Defect studies by X-ray diffraction, electrical and optical properties of layer type tungsten mixed molybdenum sulphoselenide

Layer type tungsten mixed molybdenum sulphoselenide, Mo0.5W0.5SxSe2−x (0≤x≤2) starting from ternary Mo0.5W0.5Se2 compound and by substituting selenium with sulphur ended with Mo0.5W0.5S2 which is also a ternary compound, have been prepared and characterized. X-ray diffraction studies showed that all the compounds possess 2H-MoS2 structure with a small change in a- and c-parameters. X-ray line profile analysis has been used to calculate the microstructural defect parameters of all these compounds. It showed that sulphur and selenium rich compounds possess relatively small crystallite size (P), high r.m.s. strain (〈e2〉)1/2 and dislocation density (ρdis). However, Mo0.5W0.5SSe has large P, low (〈e2〉)1/2 and (ρ). Fractional change in interlayer spacing and proportion of the planes affected by defects increased with increasing sulphur contents in Mo0.5W0.5Se2 and it was maximum in case of Mo0.5W0.5S2. Radial distribution function (RDF) analysis showed that interatomic distances, 2.80, 3.40 and 3.86 Å corresponds to metal-chalcogen, for intra and interlayer chalcogen-chalcogen bonding, respectively. The coupling constants corresponding to these and other higher pair of atoms have also been calculated. Room temperature thermoelectric power experiments temperature variation of conductivity confirmed their p-type semiconducting behaviour. Attempts have also been made to correlate the conductivity of these compounds with their size of the crystallites. The band gaps of these compounds are in the range of 1.20 eV for Mo0.5W0.5S2 to 1.07 eV for Mo0.5W0.5Se2, respectively.