Raman studies of (Fe, V)3S4 solid solution

0038-1098/82/200605-04503.00/0 Pergamon Press Ltd.

Solid State Communications, Vol. 42, No. 8, pp. 605-608, 1982. Printed in Great Britain.

RAMAN STUDIES OF (Fe, V)sS4 SOLID SOLUTION M. Ishii, H. Wada, H. Nozaki and I. Kawada National Institute for Research in Inorganic Materials, 1-1 Namiki, Sakura-nura, Niihari-gun, Ibaraki, 305, Japan

(Received 8 January 1982 by y. Toyozawa) Raman scattering of FexV3-xS4 with 0 ~
L,,b O METAL ATOM

I-I VACANCY Fig. 1. Schematic model of V3S4-type structure. The sulfur layers are omitted for convenience. heating in an H2S atmosphere at 1100 ° C for 4 h, then the composition of the vanadium sulfide thus obtained was controlled by the use of the sealed tube method. The composition of the synthesized sample was stoichiometric (VS1.333). The samples of the (Fe, V)aS4 solid solution were synthesized according to the method described in the previous papers [2-5]. Five samples of FexV3_xS4 w i t h x = 0.51, 1.00, 1.50, 1.80 and 2.00 were used for the measurements. Raman spectra were measured for the pressed pellets of the powder samples. A Spex Ramalog 4 Raman spectrometer with 514.5 nm excitation of an Ar ion laser was used for the measurements. Observed spectra at 70 K are reproduced in Fig. 2. 3. RESULTS AND DISCUSSION 3.1. VsS4

2. EXPERIMENTAL As for the synthesis of the sample of V3S4, a starting sulfide was prepared from VOSO4-3H20 by

VsS4 crystallizes to the monoclinic space group

I2/m (CSzh)and the primitive unit cell contains one formula unit (VaS4) [1 ]. Based on this crystal structure, 9 Raman active modes of 6Ag + 3Bg are expected. 605

RAMAN STUDIES OF (Fe, V)3S4 SOLID SOLUTION

606

Table 1. Observed Raman frequencies (cm -1) at 300 and 70 K, and calculated frequencies (cm -1) based on the simple model using three bond stretching type force constants: K(VS) = 0.74 md A -1 , K(VV) = 0.70 md A -1 and K(SS) = 0.09 md A -1 Obs.

Calc.

300 K

70K

Ag

400 366

405 370 343 337 288

398 365

232 204 170

230 183

334 280

337

Bg

342

287 265

225 187

166

Ag and Bg modes correspond to the vibrations parallel and perpendicular to ac plane, respectively (Fig. 1). Observed Raman frequencies at 70 and 300 K are listed in Table 1. As shown in this table, six Raman bands were observed in the frequency region below 500 cm -1 at 300 K. At 70 K, a rather broad band of 334 cm -1 observed at 300 K splits into two bands, and a peak at 343 cm -1 and a shoulder at 337 cm -1 are observed. In addition, a shoulder band is observed at about 170 cm -1 at 70 K, the corresponding band of which may not be separated from the wing of Rayleigh scattering at 300 K (see Fig. 2). Five bands of 400, 366,280,225 and 187 cm -1 observed at 300 K shift to higher frequencies at 70 K. Of these, the frequency shift of the 187 cm -~ band is 17 cm -~ between 300 and 70 K and more than twice as large as those of others. The structure of V3S4 is of the distorted NiAs-type, and the stability of this type structure for the first series transition-metal chalcogenides arises from the metal-metal bonding involved in the structure [13]. Actually, the average value of metallic V - V bond distances in V3S4 is about 2.92 A and only about 8% larger than twice the atomic radius of vanadium [ 1]. As to the Raman frequencies which are directly related to the metal-metal interaction, Shin et al. [11] reported that those in Ti2Os (Ale modes) are sensitive to the change of the c/a ratio. From these facts, it may be reasonable to consider that the 187 cm -~ band, which is sensitive to temperature, i.e. the change of lattice constants, is related to V - V bond in V3S4. In order to confirm this consideration and to assign other bands, the lattice vibration frequencies were calculated based on a simple model. Only three force constants of the bond stretching types, K(VS) [d(V-S) = 2.33-2.49 A], K(VV) [d(V-V) = 2.91-2.94 A] and K(SS) [d(S-S) =

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317-3.63 A], were used in the calculation, where d denotes the interatomic distance. The values of K(VS) and K(SS) were estimated from those of K(TiS) and K(SS) obtained from the same type calculation for TiS2, respectively [14]. The value of K(VV) was also estimated from that of the metal-metal bond stretching force constants of the transition metal compounds [ 10]. The calculated frequencies for the force constant set of K(VS) = 0.74 md A -1 , K(VV) = 0.70 md A -1 and K(SS) = 0.09 md A -1 are also listed in Tabe 1 together with the observed Raman frequencies. As can be seen from this table, the result of the calculation well explained the observation except the case of the calculated frequency of 265 cm -1 o f B e symmetry, and the above mentioned 187 cm -1 band was assigned to the lowest frequency Ag mode. As shown schematically based on the calculation in Fig. 3(a), this mode is the stretching mode of the shortest V - V bonds (2.919 A) [1 ]. Therefore, the frequency of this mode may directly reflect the strenth of the interaction between V atoms in the metal-full layer and those in the metaldeficient layer through d electrons. These results are consistent with the above described consideration, and show that metal-metal interaction in V3S4 is sensitive to temperature, i.e. the change of the lattice constants.

3.2. Fex V3-xS4 (0 < x <~2) As shown in Fig. 2, the features of the observed Raman spectra of FexV3_~S4 at 70 K change with the value of x. The observed spectra at 300 K were essentially identical with those at 70 K for each solid solution sample, though all of the observed frequencies shifted to higher ones with decreasing temperature. For each sample, the most significant frequency shift was observed for the lowest frequency band at about 200 cm -1 (Fig. 2). As compared with the above described results for V3S4, this band can be assigned to the metal-metal bond stretching type mode shown in Fig. 3(a). Atomic weights of Fe and V atoms are approximately equal to each other, therefore, as mentioned above, the frequency of this band is considered to reflect directly the interaction between the metal atoms in the metaldeficient layer and those in the metal-full layer. In Fig. 3(b), the observed frequencies of this lowest Ag mode at 300 and 70 K are plotted against the composition. As can be seen from this figure and Fig. 2, the frequency of this mode at 70 K shifts to lower frequency with increasing x value between x = 0 and x = 1, on the contrary, in the composition range between x = 1 and x = 2, it shifts to higher one with increasing x. The feature of the composition dependence of the frequency at 300K also shows a similar tendency to that at 70 K as shown in Fig. 3(b).

RAMAN STUDIES OF (Fe, V)aS4 SOLID SOLUTION

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2o]~j/1%S41 no. X = O

2y~/

X=0.51

370 3 405

337

/ X=I.0 a~a, ' '

,as

.-jJ 345

560

460

300

200

1()0

cm-~

Fig. 2. Raman spectra of FexVa_xS 4 at 70 K with 514.5 nm excitation.

TE220 1 0 0 z

~180 (

0

0

0 0

0 160 I

1

(a)

x

(b)

Fig. 3. Metal-metal stretching mode and the composition dependence o f its frequency. (a) Displacements o f each atom for the lowest frequency mode of Ag symmetry in VaS4. Open circles denote the sulfur atoms and black ones denote the vanadium atoms. (b) Observed metal-metal stretching frequency (the lowest Ag mode) plotted against the composition (x) in FexVa_xS 4. Open and black circles denote the observed frequencies at 300 and 70 K, respectively.

summarized as follows: (a) In the composition range between x = 0 and x = 1, the substitutions of Fe atoms for V atoms in the metal-deficient layer take place, (b) In the composition range between x = 1 and x = 2, the metal sites in the metal-deficient layer are occupied by Fe atoms, and parts o f V atoms in the metal-full layer are substituted by Fe atoms. Based on these facts, our Raman spectroscopic observations described above are considered to suggest the following results: (1) In the composition range between x = 0 and x = 1, the interaction between the metal atoms in the metal-deficient layer and those in the metal-fun layer becomes weak with increasing Fe substitution in the metal-deficient layer, (2) In the composition range between x = 1 and x = 2, such interaction becomes strong again with increasing Fe substitution in the metal-fuU layer. Recent investigation on the magnetic susceptibility and M6ssbauer effect of (Fe, V)aS4 [12] suggests that, in the range between x = 1 and x = 2, the electronic band structure changes as compared with that in the range between x = 0 and x = 1, and the metal-metal interaction becomes strong. The results of this investigation support our results. The value of the lattice constant c, the axis o f which is nearly parallel to metal-metal bonds between metal-deficient and metal-full layers, decreases rapidly with increasing x in the composition range between x = 1 and x = 2 [4], and this fact is also consistent with our results. The observed Raman frequencies in the region from 450 to 210 cm -1 become weak and broad, and shift continuously to the lower frequencies with increasing x in the composition range between x = 0 and x = 1, as shown in Fig. 2. However, for the spectra of x = 1.5, a broad feature at about 245 cm -1 appears discontinuously. In addition, in the spectra o f x = 1.8 and x = 2.0, two bands at 383 and 345 cm -1 appear rather discontinuously, and become strong with increasing x (Fig. 2). These discontinuous changes between x = 1.0 and x = 1.5 may correspond with the fact that Fe substitutions in the metal-full layer take place for the samples with x > 1. The behaviour of the two bands at 383 and 345 cm -1 may suggest some orderings of Fe atoms in the metal-full layer in the samples o f x = 1.8 and x = 2.0. The ordering of Fe atoms in the metalfull layer is also suggested from the results o f the magnetic measurements [ 12]. REFERENCES 1.

As mentioned before, the site distributions of Fe atoms were studied for F e V 2 S 4 and F e 2 V S 4 [ 7 - 9 , 12]. From these results reported up to now, the site distributions o f Fe atoms in this solid solution are

607

2. 3. 4. 5.

I. Kawada, M. Nakano-Onoda, M. Ishii, M. M. Nakahira, or. Solid State Chem. 1 5 , 2 4 6 H. Wada, Bull. Chem. Soc. Japan 5 1 , 1 3 6 8 H. Wada, Bull. Chem. Soc. Japan 52, 2918 H. Wada, Bull. Chem. Soc. Japan 52, 2130 H. Wada, Bull. Chem. Soc. Japan 53, 1173

Saeki & (1975). (1978). (1979). (1979). (1980).

608 6. 7. 8. 9. 10.

RAMAN STUDIES OF (Fe, V)3S4 SOLID SOLUTION Y. Oka, K. Kosuge & S. Kachi, Mat. Res. Bull. 15, 521 (1980). S. Muranaka & T. Takada, J. Solid State Chem. 14, 291 (1975). B.L. Morris, V. Johnson, R.H. Plonvnick & A. Wold, J. Appl. Phys. 40, 1299 (1969). I. Kawada & H. Wada, Physica 105B, 223 (1981). T.G. Spiro, Progress in Inorganic Chemistry (Edited by S.T. Lippard), Vol. 11, p. 1. Interscience, New York (1971).

11. 12. 13.

14.

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S.H. Shin, F. PoUak & P.M. Raccah, J. Solid State Chem. 12,294 (1975). H. Nozaki (to be published). F. Jellinek, Proceedings of the Fifth Materials Research Symposium (Edited by R.S. Roth & S.J. Schneider), p. 625. N.B.S. Special Publication 364, Solid State Chemistry (1972). M. Ishii (unpublished work).