- Email: [email protected]
The thallous chalcogenides Tl6X4Y (X = Cl, Br, I; Y = S, Se)
Mat. Res. Bull., Vol. 19. pp. 599-605, 1984. Printed in the USA. 0025-5408/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.
THE THALLOUS CHALCOGENIDES TI6X4Y
(X = Cl, Br, I ; Y = S, Se)
Roger Blachnik Anorganische Chemie, F.B. Biologie-Chemie U n i v e r s i t ~ t OsnabrUck, P.O. Box 4469, D-4500 OsnabrUck, F.R.G. Henning Arthur Dreisbach Anorganische Chemie, U n i v e r s i t ~ t Siegen Siegen, F.R.G. Josef Pelzl I n s t i t u t f~r Experimentalphysik Ruhr-Universit~t Bochum, F.R.G. (Received February 14, 1984; Communicated by A. Rabenau)
ABSTRACT Single c r y s t a l s of TI6X~Y(X = Cl, Br, I ; Y = S, Se) were prepared by the Bridgman technique. The compounds were characterized by complete X-ray s t r u c t u r a l analyses. A l l are i s o t y p i c , space group P4/mnc, with a = 843.3(2) pm, c = 917.2(2) pm f o r TI~C14S, a = 872.1(2) pm, c = 932.8(I) pm f o r TI6Br4S, a = 917.6(3) pm, c = 960.8(I) pm f o r T1614S and a = 917.8(3) pm, c = 967.5(I) pm f o r TI61~Se. FIR spectra as well as photoacoustic spectra are given. INTRODUCTION Recently we reported ( I ) the preparation and c r y s t a l structure of the compound T I 6 C I ~ . The work was part of an i n v e s t i g a t i o n the phase diagrams of the series TIX-TI2Y (X = CI, Br or I and Y = S, Se or Te). Single c r y s t a l s of several other compounds of the type TI~X4Y could by prepared. Some of t h e i r properties are reported in t h i s paper. EXPERIMENTAL AND RESULTS The compounds were prepared by melting a stoichiometric mixture of the binary components in s i l i c a ampouls and annealing f o r one month some degrees below the melting resp. p e r i t e c t i c temperature. Single c r y s t a l s of the compounds were obtained by the Bridgman technique from ingots, homogenized in t h i s way. The compounds c r y s t a l l i z e in a tetragonal l a t t i c e with the space group P4/mnc. The c e l l constants, obtained by least-square calculations from measurements on a Huber-Guinier camera ( r a d i a t i o n CrK~I), are collected in table I, the observed f i r s t 20 r e f l e c t i o n s in table 2. 599
R. BLACHNIK, et al.
600
Vol. 19, No. 5
TABLE 1 Cell Constants of Compounds TI6X4Y (293 K)
T16C14S TIGBr4S T1614S Tl614Se
a[pm]
c[pm]
843,3(2) 872,1(2) 917,6(3) 917,8(3)
917.2(2) 932.8(1) 960.8(1) 967.5(1)
c/a 1.09 1.07 1.05 1.05
Table 2 contains the atomic parameters with t h e i r standard deviations of T1614S, A l i s t of the observed and calculated structure factors is a v a i l a ble from the authors, TABLE 2 Atomic Parameters of T1614S(*) TI(I) x y z
0,5 0,5 0,2994(2)
i l22 l 33 12 13 23
34(1) 34(I ) 17(I) 0 0 0
TI(2)
I
S
0.3926(3) 0.1900(3) 0 23(1) 24(1) 255(9) 1(1) 0 0
0,1625(7) 0.3375 0,25 24(I) 24(I) 37(2) 3(2) 13(1) 13(1)
0,5 0,5 0 15(7) 15(7) 10(10) 0 0 0
*The Uij as multiples in pm2 x 1 0 -I refer to r 2 2 z3 z3 U, T = exp ~ x i=lj=1 ~ i ' j ) h i
hjla~ Ila: I] TABLE 3
Relevant Data of the Structure Determinations
TIrCI,S TI.DI.~ b T1614Se
cry.sta~ size [ mm]
independent I _>2~ i reflections
O,IxO,15xO.1 O. 15xO.15xO.1 O. IxO. IxO. 15
1578 1303 2126
675 420 985
R 9,8 % 7.3 % 12.9 %
I n t e n s i t y data were collected on a f o u r - c i r c l e CAD-4 (Enraf-Nonius) automatic diffractometer by the program system SDP (Enraf-Nonius) with Mo-Km: r a d i a t i o n , As example for the four structure determinations the data of T1614S
Vol.
19, N o .
5
THALLOUS CHALCOGENIDES
601
FIR i I .,"
X
+,"
II
/"
&&%% t%11%1~ii %'1~!! !:"~ ~/
'.. :..
II
":.
/ FIG. I
211%
/: xt " ' 'i
FIR-spectra of TI6X~S (X = CI, Br, I ) TI6CIz S
| i)
:
/:i:; I
L
120
71 cm -i
80
FIR
;
••
C
/
/
',,
#
I
jJJ Pf
I
/
i%
~ sss
f
P
Ei
t j~
FIR spectra of TI61~Y (Y = S, Se)
I
,2,;, :
+,
T[gJ~S
i:
Ivl I
2~o
26o
FIG. 2
7~
160 8'0
LI[}
Cm+1
602
R. B L A C H N I K , e t al .
Vol. 19, No. 5
are given. ~ t o t a l of 1303 nonsymmetry-related r e f l e c t i o n s were recorded (Om~v = 40~). The structure calculation was carried out using r e f l e c ti~M~ with I ~2 .. I n t e n s i t y values were corrected f o r absorption. The s t r u c t u r a l parameters were refined, using anisotropic temperature f a c t o r s , the residual f a c t o r was R = 0.073. The FIR absorption spectra ( f i g . I and 2) were recorded on a Bruker Fourierspectrometer (IFS 114), using Nujol mulls on pol~ethylene discs. Th raman spectr A showed only one line (TIGCl 4S 92 cm -, TI GBr4S 81 cm-~ T I G I ~ 71 cm-'). The optical absorption in the near infrared and the v i s i b l e region was investigated by photoacoustic spectroscopy (2). The home-made microcomputer controlled spectrometer operates with a high pressure xenon lamp, a grating monochromator (0.3 m focal length) and a variable temperature photoacoustic cell supplied with a condensor microphone. Fig. 3 shows photoacoustic spectra recorded from powdered samples at room temperature. The spectral s l i d width was t y p i c a l l y 8 nm. The normalized signal amplitudes, which are plotted against the photon energy are proportional to the absorption coefficient. The steep increase of the amplitudes in the range from 1.5 eV to 2.3 eV is correlated with the onset of the band gap. The variation of the gap energy with chemical composition as seen from the graph is in accordance with the concept of Goodman (3). In the case of substitutional derivation (within an isotypic series), the energy gap should be determined by the bond length and the electronegativity difference. An increase in bond length leads to a decrease in the energy gap, an increase in the electronegativity difference has the opposite effect. The e l e c t r o n e g a t i v i t y differences (Allred-Rochow) decrease in the series T I - C I > T I - B r > T I - S e = T I - S > T I - I . The weakest bond is determing the energy gap, that is the TI-S bond in the compounds TI6CI4S and TI6Br4S. Both have thus nearly i d e n t i c a l band gap values. In the compounds T l 6 1 ~ and TIGI4Se the e l e c t r o n e g a t i v i t y differences are nearly i d e n t i c a l . The gap is determined only by the bond length, longer in TIGI4Se, which thus has a lower band gap than TI61 ~ . DESCRIPTION OF THE STRUCTURE The structures of the type TI6X4Y ( f i g . 4) are b u i l t up of 32434 nets of atoms, which were also found in the compounds T15Y2X (4). Successive layers are not placed exactly above each other, but every second layer is translated approximately half of the base diagonal of the unit c e l l . With this stacking - A'TI(2) , A(X), A'TI(2) , A(X) - the layers f i t better in each other. The Yions center one half of the squares of the TI(2) nets, in such a way that a Tl(2) net, having centered squares over the center of the base of the unit cell is followed by a TI(2) net with centered squares over the base cell corners.
The nets of the halide ions at z = 0.25 and z = 0.75 are approximately centered by a 4 4 net of TI(1)atoms at z = 0.~95 and z = 0.305 resp. z : 0.695
Vol.
19, N o .
3 r~
5
THALLOUS
603
~
0.6
E
._. ~ o
CHALCOGENIDES
04
0
f~ no o 2
J4S ~__~TI6'CI4S ~TI6J4Se ~TI6BraS
•TI6
N
E 07
11
15
19
7
31
5
photon energy (ev)
FIG. 3 Photoacoustic spectra of TI6X4Y
FIG. 4 Unit c e l l of TI6X4Y
39
604
R. B L A C H N I K , e t al.
Vol. 19, No. 5
and z = 0.805. The sequence of layers is thus A'I/2AIA' I ~ A I . This stacking leads to isolated STI(2) T I ( I ) octahedra and ~ o - c l T s ~ r T ~ TI(1)X 4S ~-octahedra. As can be seen from f i g . 4 the most obvious place f o r the lone pairs of the Tl-ions is the free space in the middle of the unit c e l l . The lone pair is responsible f o r the d i s t o r t i o n of the ~-octahedra, surrounding the TI(1), which moves up (down) from z = I/4 (3/4) above (below) the plane through the four halide ion neighbours. The compounds c r y s t a l l i z e in the a n t i - T l 4 H g l 6 t~pe (5), i . e . the p o s i t i o n o [ the anions +in TI4HglG are occupied by the TI and those of the cations TI- resp. Hg by the anions I- resp. S -. The A4BIG-type is a v a r i a t i o n of the K2PtCI4 s t r u c t u r e , when the A-cations are much smaller than the anions and the B-cation is of lower valency than Pt (6). The relevant distances of the TI6X~ type are collected in table 3.
TABLE 3 Bond Distances in T16X4Y (pm)
d TI(1)-X TI(1)-Y TI(2)-X TI(2)-Y in TIX in TIX TI-Y
TIGCl 4S 319.1 279.9 323.3/331.1 292.3 332.5(CSCI) 315 (NaCI) 286
TIGBr 4S 330.0 284.3 332.9/335.9 296.0 343.6(CSCI) 329 (NaCl) 286
TI GI4Se 347.7 295.4 348.4/349.4 307.8 336 B-TII 347 (NaC1) 280/322
TI61 4S 347.0 287.6 347.3/345.9 301.0
The T l ( 1 ) - h a l i d e distances are similar to those of the thallous halides with NaCl-structure, whereas the T l ( 2 ) - h a l i d e distances are much larger and l i k e those in CsCl-type thallous halides. The same r e l a t i o n s are found in the TI-S distances (TI(1)-S = 280 pm; TI(2)-S = 295 pm), which i n d i cates that the bonds to the TI(1) atoms have a more covalent character than those to the TI(2) atoms. The FIR spectra of the TI6X~ compounds are composed of two parts, one is very s i m i l a r to the spectra of the pure halides (Tl(2) p a r t ) , lying between 60 and 120 cm-1. The IR-active vibrations of the thallous halides with CsCl structure are 80 cm-1 ( T l C l ) , 59 cm- I (TIBr) and 52 cm - 1 (TII) (7). Due to the introduction of the l i g h t e r chalcogen atoms in the TI6X~ compounds these frequencies are increased. A d d i t i o n a l l y the lines are s p l i t t e d by the d i s t o r t i o n of the CsCl-structure. tions of
The remaining part of the spectra is generated by the internal vibraisolated Tl6Y-octahedra with D4h symmetry. These species have f i v e
IR-active vibrations, of which only the t~b valence vibrations can be observed in the investigated frequency range. The frequency of the TI-S vibration in-
Vol. 19, No. 5
T H A L L O UCHALCOGENIDES S
605
creases, as expected, with decreasing atomic weight of the halide component to an extrapolated value of 193 cm-' f o r unsubstituted TIzS, which is in good agreement with a value of 200 ± I0 cm-1 f o r gaseous TIzS, reported by Strevelkov. This assumption is also supported by the TI-Se frequencies in TI61 ~e of 128 and 116 cm-1, which correspond with values of 169 cm-i reported by Zirke e.a. (9) for the valence vibration of a tetrahedral TiSe unit in amorphous Ti-Se alloys. The compounds T16C14S, TI6Br4S and T1614S, resp. TI61~S and T l 6 1 ~ e form complete series of solid solutions in t h e i r possible binary combinations. The l a t t i c e constants a and c of these solid solutions obey s t r i c t l y the Vegards r u l e . No super structures were found in samples annealed in the temperature interval between 550 and 670 K. In the compound Tl61~e the iodine atoms can be substituted by bromine up to the l i m i t i n g formula Tl612BrzSe (a = 895.7(3) pm, c = 952.2(2) pm). ACKNOWLEDGEMENTS The authors would l i k e to thank for the support of the Minister for Science and Research (NW) and the Fonds der Chemischen Industrie and Dr. Haeuseler for his helpful comments. REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9.
R. Blachnik and H.A. Dreisbach, Z. Naturforsch. 36b, 1500 (1981) R.B. Somoano, Angew. Chem. 90, 250 (1978) C.H.L. Goodman, J. Phys. Ch~. Solids 6, 305 (1958) R. Blachnik and H.A. Dreisbach, J. SolTd State Chemo, in press K. Brodersen, G. Thiele and G. G6rz, Z. anorg, a l l g . Chem. 401, 217 (1973) H.J. Berthold, D. Haas, R. Tamme, K. Brodersen, K.P. Jensen, D. Messer and G. Thiele, Z. anorg, a l l g . Chem. 456, 29 (1979) D.E. McCarthy, Appl. Optics , 2539 (I~7-f) V.F. Strevelkov, Yu. S. Rjabov and A.A. Maltsev, Vestnik Mosk. Univ. Khimiya 13, 645 (1972) J. ZirkeT-C. Dromer, A. Tausend and D. Wobig, J. Non-Cryst. Solids 24, 283 (1977)
THE THALLOUS CHALCOGENIDES TI6X4Y
(X = Cl, Br, I ; Y = S, Se)
Roger Blachnik Anorganische Chemie, F.B. Biologie-Chemie U n i v e r s i t ~ t OsnabrUck, P.O. Box 4469, D-4500 OsnabrUck, F.R.G. Henning Arthur Dreisbach Anorganische Chemie, U n i v e r s i t ~ t Siegen Siegen, F.R.G. Josef Pelzl I n s t i t u t f~r Experimentalphysik Ruhr-Universit~t Bochum, F.R.G. (Received February 14, 1984; Communicated by A. Rabenau)
ABSTRACT Single c r y s t a l s of TI6X~Y(X = Cl, Br, I ; Y = S, Se) were prepared by the Bridgman technique. The compounds were characterized by complete X-ray s t r u c t u r a l analyses. A l l are i s o t y p i c , space group P4/mnc, with a = 843.3(2) pm, c = 917.2(2) pm f o r TI~C14S, a = 872.1(2) pm, c = 932.8(I) pm f o r TI6Br4S, a = 917.6(3) pm, c = 960.8(I) pm f o r T1614S and a = 917.8(3) pm, c = 967.5(I) pm f o r TI61~Se. FIR spectra as well as photoacoustic spectra are given. INTRODUCTION Recently we reported ( I ) the preparation and c r y s t a l structure of the compound T I 6 C I ~ . The work was part of an i n v e s t i g a t i o n the phase diagrams of the series TIX-TI2Y (X = CI, Br or I and Y = S, Se or Te). Single c r y s t a l s of several other compounds of the type TI~X4Y could by prepared. Some of t h e i r properties are reported in t h i s paper. EXPERIMENTAL AND RESULTS The compounds were prepared by melting a stoichiometric mixture of the binary components in s i l i c a ampouls and annealing f o r one month some degrees below the melting resp. p e r i t e c t i c temperature. Single c r y s t a l s of the compounds were obtained by the Bridgman technique from ingots, homogenized in t h i s way. The compounds c r y s t a l l i z e in a tetragonal l a t t i c e with the space group P4/mnc. The c e l l constants, obtained by least-square calculations from measurements on a Huber-Guinier camera ( r a d i a t i o n CrK~I), are collected in table I, the observed f i r s t 20 r e f l e c t i o n s in table 2. 599
R. BLACHNIK, et al.
600
Vol. 19, No. 5
TABLE 1 Cell Constants of Compounds TI6X4Y (293 K)
T16C14S TIGBr4S T1614S Tl614Se
a[pm]
c[pm]
843,3(2) 872,1(2) 917,6(3) 917,8(3)
917.2(2) 932.8(1) 960.8(1) 967.5(1)
c/a 1.09 1.07 1.05 1.05
Table 2 contains the atomic parameters with t h e i r standard deviations of T1614S, A l i s t of the observed and calculated structure factors is a v a i l a ble from the authors, TABLE 2 Atomic Parameters of T1614S(*) TI(I) x y z
0,5 0,5 0,2994(2)
i l22 l 33 12 13 23
34(1) 34(I ) 17(I) 0 0 0
TI(2)
I
S
0.3926(3) 0.1900(3) 0 23(1) 24(1) 255(9) 1(1) 0 0
0,1625(7) 0.3375 0,25 24(I) 24(I) 37(2) 3(2) 13(1) 13(1)
0,5 0,5 0 15(7) 15(7) 10(10) 0 0 0
*The Uij as multiples in pm2 x 1 0 -I refer to r 2 2 z3 z3 U, T = exp ~ x i=lj=1 ~ i ' j ) h i
hjla~ Ila: I] TABLE 3
Relevant Data of the Structure Determinations
TIrCI,S TI.DI.~ b T1614Se
cry.sta~ size [ mm]
independent I _>2~ i reflections
O,IxO,15xO.1 O. 15xO.15xO.1 O. IxO. IxO. 15
1578 1303 2126
675 420 985
R 9,8 % 7.3 % 12.9 %
I n t e n s i t y data were collected on a f o u r - c i r c l e CAD-4 (Enraf-Nonius) automatic diffractometer by the program system SDP (Enraf-Nonius) with Mo-Km: r a d i a t i o n , As example for the four structure determinations the data of T1614S
Vol.
19, N o .
5
THALLOUS CHALCOGENIDES
601
FIR i I .,"
X
+,"
II
/"
&&%% t%11%1~ii %'1~!! !:"~ ~/
'.. :..
II
":.
/ FIG. I
211%
/: xt " ' 'i
FIR-spectra of TI6X~S (X = CI, Br, I ) TI6CIz S
| i)
:
/:i:; I
L
120
71 cm -i
80
FIR
;
••
C
/
/
',,
#
I
jJJ Pf
I
/
i%
~ sss
f
P
Ei
t j~
FIR spectra of TI61~Y (Y = S, Se)
I
,2,;, :
+,
T[gJ~S
i:
Ivl I
2~o
26o
FIG. 2
7~
160 8'0
LI[}
Cm+1
602
R. B L A C H N I K , e t al .
Vol. 19, No. 5
are given. ~ t o t a l of 1303 nonsymmetry-related r e f l e c t i o n s were recorded (Om~v = 40~). The structure calculation was carried out using r e f l e c ti~M~ with I ~2 .. I n t e n s i t y values were corrected f o r absorption. The s t r u c t u r a l parameters were refined, using anisotropic temperature f a c t o r s , the residual f a c t o r was R = 0.073. The FIR absorption spectra ( f i g . I and 2) were recorded on a Bruker Fourierspectrometer (IFS 114), using Nujol mulls on pol~ethylene discs. Th raman spectr A showed only one line (TIGCl 4S 92 cm -, TI GBr4S 81 cm-~ T I G I ~ 71 cm-'). The optical absorption in the near infrared and the v i s i b l e region was investigated by photoacoustic spectroscopy (2). The home-made microcomputer controlled spectrometer operates with a high pressure xenon lamp, a grating monochromator (0.3 m focal length) and a variable temperature photoacoustic cell supplied with a condensor microphone. Fig. 3 shows photoacoustic spectra recorded from powdered samples at room temperature. The spectral s l i d width was t y p i c a l l y 8 nm. The normalized signal amplitudes, which are plotted against the photon energy are proportional to the absorption coefficient. The steep increase of the amplitudes in the range from 1.5 eV to 2.3 eV is correlated with the onset of the band gap. The variation of the gap energy with chemical composition as seen from the graph is in accordance with the concept of Goodman (3). In the case of substitutional derivation (within an isotypic series), the energy gap should be determined by the bond length and the electronegativity difference. An increase in bond length leads to a decrease in the energy gap, an increase in the electronegativity difference has the opposite effect. The e l e c t r o n e g a t i v i t y differences (Allred-Rochow) decrease in the series T I - C I > T I - B r > T I - S e = T I - S > T I - I . The weakest bond is determing the energy gap, that is the TI-S bond in the compounds TI6CI4S and TI6Br4S. Both have thus nearly i d e n t i c a l band gap values. In the compounds T l 6 1 ~ and TIGI4Se the e l e c t r o n e g a t i v i t y differences are nearly i d e n t i c a l . The gap is determined only by the bond length, longer in TIGI4Se, which thus has a lower band gap than TI61 ~ . DESCRIPTION OF THE STRUCTURE The structures of the type TI6X4Y ( f i g . 4) are b u i l t up of 32434 nets of atoms, which were also found in the compounds T15Y2X (4). Successive layers are not placed exactly above each other, but every second layer is translated approximately half of the base diagonal of the unit c e l l . With this stacking - A'TI(2) , A(X), A'TI(2) , A(X) - the layers f i t better in each other. The Yions center one half of the squares of the TI(2) nets, in such a way that a Tl(2) net, having centered squares over the center of the base of the unit cell is followed by a TI(2) net with centered squares over the base cell corners.
The nets of the halide ions at z = 0.25 and z = 0.75 are approximately centered by a 4 4 net of TI(1)atoms at z = 0.~95 and z = 0.305 resp. z : 0.695
Vol.
19, N o .
3 r~
5
THALLOUS
603
~
0.6
E
._. ~ o
CHALCOGENIDES
04
0
f~ no o 2
J4S ~__~TI6'CI4S ~TI6J4Se ~TI6BraS
•TI6
N
E 07
11
15
19
7
31
5
photon energy (ev)
FIG. 3 Photoacoustic spectra of TI6X4Y
FIG. 4 Unit c e l l of TI6X4Y
39
604
R. B L A C H N I K , e t al.
Vol. 19, No. 5
and z = 0.805. The sequence of layers is thus A'I/2AIA' I ~ A I . This stacking leads to isolated STI(2) T I ( I ) octahedra and ~ o - c l T s ~ r T ~ TI(1)X 4S ~-octahedra. As can be seen from f i g . 4 the most obvious place f o r the lone pairs of the Tl-ions is the free space in the middle of the unit c e l l . The lone pair is responsible f o r the d i s t o r t i o n of the ~-octahedra, surrounding the TI(1), which moves up (down) from z = I/4 (3/4) above (below) the plane through the four halide ion neighbours. The compounds c r y s t a l l i z e in the a n t i - T l 4 H g l 6 t~pe (5), i . e . the p o s i t i o n o [ the anions +in TI4HglG are occupied by the TI and those of the cations TI- resp. Hg by the anions I- resp. S -. The A4BIG-type is a v a r i a t i o n of the K2PtCI4 s t r u c t u r e , when the A-cations are much smaller than the anions and the B-cation is of lower valency than Pt (6). The relevant distances of the TI6X~ type are collected in table 3.
TABLE 3 Bond Distances in T16X4Y (pm)
d TI(1)-X TI(1)-Y TI(2)-X TI(2)-Y in TIX in TIX TI-Y
TIGCl 4S 319.1 279.9 323.3/331.1 292.3 332.5(CSCI) 315 (NaCI) 286
TIGBr 4S 330.0 284.3 332.9/335.9 296.0 343.6(CSCI) 329 (NaCl) 286
TI GI4Se 347.7 295.4 348.4/349.4 307.8 336 B-TII 347 (NaC1) 280/322
TI61 4S 347.0 287.6 347.3/345.9 301.0
The T l ( 1 ) - h a l i d e distances are similar to those of the thallous halides with NaCl-structure, whereas the T l ( 2 ) - h a l i d e distances are much larger and l i k e those in CsCl-type thallous halides. The same r e l a t i o n s are found in the TI-S distances (TI(1)-S = 280 pm; TI(2)-S = 295 pm), which i n d i cates that the bonds to the TI(1) atoms have a more covalent character than those to the TI(2) atoms. The FIR spectra of the TI6X~ compounds are composed of two parts, one is very s i m i l a r to the spectra of the pure halides (Tl(2) p a r t ) , lying between 60 and 120 cm-1. The IR-active vibrations of the thallous halides with CsCl structure are 80 cm-1 ( T l C l ) , 59 cm- I (TIBr) and 52 cm - 1 (TII) (7). Due to the introduction of the l i g h t e r chalcogen atoms in the TI6X~ compounds these frequencies are increased. A d d i t i o n a l l y the lines are s p l i t t e d by the d i s t o r t i o n of the CsCl-structure. tions of
The remaining part of the spectra is generated by the internal vibraisolated Tl6Y-octahedra with D4h symmetry. These species have f i v e
IR-active vibrations, of which only the t~b valence vibrations can be observed in the investigated frequency range. The frequency of the TI-S vibration in-
Vol. 19, No. 5
T H A L L O UCHALCOGENIDES S
605
creases, as expected, with decreasing atomic weight of the halide component to an extrapolated value of 193 cm-' f o r unsubstituted TIzS, which is in good agreement with a value of 200 ± I0 cm-1 f o r gaseous TIzS, reported by Strevelkov. This assumption is also supported by the TI-Se frequencies in TI61 ~e of 128 and 116 cm-1, which correspond with values of 169 cm-i reported by Zirke e.a. (9) for the valence vibration of a tetrahedral TiSe unit in amorphous Ti-Se alloys. The compounds T16C14S, TI6Br4S and T1614S, resp. TI61~S and T l 6 1 ~ e form complete series of solid solutions in t h e i r possible binary combinations. The l a t t i c e constants a and c of these solid solutions obey s t r i c t l y the Vegards r u l e . No super structures were found in samples annealed in the temperature interval between 550 and 670 K. In the compound Tl61~e the iodine atoms can be substituted by bromine up to the l i m i t i n g formula Tl612BrzSe (a = 895.7(3) pm, c = 952.2(2) pm). ACKNOWLEDGEMENTS The authors would l i k e to thank for the support of the Minister for Science and Research (NW) and the Fonds der Chemischen Industrie and Dr. Haeuseler for his helpful comments. REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9.
R. Blachnik and H.A. Dreisbach, Z. Naturforsch. 36b, 1500 (1981) R.B. Somoano, Angew. Chem. 90, 250 (1978) C.H.L. Goodman, J. Phys. Ch~. Solids 6, 305 (1958) R. Blachnik and H.A. Dreisbach, J. SolTd State Chemo, in press K. Brodersen, G. Thiele and G. G6rz, Z. anorg, a l l g . Chem. 401, 217 (1973) H.J. Berthold, D. Haas, R. Tamme, K. Brodersen, K.P. Jensen, D. Messer and G. Thiele, Z. anorg, a l l g . Chem. 456, 29 (1979) D.E. McCarthy, Appl. Optics , 2539 (I~7-f) V.F. Strevelkov, Yu. S. Rjabov and A.A. Maltsev, Vestnik Mosk. Univ. Khimiya 13, 645 (1972) J. ZirkeT-C. Dromer, A. Tausend and D. Wobig, J. Non-Cryst. Solids 24, 283 (1977)