Influence of Vanadium on the Defect Structure and Thermoelectric Properties of GeTe

The development of new thermoelectric materials based on GeTe is associated with reducing the hole concentration and thermal conductivity. The objects of the present study are GeTe-based solid solutions in the Ge-V-Te ternary system. The goal of the work is to study the character of the change in the structure, mechanical and thermoelectric properties of GeTe under introduction of VTe. The electrical conductivity σ, Seebeck coefficient S, and Hall coefficient RH were measured in the range of 300 K to 850 K on cast samples and samples prepared by hot pressing; the thermal conductivity λ was measured at room temperature. It was found that the dependences of the unit cell parameters, microhardness, σ, RH, λ, and S on the VTe content exhibit nonmonotonic behavior. The experimental results were interpreted taking into consideration the complex mechanisms of defect formation in the GeTe crystal lattice under introduction of VTe, the existence of nonstoichiometric vacancies, and percolation effects. It was established that introduction of VTe into GeTe leads to a significant decrease in λ and hole concentration p. The maximum room-temperature values of thermoelectric power factor P and thermoelectric figure of merit Z corresponded to ~2 mol.% VTe. With increasing temperature up to ~550 K, P increases, and the maximum value of P is shifted to ~3 mol.% VTe. The values of P and Z obtained for the cast and pressed samples were practically the same.