Crystal and electronic structure of TlxV6S8: relation between thallium abundance and the thallium environment

Journal of AUoys and Compounds, 178 (1992) 193-204 JAL 5037

193

Crystal and electronic structure of TIxV6Ss: relation b e t w e e n thallium a b u n d a n c e and the thallium e n v i r o n m e n t W. Bensch and J. Koy Institute for Inorganic Chemistry, University of Frankfurt Niederurseler Hang, D-O000 Frankfurt 50, (FRG)

M. Wesemann Fritz Haber Institute, Faradayweg 4, D-IO00 Berlin 33 (FRG) (Received June 28, 1991; in final form August 7, 1991)

Abstract Low-temperature single-crystal X-ray studies of TI~V~Sa ( x = 0.78 and 0.033) and analysis of the anisotropic displacement parameters U , reveal that the TI atoms are statically disordered within the hexagonal channels, even at 100 K. In Tlo.7aV~Sa the average environment of the T1 atoms may be described by a 3 + 6 coordination whereas in Tlo.0aaV6Sa the TI atoms are in a trigonal antiprismatic environment. Core level photoelectron spectroscopy was used to study the changes in the electronic structure which arise from variation of the T1 concentration. The photoemission data reveal that the Tl atoms are not ionic in either compound. The T1 atom in the Tl-poor sample exhibits a lower charge density than that in the Tl-rich compound.

1. I n t r o d u c t i o n

TlxV6Ss crystallizes in the Nb3Se4 structure with the Tl atoms located in the large hexagonal channels [1-3]. The range of homogeneity depends on the preparation method: x = 0 . 5 - 0 . 8 5 for thermal preparation [41 and x = 0.17-0.8 for electrochemical deintercalation or reintercalation [5]. With I2/CHaCN or FeCla/AICla it was claimed that T1 could be completely removed without destroying the crystal structure [6 l- The T1 atoms within the channel exhibit an unusually large atomic displacement component (ADP) U~a at room temperature. This was interpreted as being due to the high mobility of the T1 ions within the channel [5]. A phase transition at around 150-160 K was postulated from the results of X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), differential scanning calorimetry (DSC) and magnetic susceptibility measurements [7, 8]. The present contribution focuses on the following points: at low temperature the possible mobility of TI should be frozen in and the crystallographic site then resolved should be different for TIxVBSa with x = 0 . 7 8 and 0.033. The aim of the structural work is to relate the "ionicity" of the T1 atoms with their coordination within the hexagonal channels and to investigate the order-disorder phase transition at low temperatures.

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194 X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) m e a s u r e m e n t s of the Tl-rich as well as of the Tl-poor samples will provide information about the bonding state o f the T1 atoms.

2. E x p e r i m e n t a l d e t a i l s Single-crystal X-ray studies were p e r f o r m e d on a STOE AED II singlecrystal diffractometer equipped with a low-temperature device (Oxford Instruments). The intensities of a single crystal with dimensions 0.01 x 0 . 0 1 X0.25 m m 3 were collected at four different t e m p e r a t u r e s 293, 200, 170 and 100 K - for the Tl-rich c o m p o u n d with x = 0 . 7 8 and at 293 and 100 K for the Tl-poor sample, at a wavelength A = 0 . 7 1 0 7 / ~ with the 12-scan technique. The intensities were c o r r e c t e d for Lorentz and polarization effects, and a numerical absorption correction was applied. The crystal structures were refined in the space group P63/m. All calculations were p e r f o r m e d using the p r o g r a m SHELXTL Plus [9]. Technical details of the data acquisition are summarized in Table 1. Atomic absorption s p e c t r o m e t r y (AAS) was p e r f o r m e d on a P e r k i n - E l m e r flameless spectrometer. The photoemission experi m ent s were carried out in a L e y b o l d - H e r a e u s ultrahigh-vacuum environment with a base pressure of 5 - 1 0 - ' ° mbar. Wide-scan XPS spectra were obtained with a constant retardation ratio of 4. High-resolution XPS spectra were m e a s u r e d with the analyser at a fixed transmission ener gy o f 50 eV. The binding energies are given relative to Au 4 f ~ (84.0 eV). A clean sample surface was prepared by s u b s eq u en t mechanical cleavage until the surface composition was within 10% o f the bulk stoichiometry.

2.1. P r e p a r a t i o n Tlo.78V6S8 was p r e p a r e d by reacting weighed amounts of V3S4 with T1 metal in an evacuated silica ampoule [7]. A batch of the sample with x = 0.78 was deintercalated at r o o m t e m p e r a t u r e using a solution of I2/CH3CN (20 mg I2 in 40 ml CHsCN). The final composition Tlo.o33V~Ss was reached after a reacting time of 48 h. AAS analysis of the deintercalated T1 confirmed the above composition [10]. TABLE 1 Technical details of the data acquisition for TI0.~sV6S8 and Tlo.033VBSs

T(K)

28~ Umque

Tlo.TsV6Ss

Tlooz~V6S8

293

200

170

100

293

100

55 448 220

65 3464 333

55 1183 220

55 1183 220

55 691 218

55 687 217

195

3. R e s u l t s

3.1. H o m o g e n e i t y r a n g e Attempts to prepare TI~V~Ss with x < 0.7 via a high-temperature reaction of V3S4 with elemental T1 leads in general to inhomogeneous products. The synthesis of h o m o g e n e o u s TI~V6Ss according to the p r o c e d u r e described by Eckert et al. [5 ] was also unsuccessful i f x < 0.7. A sample with the composition TIo.sV6Ss consisting only of black hexagonal-shaped needles was heated in an evacuated silica ampoule for 6 days at 1273 K. The X-ray pow der diffraction pattern of the p r o d u c t reveals that TI~V6Ss was partially d e c o m p o s e d to the binary c o m p o u n d s VaS4 and TleS. The chemical deintercalation of T1 leads to metastable p roduct s with the lowest TI content of x = 0.02. All attempts to completely r em ove the Tl were unsuccessful. This observation is in contrast to an earlier study [6]. 3.2. X - r a y structure d e t e r m i n a t i o n Details of the structure determination as well as the final results are summarized in Table 2. 3.2.1. Tlo.78V6Ss A projection of the crystal structure onto the a l - a 2 plane is displayed in Fig. 1. The main features of the structure are triple chains of face-sharing TABLE 2 Experimental details and refinement results for T10.07sV6Ss and T10.0aaV6Ssa T(K)

Tlo.o7sV6Ss

Tlo.oo3V6S8

293

200

170

100

293

100

a (/~.) c (/~.) V (/~a) c:a /~(mm- ') Rint

9.204(1) 3.3058(5) 242.5 0.359 20.34 1.09

9.198(1) 3.2989(5) 241.7 0.359 20.41 1.56

9.198(1) 3.2977(8) 241.7 0.358 20.41 1.45

9.186(3) 3.291(2) 240.5 0.358 20.51 3.44

9.137(1) 3.3071(3) 239.1 0.362 7.63 2.05

9.106(2) 3.2979(7) 236.8 0.362 7.70 2.23

6 F ( e - /~-a)

0.80 0.68

0.93 -0.86

0.78 -0.94

1.62 -0.94

0.97 - 1.05

0.94 - 1.05

gb y~ Rd wR GOOF T1 U~3 (/?k2)

0.010(1) 0.00058 2.11 2.11 1.12 0.50(2)

0.010(1) 0.00006 2.17 2.00 2.72 0.47(1)

0.010(1) 0.0002 2.12 2.12 1.95 0.45(1)

0.013(1) 0.00004 2.56 2.19 2.04 0.39(1)

0.0025(3) 0.000004 2.95 1.79 1.37 0.04(3)

-0.000001 2.92 2.02 1.06 0.08(8)

~Standard deviations are given in parentheses. bg: extinction coefficient in F * = F c /(1 + O.O02xgxFc2 /sin20) °'28. Cy: coefficient in the weighting scheme w = 1 / ( t r 2 ( F ) + y F 2 ) . dReflections with I > t r ( / ) suppressed. Refinement parameters: 18 for T10.78V~Ss and 19 for Tlo.o3aV~Ss.

196

r

(a)

v

4

Fig. 1. Projection of the crystal structure of Tl=VsSs onto the al-a~ plane (a); the triple chain composed of face-sharing VS6 octahedra. (b) Open circles, V; hatched circles, S. VS8 o ctah ed r a which are c o n n e c t e d via c o m m o n edges leadings to a threedimensional network with large hexagonal channels parallel to the crystallographic c axis. The V atoms are displaced from the centres of the octahedra towards the c o m m o n edges. V-V zigzag chains are f o r m e d with V-V distances of 2 . 8 6 4 ( 2 ) / ~ within and 3.150 ( 1 ) / ~ bet w e en the chains. The Tl atoms are located within the channels. The isotropic refinement of V and S showed the highest difference electron densities at 0,0,0.25 and at 0,0,0. To determine the site occupation f a c t or (SOF) of T1, the following p r o c e d u r e was applied: Tl was fixed at either 0,0,0.25 or 0,0,0.0. The isotropic displacement factor as well as the SOF w ere allowed to vary freely. The isotropic model leads to comparable values o f Uuo and S O F = 0.065(4). The anisotropic refinement with T1 at z - - 0 . 0 leads to an unreasonably high U83 c o m p o n e n t o f 1.39(15). F o r z = 0 . 2 5 U83 r e a c h e d 0.504(16). Refinements with split T1 sites or with T1 at a fixed z coordinate n e a r 0.0 result in higher values o f R, unreasonably high Usa c o m p o n e n t s and distinct difference electron densities located at 0,0,0.25. Models with a freely varying z p a r a m e t e r led

197 g e n e r a l l y t o v a l u e s o f R w h i c h a r e i d e n t i c a l w i t h t h o s e o b t a i n e d f o r T1 in z = 0.25. T h e s e o b s e r v a t i o n s a r e r e f l e c t e d in e l e c t r o n d e n s i t y m a p s o f t h e c h a n n e l ( F i g . 2 ( a ) ) . T h e e l e c t r o n d e n s i t y is s m e a r e d o u t w i t h i n t h e c h a n n e l w i t h maxima located at 0,0,0.25 (48.5/~-3). The difference between the values at the positions z=0.25 a n d z = O . O is s m a l l ( 5 . 2 e - / ~ - s ) .

\

/ /

\ \ P~

,

J

/

\

\

j...

.

(;

/ \

\

/

\

/ /

\\

/ I

__J._

(a)

"/

,, Z='.75

z--' .25

\

/

\ '~.3 \'

/ '/ /

/

/ I

Co)

_

I J,

]

z =-.25

\

l I

C'

I

\ I

~

z=.75

I

I

I J~

Fig. 2. (a) Electron density distribution in Tl0.vsVeSs within the hexagonal channel at RT. Centre of projection: 0,0,0.25. Levels are given in e - . ~ - 8 : - 3 (dashed), 5, 20, 30, 35, 45 and 47. The environment of the T1 atom is shown schematically. Co) Electron density distribution in Tlo.~sVsSs within the hexagonal channel at 100 K. Centre of projection: 0,0,0.25. Levels are given in e - ~ - ~ : - 3 (dashed), 5, 20, 35, 45, 50 and 54. The environment of the T1 atom is shown schematically.

198

The environment of the T1 atoms at z = 0 . 2 5 is quite irregular: three S(1) atoms are at the same height (z--0.25) at a distance of 3.000(1) /~ (trigonal planar coordination) and six S(1) atoms are at 3.425(1) /~. In Tl sulphides like T12S~ or T14S3 the T1--S distances range from 3.04 to 3.49/~ and 2.51 to 3.36 /~ respectively [11, 12]. The coordination of T1 is best described as a 3 + 6 coordination. The lattice parameters show only a slight decrease with decreasing temperature (Table 2), and the c : a ratio remains constant. At 100 K the Usa component of Tl is reduced by only about 20% to 0.394. If the high ADP Usa is due to the large thermal vibration of the T1 atoms it should tend to zero when extrapolated to 0 K. As can be seen from Fig. 3, the value of Uss extrapolated to 0 K exhibits a value clearly different from zero. It should be noted that the U,, of T1 and the U~ components of the V and S atoms behave fairly normally. At 100 K, the electron density map reveals an increased "localization" of the T1 atoms at z = 0.25 (Fig. 2(b)). The increase in this maximum is about 8.5 e - /~-s. This indicates a further preference for z = 0 . 2 5 . The difference between the values at z = 0 . 2 5 and z = O . O is about 6.5 e - /~-a. The distance within the V-V zigzag chains decreases by about 0.02 /~ between room temperature (RT) (V-V= 2.864(2) /~) and 100 K, reaching 2.846(2)/~. The separation between the chains remains constant. Important bond distances are summarized in Table 3. 3.2.2.

Tlo.o33V688

Removal of T1 leads to a reduction of the a axis by about 0.7%, whereas the c axis remains unchanged (Table 1). The lattice is rigid in the c direction, but less constrained in the basal a - b plane. This is reflected by the larger c : a ratio of 0.362. The reason for such a large anisotropic change in the lattice parameters must be the result of electronic effects. The refined lattice parameters are identical with those reported for VsST.a [6]. The lowest T1 content determined with single-crystal X-ray work was x = 0.024, in excellent agreement with the AAS results. 0,55

I

Tl-Ull

0,44

TI

U33

r'-I

c~ .<

0.33

k~

:=0.22 0.11 0

'

0

610

h

i

I

120

, II!

I

180

I~

I

240

,

,

300

Temperature [K3 Fig. 3. Variation with temperature of the anisotropic displacement parameters Uu (~k2) of TI in Tlo.~sV6Ss.

199 TABLE 3 Important bond distances (/~) in Tlo.TsV6Ss and Tlo.o33V6S8a TlovsV6S8

RT

200 K

150 K

100 K

T1-S1 (3 X) -S1 (6 ×) V-V V--S1 V-S1 V-S1 (2 X) V-S2 (2 X) V--Smax-- V--Stain (V-S>

3.000(1) 3.425(1) 3.306(1) 3.150(1) 2.864(2) 2.530(2) 2.359(2) 2.354(1) 2.458(1) 0.176 2.419

2.995(1) 3.419(1) 3.299(1) 3.152(1) 2.855(2) 2.529(2) 2.359(2) 2.352(1) 2.456(1) 0.177 2.417

2.994(1) 3.418(1) 3.298(1) 3.154(1) 2.853(2) 2.529(2) 2.359(2) 2.352(1) 2.456(1) 0.177 2.417

2.988(2) 3.411(1) 3.291(1) 3.150(1) 2.846(2) 2.526(2) 2.356(2) 2.350(2) 2.453(1) 0.176 2.415

Tlo.o33V6Ss

RT

100 K

T1-S1 (3 x ) -S1 (3 x ) V-V

2.996(4) 3.230(11) 3.307(1) 3.127(1) 2.867(2) 2.503(2) 2.362(2) 2.343(1) 2.448(1) 0.160 2.408

2.990(7) 3.198(17) 3.298(1) 3.116(1) 2.858(2) 2.496(2) 2.356(2) 2.338(1) 2.440(1) 0.158 2.401

V-S1 V-S1 V-S1 (2×) V-S1 (2 x) V-S~x - V-Stain

aEstimated standard deviations are given in parentheses. T h e i s o t r o p i c r e f i n e m e n t o f the V6Ss h o s t lattice s h o w e d the h i g h e s t d i f f e r e n c e in e l e c t r o n densities at 0 , 0 , 0 . 1 1 7 . T h e z c o o r d i n a t e of T1 w a s refined to 0 . 1 3 3 ( 9 ) a n d the S O F r e a c h e d 0 . 0 2 8 ( 4 ) . At RT U33 r e a c h e d 0 . 0 4 0 ( 2 9 ) . A t t e m p t s to p l a c e T1 at different z c o o r d i n a t e d r e s u l t e d in an i n c r e a s e b y a f a c t o r o f 16 a n d 9 f o r z = O a n d z = 0 . 2 5 respectively. T h e s e o b s e r v a t i o n s led us to c o n c l u d e t h a t t h e f a v o u r e d z p o s i t i o n o f the T1 a t o m s is a r o u n d 0.13. This lead to a less d i s t o r t e d e n v i r o n m e n t of t h e Tl a t o m s with t h r e e S(1) a t o m s at 2 . 9 9 6 ( 4 ) / ~ a n d a n o t h e r t h r e e S(1) a t o m s at 3 . 2 3 ( 1 ) /~. T h r e e f u r t h e r S(1) a t o m s are at a d i s t a n c e of 3 . 6 0 3 ( 1 ) / ~ . T h e c o o r d i n a t i o n p o l y h e d r o n m a y b e d e s c r i b e d as a d i s t o r t e d t r i g o n a l a n t i p r i s m . T h e d i s t a n c e within t h e V - V zigzag c h a i n s is c o m p a r a b l e with t h e value o b t a i n e d f o r t h e Tl-rich c o m p o u n d . At 100 K t h e d i s t a n c e is r e d u c e d b y 0 . 0 0 9 /~. T h e s e p a r a t i o n b e t w e e n the c h a i n s is significantly r e d u c e d f r o m 3 . 1 2 7 ( 1 ) /~ at RT to 3 . 1 1 6 ( 1 ) /~ at 100 K. As c a n b e s e e n f r o m T a b l e 3, t h e VS6 o c t a h e d r a a r e less d i s t o r t e d c o m p a r e d w i t h t h o s e in Tlo.TsV6Ss. W h e r e a s in t h e Tl-rich c o m p o u n d t h e Uss c o m p o n e n t is r e d u c e d with falling t e m p e r a t u r e , in t h e T l - p o o r c o m p o u n d U3~ i n c r e a s e s b y a f a c t o r o f a b o u t 2, r e a c h i n g 0 . 0 7 5 ( 8 5 ) . Again, t h e Us3 c o m p o n e n t o f T1 i n d i c a t e s a

200

static disorder of the T1 atoms, U,~ of TI and the Uii com ponent s of the o t her atoms exhibit the e x p e c t e d behaviour (Fig. 4). Electron density m aps plotted at RT and at 100 K showed unusual features within the hexagonal channel. Maxima are observed at z = 0.00 and z = 0 . 5 (marked by o p e n arrows in Figs. 5(a) and 5(b) respectively: 1.5 e /~-3 at RT and 2 e - /~-3 at 100 K). Between these two z coordinates two more p r o n o u n c e d maxima (about 3.1 e - /~-3 at RT and 3.7 e - /~-a at 100 K -- mar k ed by asterisks) are observed with z - 0 . 2 5 , but with x and y coordinates clearly shifted from the special value of 0.0. This observation suggests that some of the T1 atoms are shifted towards the S1 atoms of the VS6 octahedra. The refined z coordinate of 0.13 therefore reflects this complex situation.

3.3. X-ray photoelectron spectroscopy 3.3.1. Tl 4f5/2 a n d 4f7/2 c o r e level spectra The XPS s pect r a of the T1 4f region for TIo.TaV6S8 and T10.o3aV~Sa are displayed in Fig. 6. The binding energies for the different core levels are summarized in Table 4. The binding energy of the T1 4fT/e and 4fs/e core levels for the Tl-rich c o m p o u n d are 118.1 eV and near the value for metallic T1 (117.8 eV [ 13 ]). Reference lines for elemental T1 and TIF are also displayed. The line shapes of the 4f~/2 and 4f~/2 emissions are different from a purely metallic system as a c o n s e q u e n c e of the different core hole couplings in the two systems TI=V6Sa and thallium metal with delocalized valence electrons [141. F o r the Tl-poor c o m p o u n d the T1 4f7~ core level is shifted by about 1.5 eV to higher binding energy, reaching 119.6 eV. Furthermore, the line shape of the c o r e levels is more symmetric, indicating a m ore "i oni c" c h a r a c t e r of the T1 atoms. Comparing the obtained binding energy with ionic model c o m p o u n d s like TlF (4f7/~:122.0 eV) it can be deduced, however, that 0.12 TI-U33 TI-U11

0.096 r--1

(w 0.072 .< ~=0.048 0.024 0

0

i

610

I 120

Temperature

I 180

I 240

300

[K]

Fig. 4. Variation with t e m p e r a t u r e of t h e anisotropic displacement p a r a m e t e r s Uii (J~2) of T1 in Tlo.oa3V6Ss.

201

,

(a)

/ A '",

Z~ ~ 2 5

\

Z:.25

Z=.75

1~

C=

Co)

Z~ .25

Z'.25

Z='.75

1~

Fig. 5. (a) E l e c t r o n d e n s i t y d i s t r i b u t i o n in Tlo.0aaVsS a w i t h i n t h e h e x a g o n a l c h a n n e l at RT. C e n t r e of p r o j e c t i o n : 0 , 0 , 0 . 1 3 . L e v e l s a r e g i v e n in e - / ~ - 3 : - 2 . 5 ( d a s h e d ) , 1, 1.5, 2 a n d 3. T h e o p e n a r r o w s i n d i c a t e e ] e c t r o n d e n s i t y m a x i m a (1.5 e - / ~ - a ) at z = 0 . 0 a n d z = 0.5 respectively. T h e a s t e r i s k i n d i c a t e s a n e l e c t r o n d e n s i t y m a x i m u m (3.1 e - /~-a) at z = 0 . 2 5 b u t w i t h x a n d y c o o r d i n a t e s clearly s h i f t e d f r o m t h e s p e c i a l v a l u e 0. T h e e n v i r o n m e n t o f t h e T1 a t o m is s h o w n s c h e m a t i c a l l y . (b) E l e c t r o n d e n s i t y d i s t r i b u t i o n in T10.0aaVaSa w i t h i n t h e h e x a g o n a l c h a n n e l at 1 0 0 K. C e n t r e o f p r o j e c t i o n : 0, 0, 0 . 1 3 . L e v e l s a r e g i v e n in e - / ~ - a : - 2 . 5 ( d a s h e d ) , 1, 1.5, 2, 3 a n d 3.7. T h e o p e n a r r o w s i n d i c a t e e ] e c t r o n d e n s i t y m a x i m a (2 e - /~-a) a t z = 0 . 0 a n d z = 0.5 r e s p e c t i v e l y . T h e a s t e r i s k i n d i c a t e s a n e l e c t r o n d e n s i t y m a x i m u m ( 3 . 7 e - / ~ - a ) at z = 0.25 b u t w i t h x a n d y c o o r d i n a t e s clearly s h i f t e d f r o m t h e s p e c i a l v a l u e 0. T h e e n v i r o n m e n t o f t h e T1 a t o m is s h o w n s c h e m a t i c a l l y .

202

XPS T[ 4f ,TL+

,T[° .8

24t

IA

-- 1E

O1"~8

124

120

1"i6

1"i2

8.L(eV) Fig. 6. XPS T1 4 f ~ and 4fv~ core-level spectra of T10.TsVsS8 (A) and T10.038V6Ss (B). The lines for elemental T1 and T1+ (T1F) are indicated for comparison. TABLE 4 Core level binding energies (eV) in T10.7sVsS8 and Tlo.033VsS8 Compound

Tlo.vsV6Ss

TIo.o38V6S8

T1 4ds/2 4d~

386.1 406.8

387.5 408.2

T1 4fv~ 4f5~

118.1 122.4

119.6 123.9

V 2pare

513 516.2

516.2 513

O ls

532 534.3

532

the T1 atoms in the ternary compounds still carry only a small charge and are in both cases in a formal oxidation state significantly below + 1. 3.3.2. V 2p3/2 core level s p e c t r a The V 2p8/2 core level region is displayed in Fig. 7. Reference lines for elemental V, VS and for V205 are also shown. In general, the V 2p8~ spectra for both the Tl-rich and Tl-poor sample consist of two components. The more intense line in the Tl-rich compound exhibits a binding energy of 513 eV situated between elemental V and vanadium monosulphide. The less intense core level is located at a binding energy of 516.2 eV, which is near the binding energy of vanadium oxide. In the Tl-poor sample the intensity ratio is reversed. The XPS spectra of both compounds exhibit an O l s signal (not displayed here) with a binding energy of 532 eV, which is far too high for a (vanadium) oxide (530.5 eV), which suggests that it is "molecular" oxygen. Attempts to remove the oxygen by sputter cleaning of the samples were unsuccessful.

203 XPS

V2p

,lOO {¢205

yS

,v

i

i

120' & 80. 40 0

i

520

516

512

508

B.[.(~)

Fig. 7. XPS V 2p3~ core-level spectra for Tlo.TsV6Sa (A) and TIo.o33V6S8 (B). The ~nes for elemental V, VS and V205 are displayed for comparison.

T h e influence o f t h e o x y g e n on the overall e l e c t r o n i c s t r u c t u r e m u s t b e small, b e c a u s e n e i t h e r S 2 p p e a k s typical f o r s u r f a c e s u l p h a t e s or s u l p h i t e s n o r T1 lines ( 4 f or 4d) t y p i c a l f o r T1 o x i d e s are o b s e r v a b l e .

4. D i s c u s s i o n T h e o b s e r v e d t r i g o n a l p l a n a r e n v i r o n m e n t of the T1 a t o m s in the Tl-rich s a m p l e is in a c c o r d a n c e with c o v a l e n t b o n d s b e t w e e n t h e T1 a t o m s a n d the V6Ss h o s t lattice. One w o u l d e x p e c t a n e n v i r o n m e n t with a c o o r d i n a t i o n n u m b e r as h i g h a s p o s s i b l e if t h e T1 a t o m s w e r e s p h e r i c a l ions. If the T1 a t o m s are p l a c e d at a different p o s i t i o n within t h e c h a n n e l t h e r e is a significant i n c r e a s e o f the ADP U33, w h i c h is a n i n d i c a t i o n o f a " w r o n g l y a v e r a g e d " e n v i r o n m e n t . T h e d e c r e a s e in t h e U33 with d e c r e a s i n g t e m p e r a t u r e c o i n c i d e s with a n i n c r e a s e in the e l e c t r o n d e n s i t y at z = 0 . 2 5 . T h e Uii vs. t e m p e r a t u r e c u r v e s clearly d e m o n s t r a t e t h a t only t h e U33 of t h e T1 a t o m do n o t b e h a v e normally. This s u g g e s t s t h a t the T1 a t o m s a r e m a i n l y d i s o r d e r e d in o n e d i m e n s i o n a n d t h e c o r r e l a t i o n b e t w e e n the T1 a t o m s in n e i g h b o u r i n g c h a n n e l s is s m a l l or negligible. If T1 is p r e s e n t a s T1 + or T1s+ o n e w o u l d e x p e c t t h a t t h e r e p u l s i v e f o r c e s b e t w e e n t h e i o n s w o u l d r e s u l t in an o r d e r i n g within t h e c h a n n e l s with a m a x i m u m d i s t a n c e b e t w e e n t h e ions. In t h e T l - p o o r s a m p l e t h e T1 a t o m s c h a n g e t h e i r a v e r a g e d p o s i t i o n within t h e c h a n n e l s . T h e " t r u e " p o s i t i o n is d i s p l a c e d f r o m t h e c e n t r e o f t h e c h a n n e l t o w a r d s the c h a n n e l wall, as c a n b e s e e n in the e l e c t r o n d e n s i t y m a p s . U n f o r t u n a t e l y , all a t t e m p t s to refine t h e T1 in this n e w p o s i t i o n w e r e uns u c c e s s f u l o w i n g to h i g h c o r r e l a t i o n s . T h e T1 e n v i r o n m e n t is b e s t d e s c r i b e d with a d i s t o r t e d t r i g o n a l a n t i p r i s m . T h e i n c r e a s e d c o o r d i n a t i o n n u m b e r o f Tl is in a c c o r d a n c e with the a s s u m p t i o n o f a n i n c r e a s e d ionicity. As in t h e Tl-rich c o m p o u n d , t h e analysis o f t h e A D P s r e v e a l t h a t t h e T1 a t o m s a r e one-dimensionally disordered.

204 The results of the crystal s t r u c t u r e studies are in g o o d a g r e e m e n t with the results o f t h e XPS investigations. The a s y m m e t r y and position o f the c o r e levels s u g g e s t that the i n t e r a c t i o n b e t w e e n T1 a n d the V6S s h o s t lattice is c o v a l e n t in the Tl-rich c o m p o u n d a n d the c h a r g e o f T1 is clearly b e l o w + 1. The shift o f the binding e n e r g y of the 4f level to h i g h e r energies in the T l - p o o r s a m p l e is indicative of a different e n v i r o n m e n t . The c h a n g e of the binding e n e r g y as well as the m o r e s y m m e t r i c s h a p e o f the 4f core level lines d e m o n s t r a t e t h a t the T1 in Tlo.0aaV6S8 is m o r e " i o n i c " , b u t the c h a r g e is still less t h a n + 1. B o t h c o m p o u n d s exhibit an S 2p binding e n e r g y w h i c h is typical for transition m e t a l sulphides.

5. C o n c l u s i o n s The T1 a t o m s are d i s o r d e r e d within the h e x a g o n a l channels. The e x a c t t y p e o f the d i s o r d e r o f the T1 a t o m s c o u l d n o t be d e t e r m i n e d b y the lowt e m p e r a t u r e X-ray studies. The a b o v e - m e n t i o n e d results do n o t e x c l u d e a p h a s e transition at l o w e r t e m p e r a t u r e s as p r o p o s e d in refs. 7, 8. F u r t h e r investigations are in p r o g r e s s to o b t a i n a n insight into the l o w - t e m p e r a t u r e p r o p e r t i e s o f the TlxVsS8 phase. The n a t u r e o f t h e drastic c h a n g e in the V 2p c o r e level s p e c t r u m b e t w e e n the Tl-rich a n d T l - p o o r s a m p l e s w a s stated, b u t it is as yet unclear. The origin of the O l s signal a n d its influence are t h e s u b j e c t s of further e x p e r i m e n t s .

Acknowledgment Helpful d i s c u s s i o n s with Prof. Dr. R. Schl6gl as well as his financial a n d t e c h n i c a l s u p p o r t are gratefully a c k n o w l e d g e d .

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