Eyidence of microstructural modification in amorphous molybdenum sulphides obtained from d.c. and a.c. conductivities and electron spin resonance measurements

Amorphous molybdenum sulphides (MoS2 +x 0 ⩽ x ⩽ 1) have been prepared by thermal decomposition of synthetic ammonium thiomolybdate. Heat treatment of this compound leads to amorphous sulphides, the composition of which varies from MoS3 to MoS2. This evolution is followed by means of d.c. and a.c. conductivities as well as by electron spin resonance (ESR). The polarization conductivity is interpreted using a theoretical model, based on the transport of the electrical current by means of single-electron hops between localized sites. This approach leads to the determination of the potential energy W M of the electron trapped in its site, and the number N of sites involved in the conduction process. The evolution of W M and N as a function of the heat treatment temperature (TT) shows a pseudotransition corresponding to the values of composition near MoS2.6.(TT = 315°C). The ESR measurements confirm this behaviour, and the observed signals show sharp modifications at the same composition and at the same number of paramagnetic species. The appearance in these solids of such a pseudotransition can be explained assuming a microstructural modification which corresponds to the environment of the sites near the composition MoS2.6. In the composition domain from MoS2.7 to MoS2.4, a hypothetical new structure is proposed.