1701
Inorg. Chem. 1989, 28, 1701-1703
Contribution from the Departamento d e Quimica InorgPnica I, Facultad de Ciencias Q u h i c a s , Universidad Complutense, 28040-Madrid, Spain, and Instituto de Ciencia d e Materiales (CSIC), 28006-Madrid, Spain
Synthesis and Characterization of the New Mixed Oxides M2(GeTe)06 (M = K, Rb, Cs) M. Amadla,+ M. L. Veiga,* C. Pico,*v* M. Gaitin,* and A. Jerezt Received June 14, I988
The mixed oxides M2(GeTe)O6( M = K, Rb, Cs) were prepared by a solid-state reaction between Te(OH)6, GeOz, and MNO,. In this paper their synthesis, X-ray powder diffraction patterns, IR spectra, and thermal behaviors are described. From the X-ray analyses, a cubic symmetry was established, resulting in the cell parameters a = 9.917 ( M = K), 9.946 ( M = Rb), and 10.012 8, (M = Cs). These materials have a pyrochlore-type structure, and the atomic positions and R discrepancy factors were also calculated.
Introduction The compounds discussed in this paper seem t o possess the pyrochlore structure. The space group is Fd3m (No. 227), and t h e unit cell contains eight structural units with t h e ideal formula A2B20601,192whereas in t h e defect pyrochlore structure anion and cation vacancies exist in the lattice to give the compositions A2B2060', ( A 2 B 2 0 7 40 C x C 1 ) or AB206.3-5 Since in all the cases the existence of different types of vacancies for diffusion of the A ions in t h e structure could be observed, these compounds can be regarded as potential ionic conductors.b* Until now several syntheses with this structural type have been described where a tetravalent m e t a l or semimetal such as germanium is present. For example, cubic g e r m a n a t e pyrochlores (A2Ge207;A = G d , Lu, Y,Sc, In, T1) were prepared by Shannon and Sleight9 employing high-pressure techniques. O t h e r studies developed by Bocquillon e t a1.I0 have shown t h a t Gd2Ge207, prepared a t atmospheric pressure, has a tetragonal structure." At t h e present, t h e r e are not any references in t h e literature about mixed oxides of stoichiometry A 2 ( G e M ) O 6 adopting a pyrcchlore-type structure. On t h e other hand, t h e cell parameters for t h e derivatives A3+2B4+207,where B = Si and Ge, are lower t h a n 10 A, probably due t o t h e ionic radii of these elements in their usual oxidation s t a t e +4.12 This paper describes t h e preparation and identification by T G , IR, and powder X-ray diffraction techniques of t h e Mz(GeTe)6 ( M = K, R b , Cs) compounds. Experimental Section The mixed oxides M2(GeTe)O6 were synthesized, in porcelain crucibles, by heating in air a mixture, in a 1:1:2 molar ratio, of Te(OH)6, GeOZ,and MNO, (M = K, Rb, Cs) at 673 K for 36 h and later at 873 K for 24 h. The microcrystalline powders obtained are gray (K), dark gray (Rb), and black (Cs). The reagents (analytical grade) used in these experiments were supplied from Merck (Te(OH)6, Ge02, KNO,, and CsNO,) and Aldrich (RbNO,). These materials were characterized by chemical analysis using the standard method^.'^,'^ The oxygen content was calculated by difference. The obtained results were in good agreement with the proposed formulas. Anal. Found (calcd) for KzGeTeO6: K, 20.7 (20.89); Ge, 19.0 (19.39); Te, 34.9 (34.08). Found (calcd) for Rb2GeTeO6: Rb, 36.3 (36.59); Ge, 15.1 (15.54); Te, 27.8 (27.32). Found (calcd) for Cs2GeTeO6: Cs, 46.8 (47.30); Ge, 12.7 (12.92); Te, 23.1 (22.70). Thermogravimetric (TG) and differential thermogravimetric (DTG) curves were determined with a TG 50 microbalance attached to a Mettler TA 3000 analyzer, equipped with a TC IO processor unit. The IR absorption spectra were recorded with a Nicolet 60 SX spectrophotometer, and all the samples were compressed in KBr (4000-400 cm-I) and polyethylene (400-140 c d ) pellets. The d interplanar spacing measurements were made at room temperature in a Siemens Kristalloflex D-500 diffractometer, using Cu K a radiation and employing W ( a = 3.16524 A) as internal standard. The unit-cell parameters were refinedr5from the 20 values of the last nine reflections. X-ray diffraction intensities Po were collected by step scanning from I O to 154O in 28 with increments of 0.02O in 20 and a counting time of 3 s each step, the goniometer being controlled by a DACO-MP
't Universidad Instituto de Ciencia de Materiales. Complutense. 0020-1669/89/1328-1701$01,50/0
V2 computer, which carried out the integration of the diffraction peaks and the background correction. As the graphite monochromator is not able to resolve K a l / K a 2 , intensities were integrated over the whole reflection peak (Ka, Ka2) even for the high-angle reflections in which both peaks were clearly not overlapped. The intensities were calculated with the program L a z y - P u l ~ e r i x ' ~using * ~ ~scattering factors for neutral atoms, Debye-Waller, Lorentz, and polarization factors, and correction for anomalous dispersion. The arbitrarily chosen Debye-Waller factors, Bj, for calculation of the intensities were 0.32 and 0.80 A2 for Cs and 0 atoms respectively. The values for K (0.71 A) Ge (0.62 %.)R b (0.51 A) and Te (0.34 AZ)were obtained from those of Cs and 0 by interpolation of the corresponding atomic masses. The discrepancy factors between observed and calculated intensities were computed from R = C([I,'12- I c ' 1 2 ] ) / ~ I o ' fIo2 , = KIt0, and K = ~ I c / ~ I(Po ' o= measured intensities, I, = corrected measured intensities, and I , = calculated intensities). Runs of the program were performed in a first approach with step increments of 0.01 for the values of x and u positional parameters and, later, with increments of 0.001. The x and u values (coordinates for M and 0 atoms, respectively) taken for each compound were those that gave the smallest R values. Densities were determined by a pycnometric method (estimated error less than 0.2%) with CC1, as immersion liquid.
+
Results and Discussion Chemical analysis results support t h e proposed stoichiometry M2(GeTe)O6for all t h e phases (M = K, Rb, Cs). The crystallographic study of these materials was performed by means of X-ray powder diffraction patterns a n d d spacings, I, and I, values, assignments, and cell parameters a r e listed in Table I. From these results, t h e experimental interplanar spacings could be indexed as a face-centered cubic cell (Fd3m) with crystallographic pa-
O'Keeffe, 0.;Hyde, B. G. Philos. Trans. R . SOC.London 1980, 295, 553. Horowitz, H. S.; Longo, J. M.; Lewandowski, J. T. Mater. Res. Bull. 1981, 16, 489. Babel, D.; Pansewang, G.; Viebahn, W. Z . Naturforsch. 1967, 22E, 1219. Sleight, A. W. Mater. Res. Bull. 1969, 4, 377. Babel, D.; Pansewang, G.; Viebahn, W. Z . Anorg. Allg. Chem. 1972, 387, 161. Sleight, A. W.; Gulley, J. E.; Berzius, T. Ado. Chem. Ser. 1977, No. 163.
Garcia-Martin, S . ; Veiga, M. L.; Gaitin, M.; Jerez, A.; Santamaria, J.; Piso, C. Solid State Ionics 1988, 27, 195. Garcia-Martin, S . ; Veiga, M. L.; Iborra, E.; Jerez, A.; Pico, C. Mater. Res. Bull. 1988, 23, 1107. Shannon, R . D.; Sleight, A. W. Inorg. Chem. 1968, 7, 1649. Bocquillon, G.; Mongrion, J.; Loriers, J. C.R. Seances Acad. Sci. 1978, 287C. 5 .
Bocquillon, G.; Padion, J. Mater. Res. Bull. 1980, 15, 1069. Subramanian, M. A,; Aravamudam, G.;Subbarao, G. V. Prog. Solid State Chem. 1983, 15, 5 5 . Kodama, K. Methods of Quantitative Inorganic Analysis; Interscience: New York, 1963. Amarilla, J. M. Tesis de Licenciatura, Universidad Complutense, Madrid, Spain, 1987. Jerez, A. Programas de Cilculo (Rayos X), Madrid, Spain, 1988. Yvon, R.; Jeitschko, W.; ParthC, E. J . Appl. Crystallogr. 1977, 10, 73. Martinez-Ripoll, M.; Foces, C.; Cano, F. H. I. Rocasolano (CSIC), Madrid, Spain, 1987.
0 1989 American Chemical Society
1702 Inorganic Chemistry, Vol. 28, No. 9, 1989
Amarilla et al.
Table 1. X-ray Diffraction Data for M2(GeTe)06 (M = K, Rb, Cs)
M = Rb
M=K
hkl Ill 220 31 1 222 400 331 51 1 333 440 531 533 622 444 55 1 71 1 553 73 1 800 75 1 555 662 840 842 93 1 844
dcxp. A 5.73 3.51 2.99 2.863 2.480 2.275 I .909
IO
747 21 969 636 89 57 214
IC
760 20 1000
650 90 50 200
M = Cs
480 30 15 260
dcxp A 5.78 3.54 3.019 2.890 2.503 2.297 1.927
82 238 975 361 35 16 268
314 50 89 150 19 35
320 50 91 150 20 35
1.770 1.692 1.527 1.509 1.445 1.402
27 1 50 96 94 15 27
275 50 95 90 14 20
dexp, ‘4 5.47 3.52 2.999 2.871 2.487 2.282 1.914
235 1 I6 963 47 1 35 17 252
IO
IC
200 100 1000
A
v,A’
d,,,, Mg m-’ dexp,Mg m-’ space group
z
u(48f)“ x(32e)‘ R
IC
80 200 1000
370 30 15 270
1.753 1.676 1.512 1.495 1.43 1 1.389
274
275
111
100
76 167 31 63
75 170 30 60
1.758 1.681 1.517 1.499 1.436 1.393
1.291
95
95
1.295
139
140
1.303
119
120
1.240 1.145
36 32
31 30
1.243
...
41 ...
40 5
1.252
27
...
...
20 5
1.138 1.109 1.082 1.040 1.012
41 33 13 24 22
40 30 14 22 20
1.141 1.112
45 24
45 20
1.148 1.119
50 20
50 20
1.050 1.022
34 39
30 40
...
...
1
1.043 1.015
38 40
40 40
Table 11. Crystallographic Results from X-ray Diffraction Data for M2(GeTe)06 a,
IO
M=K 9.917 ( I ) 975.39 (0) 4.457 4.40 Fd3m 8 0.424 0.101 0.022
M = Rb 9.946 (1) 983.98 (1) 5.044 4.98 Fd3m 8 0.426 0.113 0.039
M = Cs 10.012 (1) 1003.62 (0) 5.573 5.56 Fd3m 8 0.433 0.120 0.042
u = coordinate for 0 atom: x = coordinate for M atom.
x ...
0’
...
0’
0’)
’
0111
1
0”
O l l
o l l l
Figure 2. Coordination polyhedron around M (=K, Rb, Cs) atoms. Table 111. Interatomic Distances (in A) and Angles (in deg), Calculated from the Experimental u Values, for the Coordination Polyhedra around B (=Ge, Te) Atoms
M=K
n’
’
Figure 1. Coordination polyhedron around B (=Ge, Te) atoms.
rameters of about 9.9-10.0 8, (Table 11). Peak intensities, when measured, a n d the systematic absences agree with a pyrochlore structure.. N o n e of the X-ray diffraction patterns showed superlattice reflections, and all the observed reflections were assigned according to t h e symmetry a n d cell parameters cited above. T h e lowest discrepancy factors R were obtained for M = (K, R b , Cs) in t h e 32(e) positions (origin a t center 3m);Ge a n d Te are randomly distributed in 16(d) and oxygen atoms in 48(f). The observed interatomic distances a r e comparable with the sums of t h e radii of Shannon.ls Figures 1 a n d 2 show t h e coordination polyhedra around the metal atoms (B and M) on the basis of the suggested structure, and the respective interatomic distances and angles a r e gathered in Tables I11 a n d IV. B atoms ( G e or T e ) a r e in a slightly distorted octahedral coordination, which becomes more regular when t h e radius of the
M=Rb
M=Cs
B-0’ B-0” B-0 (radii sums)
Distances 1.91 1.91 1.91
1.91 1.91 1.91
1.89 1.89 1.91
Oi-B-Oi Oi-B-Oii Oii-B-Oii
Angles 95.2 180 84.8
94.5 180 85.5
91.8 180 88.2
Table IV. Interatomic Distances (in A) and Angles (in deg), Calculated from the Experimental x and u Values, for the Coordination Polyhedra around M (=K, Rb, Cs) Atoms
M-0‘ M-0“ M-Oiii M-0 (radii sums)
0i-M-O’ Oi-M-Oii Oi-M-Oi” Oii-M-Oii Oii-M-Oiii
M=K Distances 3.22 2.75 3.25 2.74 Angles 81.2 88.9 167 83.1 136 99.5 49.9 110
(18) Shannon, R .
D.Acta Crysrallogr. 1976, A32, 751.
OLii-M-Oiii
80.3
M=Rb
M=Cs
3.12 2.88 (3.41) 2.88
3.14 3.03 (3.53) 3.03
85.5 89.7 174
88.2 90.0
...
...
94.1
91.9
...
...
...
177
...
Inorganic Chemistry, Vol. 28, No. 9, 1989 1703
M2(GeTe)06 Table V. IR Spectroscopic Data (cm-') for M2B206(M = K, Rb, Cs; B = Ge, Te)" vI(B-0)
v,(M-O)
745 740 720
620 615 580
K,(GeTe)On
0-B-0)
v,(O-M-O), Vs(M-BO6)
460 465 470
420 420 415
~ q (
V6 (
0-M-0) 345
350 355
" All absorptions shown are strong quencies ul, us, and v6 diminish, v4 increases with increasing atomic magnitude, and v3 becomes higher when the a cell parameter diminishes. Figure 3 shows TG and DTG curves for the initial mixtures of Te(OH)6, Ge02, and KN03. When the alkali-metal reagents are RbN03 or CsN03, the behaviors of the thermal decomposition processes for these mixtures are closely similar. All the reactions can be associated with the loss of 3 H 2 0 per formula weight, two molecules being lost near 473 K and the third being progressively eliminated at higher temperatures overlapping with the melting process of metal nitrate (573 K for KN03). At this temperature a significant weight loss is observed, which was attributed to N O + NOz elimination from the reaction media24by several undefined steps. The process finished toward 923 K. The total weight loss measured is equivalent to 1 mol of dinitrogen pentoxide. The successive transformations undergone by the initial mixtures during thermal analysis can be expressed as
AW
I
'
1
loo
I
I
1
300
I
I
500
T T
Figure 3. T G and DTG curves of the initial mixture leading to K(GeTe)O6.
M cation is increased as suggested by the 0-B-0 angles. In all cases the observed B-0 distances are in accord with those calculated from the sums of ionic radii. On the other hand, the coordination polyhedra around the alkali-metal cation (M) can be described as 3 + 3 3 for K (Figure 2); that is, there are three parallel planes of oxygen atoms, two of them (0' and 0")being related to two opposite faces of a distorted-octahedral arrangement and the third set of oxygen atoms (Oiii)being located toward the middle of the edges Oii-Oiiand at longer distances from K than the preceding ones. In Rb and Cs compounds these distances are considerably greater than the sums of respective ionic radii, and this fact suggests that coordination in these cases is reduced to 3 + 3 (Oi and 0"). Again, the polyhedron around Cs is more regular. In Table V are shown the IR absorption bands that are present in the range 745-345 cm-I and their corresponding assignments, according to the previous spectroscopic analysis for the other pyroclore-type compound^.^^-^^ As can be observed, the fre-
+
(19) Vandenborre, M. T.; Husson, E.; Fourquet, J. L. Spectrochim. Acta 1982, 38A, 991.
Te03 + GeO,
+ 2MN03
-NO,-NO1
Mz(GeTe)O6
which are similar to those of other related systems previously described by us.25 On the other hand, the cgmpition of the solid phases, isolated at the end of this reaction, seem to confirm the suggested stoichiometry.
Acknowledgment. We are indebted to Dr. P. Tigeras (CSIC) for the recording of IR spectra. This work was financially supported by the CAICYT (Grant PB 85-0057). Registry No. Te(OH),, 7803-68-1; Ge02, 13 10-53-8; KNO,, 775779-1; R b N 0 3 , 13126-12-0; CsNO,, 7789-18-6; K2GeTe06, 119696-12-7; Rb2GeTe06, 119696-13-8; Cs2GeTe06, 119696-14-9.
(20) Vandenborre, M. T.; Husson, E.; Chatry, J. P.; Michel, D. J . Raman Spectrosc. 1983, 14, 2. (21) Vandenborre, M. T.; Husson, E.; Bresset, H.Spectrochim. Acta 1981, 37A, 113. (22) Murphy, D. W.; Dye, J. L.; Zahurvak, S. M. Inorg. Chem. 1983, 22, 3681. (23) McCanley, R. A. J . Opt. SOC.Am. 1973, 63, 721. (24) Veiga, M. L.; Jerez, A,; Gaitan, M.; Pico, C. Thermochim. Acta 1988, 124, 25. (25) Garcia-Martin, S.;Veiga, M. L.; Gaitin, M.; Jerez, A,; Pico, C. J . Chem. SOC.,Dalton Trans. 1988, 2141.
