Bull. Mater. Sci., Vol. 22, No. 2, April 1999, pp. 103-108. © Indian Academy of Sciences.

X-ray studies on microstructural characterization of substituted mixed diselenide; W0.6sMo35_xTaxSe2 (0 _< x <_ 0-35)

DEBASHIS PALIT*, S K SRIVASTAVA* and B K SAMANTAY RAY* Department of Chemistry, ~ Department of Physics, Indian Institute of Technology, Kharagpur 721 302, India * Present address: Department of Chemistry, University of Chitlagong, Bangladesh

MS received 2 November 1998; revised 19 January 1999

Abstract. The wide angle X-ray diffraction on W0~sMos_xTaxSe2 (0

Keywords. ; ; microstructure parameters.

1. Introduction their intercalation compounds are useful as high tempe- rature and high pressure lubricants, electrode materials During the past few years there has been a great deal and in secondary batteries, solar energy conversion pur- of research in the study of the properties of materials poses and in the field of catalysis etc. In this regard, having layered type of structure. Probably the greatest MoSe2, WSe 2 and TaSe 2 constitutes structurally and attention has been focussed on transition metal dichalco- chemically well defined family of compounds which find genides (TX2: T = group IV, V and VI transition metals, various applications for modern purposes (Srivastava and X = S, Se, Te) because of their unique properties and Avasthi 1981, 1985). Their results from which covered a wide range of spectrum e.g. semi- the hexagonaily packed atoms in the sequence X-T-X, conducting, insulators, metallic and superconducting X-T-X. Within each X-T-X layer the T is either in (Wilson and Yoffe 1969; Subba Rao and Schafer 1979; trigonal prismatic coordination or in octahedral coordi- Srivastava and Avasthi 1981, 1985, 1993; Srivastava nation. The typical layer type structure of MoSe 2 and 1989; Young and Frindt 1996; Srivastava et aI 1997). WSe2 and identical radii of Mo and W suggested the The distinctive feature of these two dimensional materials formation of isomorphous MoxWt_,Se 2 (0

103 104 Debashis Palit, S K Srivastava and B K Samantay Ray ranging from 0-90 to 0.99. Further studies by Palit et al where (1997) have shown that W.65Moo.3sSe2 and W0.65Ta03sSe2 -/~/n~ with MoS 2 structure could be combined together advan- S= tageously in the form of a single phase W~reMo3s_~Tafie 2 d ~ (0 < x < 0.35) solid solutions. The unique feature common Pv is the apparent crystallite size which is obtained by to these materials is their weak interlayer bonding. This the method of variance and PF the true crystallite size introduces the structural anisotropy which directly affects as determined by the method of Fourier analysis and their properties. In addition, it results in the variability both are related as of interlayer spacing of some of the planes. Because of these peculiarities the study of microstructure parameters like mean fractional change in interlayer spacing, fractions Pv - PF + ' (2) of the planes affected by defects, dislocation density, where flj is the integral width of the defect profile. Thus root mean square strain, crystallite size anisotropy and a plot of W20 versus A (20) will be linear and the slope stacking fault probability of these compounds using X-ray will give apparent crystallite size Pv and the intercept, diffraction have been made in the present investigation. ((e2)-f12/~2). Thus knowing the values of P~ and Pv, The calculation of the interatomic distances, mean square the value of fld can be obtained. displacements and coupling constants for different pairs If g be the mean fractional change in the interlayer of atoms (M-M, M-X and X-X intra- and interlayer) spacing in the direction of d~ 2 and ~ is the proportion based on radial distribution analysis is presented. of the planes affected by such disorder, then one can write, 2. Sample preparation and characterization g = ~ cot- , (3) In the present work, Wo.65Moa5_xTaxSe 2 (0<-x<0-35) compounds were synthesized directly from the elements and and the method of preparation was quite similar to that already reported by Palit et al (1997). Appropriate fld (4) amounts of molybdenum, tungsten, tantalum and ~' -- sin2 n lg ' (all 99.99% pure and from Aldrich) powders were weighed where, A is the measure of peak shift from the centroid accurately to give the desired composition, mixed inti- of the diffraction profile and is determined following mately to give a homogeneous mixture and then placed the method described by Mitra (1964). inside a quartz tube and vacuum sealed. The heating of The dislocation density, p as shown by Williamson the sample was carried out at 400°C and 750°C for 96 h and Smallman (1956), can be written as and 48 h, respectively. A loose product resulted having a considerably larger volume than the reacting elements. 2(3(d)) ,/2 This was mixed well mechanically and subsequently P bp ' (5) placed inside the furnace and heated at 1000°C for 40 h followed by slow cooling to room temperature. where b is the Burger's vector associated with Burger's All these compounds were ground at room temperature circuit. If it is assumed that b = a, the lattice parameter and passed through 200 mesh sieve. However, it is to of the sample, then p can be easily calculated. be noted that by grinding of the samples one introduces The stacking fault probability ct is the fraction of the additional strains and lattice imperfections in the crys- layers undergoing stacking sequence faults in a given tallites. For this reason all the sample powders were crystal and hence one fault is expected to be found tn annealed in evacuated ampoules at 1000°C for 24h. 1/a layers. Thus according to Warren (1968) a can be X-ray diffractograms of all the samples were recorded calculated based on the following equation on a Philips 1729 diffractometer using CuKa radiations. Accurate lattice parameters were obtained by least square _ 3__! (6) method. The microstructural parameters have been cal- culated while considering 002 reflection in the diffraction pattern. Assuming that the broadening of the X-ray line profile is due to the presence of size broadening and where (dAn/d) is the initial slope of the Fourier co- broadening arising in the variance of interlayer spacing efficient (A) versus order (n) of the curve, c the lattice (Mitra 1964) the variance of the line profile, W(20) can parameter. It may be noted that the probability of growth be written as fault fl is negligible. The crystallite size anisotropy for all these compounds JlA(20) &l 2 was calculated by finding out the relative change in the W(~ = 2~ 2 Pv cos 0 + -----~,cos2 (1) true crystallite size i.e. Fourier for 002 and 103 reflections. X-ray studies of Wo.~sMo~s_TaeSe 2 105

For radial distribution analysis (RDF) and differential strain ((e2)~n), dislocation density p, crystallite size aniso- radial analysis (DRDF) the intensities as observed for tropy (P~/P~o3) and-stacking fault probability (ct) for each of the samples were corrected for respective back- Wo.65Mo0.3s_xTa#Se2 (0

Table 1. Lattice parameters and other crystallographic data for W0.6sMOo.35_~TaxSe2 (0 ~ x ~ 0-35).

W~r~M%3s_ ~TaxSe2 a (A) c (A) c/a v (A)3

x = 0-00 3.269 + 0-001 12.959 + 0.002 3.959 119,85 x=O.05 3.279+0.001 12-904+0-003 3.934 120.19 x = 0- ! 5 3-294 + 0.002 12.843 + 0.001 3.899 120.68 x=0-25 3.305+0.003 12.832+0.002 3.882 121.39 x = (~35 3-345 + 0.002 12.802 + 0.001 3.827 124.05

Table 2. Microstructural and layer disorder parameters of W0.65Moo.35_xTaxSe2 (0 _

By /'~ ((e2)),/2 p (1015) Wo.65Moo.3s_~Ta.,Se~ (A) (10-3) lines m -2 g ~, p~,2/P~oa a

x = 0.00 280 295 2.69 0.96 0.04 0-001 2.10 0.02 x = 0.05 260 280 3.27 1.40 0.05 0-003 3.11 0.04 x = 0.15 220 250 4.80 2.20 0.10 0.004 4.44 0.08 x = 0.25 275 295 3.09 1- I 1 0.05 0-002 4-83 0.07 x = 0.35 280 310 5.83 1.95 0.02 0-007 I 1.0 0.18 X-ray studies of Wo.65Mo35_xTaxSe 2 107

fiB)

~) I:T

I I I I I I 2 3 /-, S 6 7 8 9 2 3 /. 5 6 7 O rCA) ,.(~)

Figure 2. (A) Radial distribution function (RDF) and (B) diflerential radial distribution function (DRDF) curves for Wo.65Moo.35_xTa,Se2; (a) x=0, (b) x=0.05. (c) x=0.15, (d) x=0.25 and (e) x=0.35.

Table 3. Coupling constant and mean square displacement data of Wo.65Moo.35_xTaxSe2 (0 _

c.c. peak ~2 (/~) peak

Wo.65Moo.35_xTa Se2 I 2 3 4 5 1 2 3 4 5

x=0-00 0-21 0-32 0-35 0-58 1.00 1.73 2.76 1.08 1.08 1.38 x=0.05 0.20 0.38 0.54 0.60 1.00 1-08 1-55 3.00 3.24 5.54 x=0.15 0.25 0.36 0.56 0.56 1.00 !.08 1.55 2.43 2.43 4.32 x=0.25 0.25 0.56 0.56 0.61 1-00 1.08 2.43 2.43 2.65 4.32 x=0.35 0.20 0-43 0.52 0-62 1:00 1-38 3.00 3.63 4.32 6.93

confirmed that the M-Se bond corresponding to this References interatomic distance according to White and Lucovsky (1972) is strong and covalent. The coupling constant Agarwal M K and Wani P A 1979 Mater. Res. Bull. 14 825 Agarwal M K, Patel P D, Patel J V and Kshatriya J D 1984 values are relatively much greater corresponding to M-Se Bull Mater. Sci. 6 549 and Se-Se bonds for the second and third peaks in RDF Bissessur D C, DeGroot J L, Schindler and Kannewurf C R curves suggesting thereby a weak interlayer bonding. 1993 J. Chem. Soc. Commun. 687 These studies also further supported the suggestion of Brixner L H 1963 J. Electrochem. Soc. U0 289 White and Lucovsky (1972) that there exists a Se-Se Brixner L H and Teufer G 1963 Inorg. Chem. 2 992 interlayer bonding. Cromer D T and Weber J T 1971 Acta Crystallogr. 18 73 108 Debashis Palit, S K Srivastava and B K Samantay Ray

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