Vibrational spectroscopic investigations of snse8-n molecules

Journal

of Molecular

Elsevier Scientific

VIBRATIONAL MOLECULES

RISTO

Structure,

Publishing

LAITINEN*

SPECTROSCOPIC

and RALF

Institut fir Anorganische Berlin I2 (Bundesrepublik (Received

30 January

68 (1980) 19-32 Company, Amsterdam

-

Printed

in The Netherlands

INVESTIGATIONS

OF S, Se8 -n

STEUDEL

und Analytische Deutschland)

Chemie,

Technische

UnioersitGt

Berlin,

D-l 000

1980)

ABSTRACT The fundamental vibrations of 13 cyclic S, Se, -n (n = 7-2) molecules have been calculated using a modified Urey-Bradley force field with 9-14 independent force constants whose values have been adapted from those of Se, and S, _ Calculated wavenumbers are compared to those obtained by Raman spectroscopy for sulfur-selenium phases prepared by recrystallizing quenched molten mixtures of the elements as previously described. Agreement between the observed spectra and calculated wavenumbers is closest by assuming the existence of seleniumselenium bonds and the absence of isolated selenium atoms in S, Se, -n molecules. It is assumed that sulfurs elenium phases are mixed crystals with the following components in varying concentrations: 1,2-&Se,, 1,2,3-&Se,, 1,2,3,4S,Se,, 1,2,5,6-&Se., , 1,2,3S,Se, and 1,2-S,Se,. The presence of S, and Se, in some of the phases is indicated by the Raman spectra. JNTRODUCTION

The existence of sulfur-selenium

formed in the molten mixtures of the elements has been established [l] and confirmed by Raman [ 21, IR [ 31, and mass spectroscopic [ 4-61 evidence. Similar compounds have also been prepared by condensation reactions between chlorosulfanes S, Cl, and hydrogen selenide [7, 81 and between the chlorides of selenium and sulfanes [9] . X-ray crystallographic investigations have shown that while the structures consist of either eight-membered [IO-131 or twelve-membered [9] crown shaped rings like those of S, , Se,, and S,, , none of the phases are ordered therefore giving rise to the hypothesis of mixed crystals. Recent Raman spectroscopic investigations [13] have confirmed this hypothesis indicating that the different S,Ses-, molecules can crystallize together in varying ratios. While the present study was under way Eysel and Sunder [ 143 concluded from their Raman spectroscopic investigations that selenium--selenium bonds do exist in the phases obtained by recrystallization

*Permanent address: Helsinki University Otaniemi, SF-02150 Espoo 15, Finland. 0022-2860/80/0000+I000/$02.25

binary compounds

of Technology,

o 1980

Department

Elsevier Scientific

of C%en:istry,

Publishing

Company

20

of quenched molten mixtures of the elements and proposed a mechanism for their formation. The modified Urey-Bradley force field (MUBFF) has turned out to be successful in describing the vibrational spectra of Ses [ 151 and Ss [ 16, 171. The same force field has been found suitable also for sulfur rings of different sizes, namely S7 [lS] and Slz [ 171, for the oxidized eight-membered ring in S80 [19], and for the heterocyclic eight-membered rings in &NH [20] and Sq(NH)4 1211. MUBFF was therefore thought to be suitable also for the calculation of the vibrational frequencies of &Se,-, molecules which was under&ken to obtain more information about the composition of the mixed sulfur-selenium crystals by comparing the calculated wavenumbers with the observed Raman lines. The following thirteen molecules have been included in the calculations: &Se, !&Se, (1,2-, 1,3-, 1,4- and 1,5-isomers), &Se3 (1,2,3-, 1,3,5- and 1,4,6-isomers), &Se, (1,2,3,4-, 1,2,5,6- and 1,3,5,7Gsomers), 1,2,3-S3Ses, and 1,2-&See. The number of independent force constants in MUBFF according to the different molecular symmetries varied between nine and fourteen. EXPERIMENTAL

The sulfur-selenium phases whose Raman spectra have been investigated were prepared by meking together sulfur and selenium in molar ratios of 7:1, 6:2, 5:3 and 4:4 as described earlier [ 131 where the phases are referred to according to their nominal formulae obtained from the original molar ratio of the elements. However, in some cases these names were found to bear little or no relation to the actual composition of the phases 1131 and as only one of the two phases formed in the equimolar melt (4:4) is included in the present study it was thought best to rename the phases in order to avoid misleading chemical information. The renaming scheme and the analytical composition of the four phases are shown in Table 1.

TABLE

1

Renaming scheme and analytical composition Old name (ref. 13)

of sulfur-selenium

phases

Molar ratio of S and Se at the beginning of the preparation

Approximate analytical composition (molar ratio S:Se) (ref. 13)

New name

7:l 6: 2 5:3 4:4

6:2

Phase I

6:2

Phase II Phase III

5:3 3:5

Phase IV

21

The Raman spectra of the phases were recorded with a Varian Gary 82 spectrometer equipped with a triple monochroma~r and a krypton laser (647.1 nm). AU spectra were measured at about -12O’C in order to avoid the decomposition of the samples in the laser beam and obtain better resalution in the spectra. MOLECULAR

ST~U~~~S

AND FORCE

FIEILD

The molecuies included in the frequency calculations can be divided inta two groups: those with SeSe bonds and those without SeSe bonds. Because accurate structural ~formation is not available, the following assumptions in the bond tenths and angles had to be made when c~~u~at~g the cartesian coordinates of the atoms: rss = 2.050 A, rssr?= 2.200 A, rSeSe= 2.350 A,, as = 108.0” and cuse= 106.0”. The observed average bond lengths and angles for S8 (orthorhombic ~sulfur) and Se8 (monoclinic a-selenium) are 2.046 A, 108.2” 1221 and 2.336 A, lOfi.7” 1231, respectively, The torsional angles have been adjusted in~~du~y for each molecule so as to effect the ring efosure (see Table 3). The definition of the force constants of the modified Urey-Bradley force field applied and their selection for the different SnSes -n species are presented in Tables 2 and 3, The values of the force coaster KI (SeSe s~etch~g~, Ii; ~be~ding at Se), YE (SeSe torsion), PI (bond-bond interaction at Se), and F1 (Se * - - Se repulsion) were taken from Se, Cl 51. The values of the force constants & (SS stretching), Hz (bending at S), Y, (SS torsion), Pz (bond-bond interaction at S), and F3 (S - - S repulsion) were taken from Ss fl7’j. The force l

TABLE

2

Definition of the Urey-Bradley Index

1 2 3 4 5 ; 8 9 10 I.1 12 13 14

Force constants

force constants used in the calculations Definition

Value (N cm-l)

SeSe stretching SSe stretching SS stretching Bending at Se Bending at S SeSe torsion SSe torsion SS torsion Bond-bond interaction at Se Bond-bond interaction at S SeSe repu~io~ SSe repulsion SS repulsion Long-range repulsion

1.34 1.62 1.89 0.02 0.08 0.015 O.O%I 0.020 0.23 0.29 0.21 0.24 0.26 0.07

22 TABLE3 Molecular symmetries, torsional angles and the selection of force constants in S,Se,_, molecules treated in this work Molecule

Molecular symmetry

Torsional angle (deg.)

Indices of force constants (See Table 2)

Number of force constants

A. Molecules without SeSe bonds cs 97.82 S,Se Cs 97.90 1,3-&Se, C, 98.85 1,4S,Se, c Z" 1,5-&Se, 99.34 Cs 98.49 1,3,5-&Se, c, 99.43 1,4,6S,Se, C,, 99.93 1,3,5,7S,Se,

2 34 57 8910121314 2 34 5 7 8 91011121314 2 34 57 8910121314 23457 8910121314 2 34 57 891011121314 23457891011121314 2457910111314

11 12 11 11 12 12 9

B. Molecules with SeSe bonds c, 96.92 1,2S,Se, c, 96.96 1,2,3-$We, C, 97.13 1,2,3,4-f%% D, 99.94 1,2,5,6-S,Se, c, 102.45 1,2,3S,Se, C, 98.33 1,2-&Se,

12345678910121314 1234567891011121314 12 345 67 891011121314 123456789101214 12345 67891011121314 12345678910111214

13 14 14 12 14 13

constants K2 (SSe stretching), Y2 (SSe torsion) and F2 (S - - *Se repulsion) have been estimated as averages of the corresponding force constants of Se, and S8 as this is a logical starting point for lack of more precise information about the sulfur-selenium bond. Only one long-range repulsion force constant, C, was used in all cases as the corresponding values of Se8 [ 151 and Ss [ 171 agree very closely. This is only to be expected as the coordinate q’ (the distance between atoms i and i+ 3) has only a small range regardless of the nature of the four atoms giving rise to the above-mentioned coordinate. The applied Urey--Bradley force field with up to 14 independent force constants can be written as follows 2V=

?

K1 (AI-)’ + 2 “t K;r(Ar)

+ ?

K3 ( Ar)2

+

H2(rA(x)2 + 2 ? H;r(rAar) + i;” Yl (rAT)2 + 2 2

?

+ ?

+

2 ?

+ ?f K2(Ar)2 + 2 ? K;r(Ar) +

Y2 (~AT)~ + 2 2

Kir(Ar)

+ ? HI

(?-A&)' +

2 “r*HI r(rAa!) +

y2r(rA7) + ? Y,(~AT)~ + 2 ?

+ 2 ? P,ArAf + 2 “t” P*ArAr’ + ?’ PI (Aql)’

Kr(rAT)+ yjr(rAT)

+

f 2 “t’ F;q, (Aq,) +

23

n14

+

1

“14

C(Aq’)2

+ 2

c

C’q’( Aq’)

where r and r’ are the lengths of adjacent bonds, q is the distance between atoms i and i + 2 (ql is the distance between two selenium atoms, q2 the distance between a sul?ur and a selenium atom and q3 the distance between two sulfur atoms), and Q’ is the distance between the atoms i and i + 3 regardless of the atom type. The summation indices are defined as follows: n1, n2, n3, number of SeSe, SSe, and SS bonds; n4, n5, number of Se and S atoms; n6, n,, n8, number of SeSe, SSe and SS torsional axes; ng, nlo, number of adjacent bonds originating from S and Se atoms; n,, , n ,2, n,3, number of Se - - - Se, S - - - Se, and S - - - S non-bonded distances q,, q2 and q3 , respectively; n,4, number of long-range distances q’ (n 14= 8 because q’ is independent of the atom type). The constants F’ and C’ were constrained by the conventional assumptions F; = -O.lJ$ and C’ = -0.1 C. All constants Ki, Hi’, and r are eliminated in the removal of the redundant coordinates ql, q2, q3 and q’. CALCULATIONS

The calculations were performed with a CD 6500 computer using the program UBZM by Schachtschneider [24] to calculate the Z-matrix elements for the force constant C, and programs GCCC, BGLZ and LSMA by Shimanouchi [25] to calculate Cartesian coordinates of the atoms, all matrices, and the wavenumbers and potential energy distribution using given UB force constants. The symmetry coordinates of the molecules belonging to different point groups were constructed in the conventional way [1921]. RESULTS

AND

DISCUSSION

The Raman spectra of the four sulfur-selenium phases are presented in Fig. 1 and in tabulated form in Table 4 together with those of Ses [26] and S8 [ 171 to make the comparison of observed wavenumbers with those calculated more straightforward. The calculatitd wavenumbers of the thirteen S, Se*-, molecules are presented in Tables 5 and 6. It can be seen from the least-squares refinement of the force constant: of Se8 [ 151, S8 and S12 [ 171, S, [ 181, S80 [ 191, S,NH [ 201 and S4(NH)4 [21] that the largest differences between the observed and calculated wavenumbers amount to about 15 cm-l. This probably represents the limit of accuracy of the present calculations. It can readily be seen from Fig. 1 that the spectra of all the phases are very similar with more marked differences between phase IV and the other phases. This led earlier to the conclusion that alI sulfur-selenium phases

24

PHASE III

, .-

!

’

I

‘.

*

:

0, ;

I

I

:!

c

1

;.

../

I

‘_,.

,’

*

1

.

PHASE

..

j

.

Jr 1

.’

!

IY 200 WAVENUMBER

Fig. 1. The Raman spectra of phases I-IV

loo km-‘)

recorded at -120

-C20°C.

formed in the melts are mixed crystals consisting of different S, Ses-n molecules in varying concentrations [133. By comparing the observed spectra with the wavenumbers calculated for the thirteen S, Se*-,, molecules mentioned above this conclusion can be confirmed, but since the wavenumbers calculated for many of the possible molecular species are fairly close together it is difficult to make extensive assignments in the observed spectra. The following general inferences are, however, possible.

25 TABLE

4

Raman spectra (in cm-‘)

of the sulfurselenium

phases I-IV

and of S, and Se, at -120

+ 20°C

Type

of vibration

S, 1171

Phase I

Phase II

Phase III

Phase IV

SS stretching

473

vs

469

vs

469 vs

vw

vw vw

vw vw vw

vw

436 429

447 439 430

448

vw

vs sh vw vw v-w

463

439

469 455 448 437 428

SSe stretching

380 m 368 sh 358 m 346 VW

362 365 359 348

m sh s vw

380 366 357 348

m sh vs vw

375 362 355

vw m m

SeSe stretching and deformation vibrations

265 259

263

s

264

vs 260 253

vs sh

218

VW

m sh

249 VW 245.5 vw 218.5 vs 214 sh

187

232 vw 218~s

231 218

v-w m

202

vs

202.5

202

vs

203 199

vw vw

179 165 151 139 130 122 112 102

w vw s m m s w vw

179 w 167 vw 151 m 140 m 130 m 122 s 111 w 104 w

178 m 166 w 151 w 139 m 130 m 123 s 112 m 103 m

175 162 154

m m vw

124 112 103 94

w m m sh

vs

[26]

268 259 253 248 240

vw w v.s w VW

127 120 112 101 94

v-w sh,w vs W w

vw

153 s

Torsional vibrations

239 VW 218.5 ws

Se,

and lattice 82.5

w 73 vw

62 vw 50s 42.5 27 w

73

w(b)

85 sh 81 m 73 sh

67 sh 51 m

50 s

49 s

34 w

34 m

32 sh

m

56w 47 m 41 m 29 m 18 vw

83 m 75 m 69 m 51 43 37 27 17

m m m m w

26 TABLE

5

Calculated wavenumbers of &Se,_, Typeof S,Se vibration VSS

475(a'.a") 473(a')

l,&S,Se,

molecules containing no SeSe bonds (in cm-‘)

l.4-S,Se,

1,5-S&e,

1.3.~S,Se,

1.4.6-S+,

1.3.5.7-S,Se,

477(a") 467(d)

467(b) 465(a)

458(a')

458(a) 448(a)

463(a")

470
446(a')

458(a")

45O(a,I 447(b ,)

451(a")

440(a") 432(a") 393(a')

%eS 390(a") 387(d)

389(b) 386(a)

383(a')

3Wb,)

395(b,) 391(a") 387(d)

390(n,)

384(a") 382(d)

382(a')

348(a") 345(a') 339(a")

342(b) 336(a)

341(b,) 336(a,)

331(a")

340
345(a') 343(H)

328(a")

330(a")

338(e) 324(c#

6sss

246(d) 241(a")

244(a')

245(b) 235(b,)

230(a') 226(a,)

220(a") 215(a') 209(a')

209(a)

205(a') 190(a") 211(b)

6SeSS

213(a") 208(d)

188(a)

170(a')

171(a")

172(a) 164(a') 163(n")

161(b) 158(a') 153(a")

154(b,) 144(a,)

6SSeS 179(d) 174(b,)

136(a')

152
195(d) 178(a") 176(d)

191(d)

192b,) 174(e)

133(b)

126(a) 121(a")

122(a') 116(a")

120(0,)

llO(a') lOl(b,)

27

TABLE 5 (continued) Type

of

S,Se

1.3-S,Se,

l.kS,Se,

l.&S,Se,

1.3.5S,Se,

1.4,6-S,Se,

1.3.5.7-S,Se,

141(d)

14O(a')

144(a,) 135(e) 128(b,) 80(&I

vibration

Ssesse 135(a') Torsion

83(a") 73(a')

83(a')

82(q)

130(0") 81(a")

72(a) 71(b) 63(a")

71
6O(a,) 55(d)

4S(b,)

a Inactive.

The wavenumbers calculated for the SS stretching vibrations (480-430 cm-r ) of all S, Se,-, molecules containing SS bonds are fairly similar and Raman lines observed in this region can indicate the presence of any of them. Also, as S8 exhibits characteristic lines at 473 and 439 cm-’ which may be hidden under the strong lines at 469 cm-l and 439-436 cm-l observed for phases I-III the presence of S, in three of the four phases cannot be excluded. In phase IV the intensities of the Raman lines belonging to SS stretching vibrations are very low and their positions have shifted slightly indicating that the concentration of molecules containing SS bonds is very small in this phase. It also makes the presence of S8 in phase IV very unlikely. In the SSe stretching region (400-320 cm-* ) the wavenumbers calculated for different S, Ses-, molecules show marked differences depending on whether or not there are two or more adjacent selenium atoms in the molecules. If there are no SeSe bonds in the molecule all the selenium atoms must be linked to two sulfur atoms. The vibrational coupling between the two adjacent SSe bonds can be expected to be fairly strong causing relatively large splitting in the wavenumbers of the symmetrical and asymmetrical stretching vibration of the structural unit S-Se-S. As can be seen from Table 5 large splittings of the calculated wavenumbers are found for those S,Ses-n molecules which contain isolated selenium atoms resulting in the distribution of the SSe stretching vibrations over a fairly wide wavenumber range (395-329 cm-l). On the other hand, if all selenium atoms are involved in SeSe bonds the sulfut-selenium bonds are separated from each other more efficiently and the vibrational coupling between the corresponding SSe stretching vibrations is smaller causing much smaller splittings. This can clearly be seen from Table 6; all SSe stretching vibrations lie between 372 and 358 cm-l. From the spectra in Fig_ 1 and the values given in Table 4 it can be seen that in all four phases the splitting of the SSe stretching vibrations is small.

28

TABLE 6 Calculated wavenumbers of SnSes-. molecules containing SeSe bonds (in cm-' ) Type of 1,2-$6Se 2 vibration

1,2,3-SsSe~

vss

478(a")

475(0) 472(a) 470(a)

466(a')

1,2,3,4-S,Se,

1,2,5,6-S,Se,

1,2-$2Se 6

472(a') 467(b) 463(a) 459(u) 457(b3)

458(a') 451(a)

452(b)

1,2,3-$3Ses

458(a) 449(a")

441(u") 435(a) USeS

364(a,b)

364(a;a")

366(a) 363(b)

372(b,) 369(bz) 360(a) 358(ba)

368(a') 362(a")

370(b) 361(a)

265(a')

~SeSe

259(a) 256(a') 249(b)

253(a")

260(a) 258(b) 251(a)

245(b2 ) 244(a)

244(a)

243(a') 237(b)

234(a") 229(a)

222(a") 6SSS

243(b) 240(u')

238(b)

228(a')

233(a) 209(a) 194(b)

221(a") 207(a' )

208(a) 216(b2) 203(b, )

5SeSS 187(a) 175(b)

206(b)

184(a') 173(a) 167(b3 )

170(a") 162(u' )

169(a) 165(a")

158(b)

156(a) 5Se~eS

223(a)

139(a' ) 129(u) 125(b)

118(a")

141(b)

142(b 3)

123(a)

122(b, ) 113(u) 112(bz )

139(a" ) 130(a')

138(a} ll6(b)

TABLE Tlkpe of vibration

6 (continued) 1,2-S,Se,

1,2,3-&Se,

1,2,3,4-&Se,

1,2,5,6S,Se,

1,2S,Se,

129(b)

sSeSeSe

112(a’) 108(a’)

109(b)

105(a) 99(a”) 97(d)

99(a)

Torsion modes

1,2,3S,Se,

73(b) 67(a)

68(a’) 62(a”)

62(a) 58(b)

62(h 56(a)

96(a) 92(b)

) 58(a”) 53(a’)

53(b) 50(a)

As there are no lines in the region; 400-385 cm-l and 345-320 cm-l which, according to Table 5, are expected for sulfm-selenium compounds containing isolated selenium atoms of the type -S-Se-S-, it can be concluded that the most abundant S,Ses-n molecules in the crystals always contain SeSe bonds and no isolated selenium atoms. This excludes all isomers presented in Table 5 as possible components of the phases I-IV. The strong Raman lines near 265 cm-l were earlier concluded to be possibly due to SSS deformation vibrations [13]. The present calculations show, however, that the SSS deformation vibrations of molecules given in Table 6 lie below 243 cm-l. Also, the highest observed deformation vibration of S8 occurs at 249 cm-l [ 171. On the other hand, SeSe stretching modes are found in the region 268-240 cm-l [ 151. It is therefore more likely that the lines observed in the region 265-250 cm- * for phases I-IV are due to SeSe stretching vibrations rather than SSS bending modes. This further indicates the presence of SeSe bonds in all phases. For Se8 the strongest SeSe stretching vibration is found at 253 cm-’ and the strongest SeSeSe deformation vibration at 112 cm-1 with the intensity ratio of 100:76 (see Table 4). If the four sulfur-selenium phases also contain Se a, it seems reasonable to expect that at least these two lines should appear in the spectra with a similar intensity ratio. In phases I-III there are no lines around 253 cm-l and the nearest SeSe stretching vibrations lie at 265-263 cm- * . In phase IV, however, there is a strong line at 260 cm-l with a shoulder at 253 cm-l. This could indicate the presence of Se, in this last phase, though according to Table 6 1,2,3-&Se,, 1,2,3,4S,Se,, 1,2,3-&Se,, and 1,2-&Se, also have their SeSe stretching vibrations near this wavenumber, making the assignment of this line speculative. There occurs a line of medium intensity in every phase at 112-111 cm-‘. In addition to Se,, these lines can be caused by 1,2,3-&Se,, 1,2,3,4S,Se,,

30

1,2,5,6-S4Se4, 1,2,3-S3SeS or 1,2-S2Se6. As the line at 253 cm-l is missing in the spectra of phases I-111 it can be assumed that the lines at 112-111 cm-* are due to SeSeSe and SSeSe deformation vibrations of the above-mentioned molecules and do not indicate the presence of Se, in these phases. In phase IV the intensity ratio of the shoulder at 253 cm-’ and the line at 112 cm-l is about 1:l which makes it possible to assign these lines at least partly to Ses. The presence of Ss in phases I-III is confirmed by the two characteristic lines at 218 and 151 cm-l which have been assigned as a, and e2 modes of the molecule. The intensity ratio of these two lines in the spectrum of Ss is 100:60 [17] _ For the three phases the observed intensity ratios of the corresponding lines are 100:61,100:54 and 100:71, respectively, agreeing closely with the ratio observed in the case of Sg. The overall intensities of the two lines decrease from phase I to phase III and virtually disappear in phase IV due to a decreasing concentration of Sa as the selenium content increases. Phase IV is unlikely to contain S,. There are also alternative ways to assign the lines at 218 and 151 cm-* (see Table 6). Therefore the assignment of these lines to Ss is based on the characteristic high intensity and the similarity of the intensity ratios of these lines in Ss and phases I-III. When comparing the spectra of the four phases presented above to the Raman spectra measured by Eysel and Sunder [ 141 it can be noted that both sets of spectra show the same general features. Thus the absence of molecules containing isolated selenium atoms is also demonstrated by this latter set even though Eysel and Sunder only note the presence of SeSe bonds in the mixed S,Ses-, molecules. The Rarnan spectioscopic results are in disagreement with the earlier mass spectroscopic investigations [13]. In the mass spectra of phases I-IV the isotopic distribution pattern of S, Se is observed. This discrepancy probably arises because upon heating the samples in vacuum extensive decomposition occurs and therefore the ions recorded in the mass spectra do not necessarily correspond to the fragmentation pattern of the original S, Se,-, molecules. CONCLUSIONS

The sulfur-selenium

binary compounds formed by melting together sulfur

and selenium or by various chemical reactions have been characterized by chemical analysis [13], mass spectroscopy [4-7,131, IR spectroscopy [3],

Raman spectroscopy [2,13,14], and X-ray crystallography [g-13]. All methods indicate that in the solid state sulfur and selenium form definite chemical compounds and it has been established by X-ray diffraction results that the prevailing molecular form is an eight-membered ring S,Ses-, . None of the phases studied so far have been stoichiometrically pure and the existence of mixed crystals of different molecular species is strongly indicated by spectroscopic and X-ray diffraction studies. --

31

In the present work more detailed information about the composition of sulfur--selenium mixtures was sought by comparing Raman spectra of four phases obtained by melting together sulfur and selenium in molar ratios 7: 1, 6:2, 5:3 and 4:4 with the wavenumbers calculated for selected S,Ses-, molecules. The calculations indicate that while the presence of sulfur-sulfur, sulfur-selenium and selenium-selenium bonds in the molecules is confirmed, no structural units of the type S-Se-S are present. According to the fundamental vibrations calculated for thirteen &Se, - ,, molecules (see Tables 5 and 6) the four sulfur-selenium phases may contain Ss, 1,2-&Se,, 1,2,3-S,Se,, 1,2,3,4-&Sea, 1,2,5,6S,Se,, 1,2,3-S3Ses, 1,2-&Se, and Se, in varying concentrations. The phases I-III contain Ss with decreasing concentration as the nominal selenium content is increasing. Phase IV contains virtually no Sg but Se, seems to be one of the major components. ACKNOWLEDGEMENTS

We are grateful to Prof. L. Niinistij for his interest in this work. Also financial aid to R.L. from The Finnish Cultural Foundation is gratefully acknowledged. REFERENCES

1 L. L. Hawes,Nature(London), 198 (1963) 1267. 2 A. T. Ward,J. Phys.Chem., 72 (1968) 4133. 3 T. Ohsaka, J. Non-Cryst. Solids, 17 (197 5) 121. 4 J. Berkowitz and W. A. Chupka, J. Chem. Phys., 45 (1966) 4289. 5 R. Cooper and J. V. Culka, J. Inorg. Nucl. Chem., 27 (1965) 755. 6 R. Cooper and J. V. Culka, J. Inorg. Nucl. Chem., 29 (1967) 1217.

7 R. Cooper and J. V. Culka, J. Inorg. Nucl. Chem., 32 (1970) 1857. 8 M. Schmidt and E. Wilhelm, Z. Naturforsch., Teil B, 25 (1970) 1348. 9 J. Weiss and W. Bachtler, Z. Naturforsch., Teil B, 28 (1973) 523.

10 T. Kawada, T. Matsumoto, H. Burzlaff and E. Hellner, Acta Crystallogr., Sect. A, 28 (1972) S61. 11 J. Weiss, Z. Anorg. Allg. Chem., 435 (1977) 113. 12 C. Calvo, R. J. Gillespie, J. E. Vekrls and H. N. Ng, Acta Crystallogr., Sect. B, 34 (1978) 911.

13 14 15 16 17 18

R. Laitinen, L. Niinistii and R. Steudel, Acta Chem. Stand. A, 33 (1979) 737. H. H. Eysel and S. Sunder, Inorg. Chem., 18 (1979) 2626. R. Steudel, Z. Naturforsch., Teil A, 30 (1975) 1481 and references cited therein. D. W. Scott, J. P. McCullough and F. H. Kruse, J. Mol. Spectrosc., 13 (1964) 313. R. Steudel and H.-J. Mlusle, Z. Naturforsch., Teil A, 33 (1978) 951. R. Steudel and F. Schuster, 3. Mol. Struct., 44 (1978) 143.

19 R. Steudel and D. F. Eggers, Spectrochim. Acta, 20 R. Steudel, J. Phys. Chem., 81 (1977) 343.

Part A, 31 (1975)

871.

21 R. Steudel and F. Rose, Spectrochim. Acta, Part A, 33 (1977) 979. 22 P. Coppens, Y. W. Yang, R. H. Blessing, W.F. Cooper and F. K. Larsen, J. Am. Chem. Sot., 99 (1977) 760. 23 P. Cherin and P. Unger, Acta Crystallogr., Sect. B, 28 (1972) 313.

24 J. H. Schachts&eider, Vibrational Analysis of Polyatomic Molecules V, Shell Development Co., Emeryville, CA, Rep. No. 213-64. 25 T. Shimanouchi, Computer Programs for Normal Coordinate Treatment of Polyatomic Molecules, Tokyo, 1968 (distributed privately). 26 J. Bojes, private communication.