The electrical and thermal conductivity and thermopower of nickel doped compounds (NixTi1−x)1+yS2 at low temperatures

Abstract

Nickel doped compounds (NixTi1−x)1+yS2 (0 ⩽ x ⩽ 0.06) were prepared by solid-state reaction, and their dc electrical and thermal conductivity and thermopower were investigated from 5 K to 310 K. The results indicated that Ni doping caused a metal-like to semiconductor-like behaviour transition; at low temperatures (T < ∼100 K) dc electrical conduction σ for (NixTi1−x)1+yS2 (x > 0) obeys Mott's 2D variable range hopping law, ln σ ∝ T−1/3, indicating that TiS2 possesses 2D transport characteristics. The appearance of Mott's 2D law could originate from potential disorder introduced by Ni substitution for Ti in S–Ti–S slabs, while the metal-to-semiconductor transition can be ascribed to de-degeneration through reduction in electron concentration due to Ni substitution. Experiments also indicated that both lattice thermal conductivity and carrier (mainly electron) thermal conductivity of the doped compounds decreased upon doping, which can be explained as the combined effects of substitution with intercalation of Ni and reduction of carrier concentration upon doping, respectively. The absolute Seebeck coefficient |S| was found to decrease after doping, which could be attributed to generation of some holes after Ni substitution for Ti. The figure of merit, ZT, of the doped compounds (NixTi1−x)1+yS2 (x > 0) decreased as compared with TiS2 due to both a large increase in their resistivity and an obvious decrease in their Seebeck coefficient.