TAYLOR, J. B., CALVERT, L. D. & WANG, Y. (1977). J. Appl. Cryst. 10, 492-494. WANG, Y., CALVERT, L. D., GABE, E. J. & TAYLOR, J. B. (1978). Acta Cryst. B34, 1962-1965. WANG, Y., GABE, E. J., CALVERT, L. D. & TAYLOR, J. B. (1976). Acta Cryst. B32, 1440--1445. WANG, Y., GABE, E. J., CALVERT, L. D. & TAYLOR, J. B. (1977). Acta Cryst. B33, 131-133. WOLFF, P. M. DE (1968). J. Appl. Cryst. 1, 108-113. YOSHIHARA, K., TAYLOR, J. B., CALVERT, L. D. & DESPAULT, J. G. (1975). J. Less-Common Met. 41,329-337.
J. Appl. Cryst. (1979). 12, 251-252
Crystal data and crystal growth of PbsGeOT.
By H. H. OTTO, Faehbereich Physik der Universitdt Regensburg, Postfach 397, D-8400 Regensburg 2, Federal Republic of Germany (Received 29 November 1978" accepted 21 December 1978)
Abstract Transparent and light brownish single crystals of PbsGeO7 were grown from a melt of composition 4.8PbO.GeOz by spontaneous crystallization. The compound melts incongruently at 1014 K. The unit cell is orthorhombic with lattice constants a=9"296 (2), b=11"546 (2), c=17"298 (5) .A,; Dm(293 K) = 8.63 (4), Dx = 8.734 Mg m- 3 for Z = 8. The space group is Pbca. Indexed X-ray powder data are given.
gold crucible until a homogeneous melt was obtained. The melt was cooled at a rate of 3K/h in a programmed tubular furnace. We obtained well-developed crystals of PbsGeOv, together with a small amount of yellow/3-PbO and of a third phase, indicating that PbsGeO7 melts incongruently. The incongruent melting point was 1014 K. The stoichiometric composition of the brownish crystals was verified by a chemical analysis using carefully separated material.
Unit-cell data Introduction Lead germanates have for some time been under investigation at the University of Regensburg because they have unusual properties and attracted considerable interest as materials for electronic applications. The phase diagram of the PbO-GeO2 binary system was first investigated by Speranskaya (1959). She reports on Pb6GeO8, Pb3GeOs, PbsGe3Oll, PbGeO3, and PbGe3OT. Our results indicate that PbsGeO7 is a stable lead germanate, but Pb6GeO8 is not.
Origin of specimens Transparent and light brownish single crystals of several mm in size were obtained by spontaneous crystallization of a melt of composition 4"8PbO.GeOe using Optipur-quality red a-PbO from Merck Chemical Corp. and Specpurequality GeOz (quartz modification) from Johnson Matthey Chemicals Ltd. To avoid the formation of metallic lead the powdered oxides were well mixed and sintered at 980 K in air. The resulting material was ground and then melted in a 0021-8898/79/020251-02501.00
Moving-crystal and Weissenberg photographs taken with Cu K~ radiation showed that the unit cell of PbsGeO~ is orthorhombic. The lattice constants determined from these photographs were refined from the indexed powder data using the program P U L V E R (Weber, unpublished). Systematic absences hkO for h odd, hOI for I odd, Okl for k odd, and h00, 0k0, 001 for h,k,I odd, respectively, were found. The probable space group is therefore Pbca (Dzh). 15 The density measured by toluene displacement in a pycnometer at 293 K was D m = 8-63 (4) Mg m-3. The calculated density is Dx = 8-734 (6) Mg m-3 for eight formula units of PbsGeOT. This is in agreement with the space group requirements regarding the multiplicities of the equipoints. The crystal data are Table 1. Crystal data for PbsGeOv PbsGeO7 Space group: Pbca (D2h) 15 a = 9"296(2) A b = 11"546 (2) c = 17-298 (5)
FW 1220.6 V = 1856"5 (12)/~3 Dr,(293 K) = 8"63 (4) Mg m- 3 Dx = 8"734 (6) Mg m - 3 Z=8
© 1979 International Union of Crystallography
252
CRYSTAL DATA s u m m a r i z e d in Table 1. The 3a errors associated with the final digit of the lattice parameters are given in parentheses.
Table 2. X-ray powder data for P b s G e O 7 2= 1.54051A
I/Io 2 5 1 ~5 0.5 1 2 4 8 1 1 10 6 4 100 3 20 50 10 2 10 15 1 1.5 1 2 1
200 13.238 15-344 15"952 19.096 19.676 d { 20.524 20.584 20.808 21-780 23.956 25.118 25.770 26.672 27.088 b 28.614 29.076 29.492 30.282 30-956
b
31.390 31.610 32.210 32.568 33.200 34.100 36.672 37.598 38.454
10
38.708
3 3 1 5
~ 39-792 d ~ 39.900 40.162 40.676
7
44-356
0"5 0"5 8 3 0.5 6 2
45.288 45.546 47.676 48-428 48.686 49.456 49.932
b
12
50.372
5
50"658
20c 13.244 15.335 15.949 19.078 19.665 20.520 20.581 20"804 21.767 23.948 25.109 25.778 26.670 27-090 28.571 29.099 29.487 30.282 30.955 30.993 31.394 31.607 32"219 32.573 33.191 34.108 36.648 37.595 38.460 38.699 38.704 38.711 39.792 39"918 40.143 4~674 44-345 44-373 45.291 45.551 47.686 48.419 48-709 49-465 49-920 50.366 50.391 50"660
do 6.682 5-770 5.551 4.644 4.508 4.324 4.311 4.265 4.077 3.7114 3.5423 3.4541 3"3393 3.2890 3.1170 3.0685 3.0261 2.9490 2.8863 2.8474 2.8280 2.7767 2.7470 2.6961 2.6270 2.4484 2.3902 2-3390 2-3242 2.2634 2.2575 2.2434 2.2162 2.0405 2.0006 1.9899 1.9060 1.8780 1.8687 1.8413 1.8249 1.8100 1.8004
dc
hkl
6.679 5.773 5.552 4.648 4.510 4-324 4.312 4.266 4.079 3.7127 3.5436 3"4530 3.3396 3"2887 3.1215 3.0661 3.0266 2-9490 2.8864 2-8829 2.8470 2.8282 2.7760 2.7466 2.6968 2.6264 2-4499 2.3904 2.3386 2.3248 2.3245 2"3240 2.2634 2.2565 2.2444 2.2163 2.0410 2.0398 2.0005 1.9897 1.9055 1.8783 1.8678 1.8410 1.8253 1.8102 1.8093 1.8004
111 020 112 200 113 004 210 122 023 114 221 213 222 132 115" 223 133" 311 040 006 041 312 224 134 321 142 206 331 117 332 144 400 315 243 402 045 227 046 245 154 317 062 246 351 063 440 406 441
d=doublet, b = b r o a d , * position ofthe strongestlines of Pb3GeOs
X-ray powder data The p o w d e r data (Table 2) were obtained with a Guinier counter diffractometer using Ge m o n o c h r o m a t i z e d Cu K s radiation ( 2 = 1.54051 A). Intensities represent relative peak heights. All strong reflections from single-crystal p h o t o graphs were found in the indexed p o w d e r diagram. The mean angle error for the lines listed in Table 2 is 0"(20)= 0.006 c, excluding very weak, b r o a d (b), and double (d) lines as well as the strongest line. The 115 reflection may be b r o a d e n e d due to coincidence with one of the strongest lines of Pb3GeO5 m a r k e d by an asterisk in Table 2.
Further problems In the Pb-rich part of the phase d i a g r a m there are further problems regarding the existence of Pb4GeO6, first reported by Phillips & Scroger (1965), and recently by Bush & Venevtsev (1978) and Breuer (1978). O u r investigation confirms the existence of a monoclinic pseudo-trigonal c o m p o u n d of greenish colour between P b s G e O 7 and P b 3 G e O s , but the composition P b a G e O 6 may be deficient in not reflecting the measured density. Weissenberg p h o t o g r a p h s of this comp o u n d show obvious disorder effects and the indexing of the X-ray p o w d e r d i a g r a m has to be done carefully. Therefore, improved data of this phase will be given soon in more detail. The a u t h o r thanks the Deutsche Forschungsgemeinschaft for financial support.
References BREUER, K. H. (1978). D i p l o m a r b e i t T H , Aachen. BUSH, A. A. & VENEVTSEV, YU. N. (1978). Kristallografiya, 23, 203-205. PHILLIPS, B. 8t. SCROGER, M. G. (1965). J. Am. Ceram. Soc. 18, 398-401. SPERANSKAYA, E. A. (1959). Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, pp. 162-163.
