Thermopower across the insulator-metal divide inNiCr2−xVxS4(0⩽x⩽2)

Abstract

Measurements of the Seebeck coefficient of phases in the series ${\mathrm{NiCr}}_{2\ensuremath{-}x}{\mathrm{V}}_{x}{\mathrm{S}}_{4}$ have been carried out over the temperature range $90\ensuremath{\leqslant}T∕K\ensuremath{\leqslant}700$. The data demonstrate that with increasing levels of vanadium incorporation, there is a change from semiconducting to metallic behavior at a critical composition $0.4\ensuremath{\leqslant}{x}_{c}\ensuremath{\leqslant}0.5$. This change in the nature of the electronic states is coincident with a structural distortion, previously identified by powder neutron diffraction, involving the formation of zigzag cation chains within a dichalcogenide slab. Analysis of the temperature dependence of the thermopower in the semiconducting and metallic regions suggests that the transport properties are strongly affected by electron-phonon coupling, which for $x\ensuremath{\leqslant}0.2$ leads to a small-polaron hopping conduction mechanism. Analysis of the conductivity data yields activation energies of 28.1(1) and 60.3(3) meV for $x=0.0$ and 0.2, respectively.