Structural, electronic and thermal properties of TexCo4Sb11.75Te0.25

Abstract

The aim of this work is the theoretical and experimental study of the influence of Te, introduced in two Wyckoff positions (2a and 24g) of the CoSb3 skutterudite structure, on the electronic and thermoelectric properties of the material. The samples with the nominal composition of TexCo4Sb11.75Te0.25, where x = 0, 0.125, 0.25, 0.5 have been prepared using FAST and HPHT methods. Powder XRD analysis confirmed that Te can be introduced into both considered structural positions at high pressure of 7.8 GPa and can partially substitute Sb, as well as occupy the void at 2a site even if low pressure of 30 MPa is applied. Electrical and thermal conductivities vary with the increase of Te content up to x = 0.25 and are almost invariant with further doping with Te. Changes in transport properties well correspond with alternations of the lattice parameter a. The electronic transport properties are coherently explained through electronic structure calculations results. Two opposing effects of Te doping were found, depending on the doping sites, since tellurium behaves either as a one-electron donor (24g site) or two-electron acceptor (2a site). The filling of structural voids with Te results in a decrease of the lattice thermal conductivity and the improvement of the power factor (α2σ). The combined effects lead to enhancement of thermoelectric figure of merit to zT = 1.02 at 673 K for Te0.25Co4Sb11.75Te0.25.