Thermopower across the insulator-metal divide in NiCr2-xVxS4 (0 2)

Measurements of the Seebeck coefficient of phases in the series NiCr _2-xV_xS₄ have been carried out over the temperature range 90=T/K=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=x_c=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=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. ©2005 The American Physical Society.