Mat. Res. Bull . , Vol. 23, pp. 107-112, 1988. Printed in the USA. 0025-5408/88 $3.00 + .00 Copyright (e) 1988 Pergamon Journals Ltd.

NEW TRANSITION METAL SILICOSELENIDES

POSSESSING CdI2-TYPE STRUCTURES +

J. Qopalakrishnan and K.S. Nanjundaswamy Solid State and Structural Chemistry Unit

Indian Inst i tute of Science, Bangalore-560012, India

(Received August 20, 1987; Communicated by C.N.R. Rao)

ABSTRACT Three d i f ferent kinds of f i rs t - row transi t ion metal si l icoselenides having C d L - r e l a t e d structures have been synthesized Vanadium and chromium

Z form VSiSe and Cr _vSiSe, which are simi lar to MPSe crysta l l iz ing

3 1~^ ~ 3 in a hexagonal structure. Manganese forms Mn3/2SiSe 3 which is isostruc- tural wi th Mn3/zSiTe 3. With Fe, Co and Ni, a new type of si l icoselenide of the formula MSi 2 Se 4 (M = Fe, Co or Ni) is obtained. These sohds crysta l l ize in the CdLz/Cr3S 4 structure where M atoms and Si-Si pairs occupy the cat ion positions. MATERIALS INDEX: Silicoselenides~ vanadium, chromium, manganese, iron, cobalt and nickel.

INTRODUCTION

Layered t ransi t ion metal dichalcogenides exhibi t ing reversible in terca la t ion of inorganic and organic species are of special interest in solid state chemistry (1,2), the prototype example being TiS wi th the Cd[ structure (3). Transit ion 2 2 metal phosphoselenides of the general formula MPSe 3 also adopt C d I _ - r e l a t e d

Z structure where P-P pairs and M atoms are ordered at Cd sites (4). In terca la t ion chemistry of MPSe~ and their thio analogues has been widely invest igated in the l i te ra ture (5,6). In view of the relat ionship between the electronic structures of solids containing P-P and Si-Si linkages (7), we expected that i t may be possible to prepare t ransi t ion metal si l icoselenides which are analogous to MPSe 3. Because of the di f ference in the valencies of P and Si, the composit ions are however not l ike ly to be the same. A [SizSe 6] unit wi th 5i-5i bonds wi l l bear six posit ive charges whi le a s imi lar [PzSe6 ] unit possesses four posit ive charges. Accordingly, we would expect the formulas of si l icoselenides to be MSiSe 3 where M is a t r iva lent metal and M3/2SiSe3 where M is a divalent metal . ]n addit ion, we would also expect the composit ion MSi~.See to be stable wi th divalent M atoms; in this case, equal amounts of M and (Si-Si) units would occupy Cd positions in the Cdl structure.

2

+Contr ibut ion No. 470 f rom the Solid State and Structural Chemistry Unit.

107

108 J . GOPALAKRISHNAN, et al. Vol. 23, No. 1

We invest igated the format ion of Cd]z - re la ted phases in M-Si-Se (M = V, Cr, Fe, Co, Ni) systems by sealed tube react ion of the elements at elevated tempe- ratures. In this paper, we describe the synthesis of several new CdI - re lated,

2 • •

f i rs t - row transi t ion metal si l icoselenides. We have succeeded in character,z ing three d i f fe rent kinds of si l icoselenides of the fo l lowing formulas-" MSiSe 3 (M = V, Cr), M~/SiSe] (M = Mn), and MSizSe 4 (M = Fe, Co, Ni).

EXPERIMENTAL

Mixtures of M, Si, and Se corresponding to the composit ion MSiSe3, M 1 +xSiSe3, M3/zSiSe] and MSi2Se 4 were reacted in evacuated sealed si l ica ampoules at 700-800°C for varying durat ion ranging upto two weeks. The inner walls of the si l ica tube was coated wi th carbon to avoid react ion wi th the container. Solid phases formed were ident i f ied by powder X-ray d i f f rac t ion. Composit ions of the compounds formed in each case were f ixed by repeated t r ia l .

X-ray powder d i f f rac t ion patterns were recorded wi th a JEOL JDX-SP powd(,r d i f f rac tomete r using nickel f i l te red CulZ.~radiat ion. Infrared spectra in KBr pel lets were recorded using a Perk in-Elemer Model 580 spectrometer.

RESULTS AND DISCUSSION

We at tempted to prepare d i f fe rent composit ions in the M-Si-Se (M = V, Cr,

TABLE 1

Lat t i ce parameters and structure types of f i rs t - row transi t ion metal si l icoselenides

Compound Lat t i ce parameters Structure type

VSiSe3 Hexagonal -" a = 6.520(8) o -MPSe3 c = 19.550(9) A

Crl+xSiSe3 Hexagonal ." _a = 6.644(8) o MPSe 3 c = 19.643(7) A

Mn3/2SiSe 3 Hexagonal • a = 6.510 (3) o Mn3/2SiTe3 c = 13.715(4) A

FeSi2Se 4 Hexagonal : a = 3.615(3) o CdI 2 c = 6.048(2) A

CoSi2Se 4 Monocl in ic = a = 5.945(5) Cr3S 4 = 3.586(8) o

B = 91.82(4) °, --_c = 11.108(8)A

NiSi25e 4 Monocl in ic : a : 6.038(9) Cr3S 4 = 3.411(8)

O

6 = 91.84(4) ° c = 11.014(8)A

Vol. 23, No. 1 SILICOSELENIDES 109

TABLE 2

X-ray powder' d i f f rac t ion data of Mn3/2SiSe 3

o o o o

hkl dobs(A) dcal(A) l./l ° hkl dobs(A) dcal(A) i / [o

002 6.862 6.858 15 206 1.774 1.775 7 004 3.423 3.429 65 008 1.712 1.714 22 112 2.945 2.940 35 I08~ 1.640 ~ 7 104 2,931 2,930 100 304J 1,643 1,647

J

201 2,755 2,760 15 220 1,626 1,627 2 114 2,358 2,361 32 222 1,584 1,583 8 006 2,290 2,286 11 313 1,482 1,480 2 205 1.965 1.968 9 224 1.469 1.470 5 213 1.932 1.931 19 226 1.324 1.325 3 300 1.880 1.879 16 14.10 1.262 1.264 10 301 ~ 1.862 ~ 25 405 1.252 1.253 9 116J 1.860 1.870 J

Mn, Fe, Co, Ni) systems taking into account the known ox idat ion states of M atoms in chalcogenides. Considering that V and Cr exist in the t r i va lent state in several chalcogenides, we expected that compounds of s to ich iometry VSiSe 3 and CrSiSe3would be formed. Indeed we could prepare single phase mater ia ls

corresponding to VSiSe_ and Cr 1 +xSiSe. (x = 0.2) possessing hexagonal structures )

simi lar to MPSe 3 analogues (Table 1). S~ince Mn is known to exist in the divalent state in chaleogenides, we ant ic ipated t ha t a compound of the formula Mn~SiSe3 to be formed. Powder d i f f rac t ion pat tern of the manganese compour~l (Fig. '1) neat ly indexes on a hexagonal system wi th a = 6.510(3) and c = 13.715(4)A (Tab le 2). The c - a x i s of Mn3DSiSe~ is t w i c e t h a t of the CdI z s t r u c t u r e and not t h r e e t i m e s as ~ VSiSe3and C~" I S~iSe3. This shows t h a t the c a t i o n layer s e q u e n c e in this compound is l ike ly to b~Xas fol lows- I Mn~/3 (Si2)il~lMn~131~zl31 Se2, where I::ldenotes cat ion vacancy. Accordingly, the com-156und-is" isoslEruc~ural wi th Mn3/zSiTe 3 (8 ) .

With Fe, Co and Ni, solid state react ion corresponding to the composit ion M..^SiSe 3 did not yield single phase mater ials. On the other hand, composit ions

) / z corresponding to MSi~ Se 4 were readi ly formed as single phases possessing Cd[ 2 -re lated structures (Fig. 1)~ Indeed the FeSizSe 4 indexes on a CdI 2 cel l wi th a= 3.615(3) and 2 = 6.0b,8(2)A (Table 3). The Co and Ni compounds of this stoichio- metry also possess CdI 2 structure but show addi t ional superstructure lines which are indicat ive of an ordering of M atoms and Si-Si units at the octahedral sites. I f we assume that there is a 1:1 type of ordering of Si-Si and M atoms in the C d l . s t r u c t u r e , MI , . (S i_ ) Iml Se . , one would e x p e c t t he s t r u c t u r e s of t h e s e sol ids would be re la ted to ~rf~/~5 , Cr 1 Crl/21~/2 I S (9,10) Accordingly we could index

• . 3 4 . 2 . . " • • •

the powder d i f f rac t ion pat tern of Co~i~ ~eZ4 and NISI2Se 4 on monochnlc unit cells re lated to the Cr:~ S 4 structure (Table b,). MSi2Se 4 reported in this invest igat ion const i tute a new series of layered mater ia ls simi lar to the transi t ion metal dichalcogenides wi th a vander Walls gap. I t would therefore be interest ing to invest igate the in terca la t ion behaviour of these compounds.

A schematic representat ion of the structures o f the new sil icoselenides synthesized is shown in Fig. 2. The unit cell parameters of the si l icoselenides

110 J . GOPALAKRISHNAN, et al. Vol. 23, No. 1

10

(o)

g

I I I I I I I I I I I

(c) s -

o ,,m

I I I I I I I I I I

(d) 8 ~_ .

I I I 1 I 1 1 I I I 15 20 25 3 0 35 40 4 5 50 55 6 0 65

Cu K,C 28 (deg)

FIG. 1 X-ray powder diffraction of (a) Crl+xSiSes, (b) Mn3/2SiSes, (c) FeSi2Se 4 and (d) NiSi2Se 4.

TABLE }

X-ray powder diffraction data of FeSi25e 4

O O

hk| dobs(A) dcal(A) I/I °

001 6.050 6.048 5 002 3.050 3.024 4 101 , 2.780 2.780 90 102 2.170 2.175 100 003 2.012 2.016 25 110 1.810 1.807 45 111 1.725 1.732 20 004 1.510 1.512 23 202 1.390 1.390 10

Vol. 23, No. 1 SILICOSELENIDES 111

TABLE 4

X-ray powder d i f f rac t ion daLa of NiSi2Se 4

0 0 0 0

hkl dobs(A) deal(A) I / l ° hkl dobs(A) dcal(A) I/1 °

002 5.510 5.504 6 114 ~ 2.003~ i01 5.365 5.369 5 204 - 2.U03 2.002J 25

101 5.195 5.221 30 301 1.993 1.992 10 703 3.175 3.181 95 301 1.972 1.970 4 103 3.100 3.091 8 213 1.945 1.946 7 200 3.010 3.017 40 213 1.903 1.905 6 110 2.965 2.969 20 015 1.855 1.850 5 502 2.692 2.682 45 006 1.830 1.834 20 712 2.643 2.631 100 303 1.795 1.790 75 113 2.488 2.498 85 310 1.734 1.734 15 013 2.308 2.298 5 312 1.665 1.667 28 ~05 ~ 2.090 ~ 206 1.595 1.591 33 211J 2.100 2.080 ~ 25 i07 1.535 1.534 10 204 2.075 2. 67 5 305 1.505 1.505 I0 714 2.030 2.034 35 220 1.488 1.485 5

are l isted in Table 1.

The presence of Si-Si bonds in these compounds is revealed by i.r. spectra. A strong band seen around 420-450 cm -1 in al l the compounds is character is t ic

(al (b) (c)

O Se

O M o Si

FIG. 2

Schematic representat ion of the structures of (a) MSiSe3(M

(b) IVln3/2SiSe 3 and (c) MSi2Se 4 (M = Fe, Co, NiL

= V, Cr),

112 J. GOPALAKRISHNAN, et al. Vol. 23, No. 1

of Si2Se 6 units with Si-Si linkage. Si2Br6 shows this band at 479 cm -~ (11). MPSe 3 analogues show similar characteristic absorption around 4145 cm -I (12).

ACKNOWLEDGEMENT

The authors thank Professor C.N.R. Rao, F.R.S., for valuable advice and encouragement.

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