Composition-induced metal-semiconductor-metal crossover in half-HeuslerFe1−xNixTiSb

Abstract

Structural and electrical transport properties of half-Heusler ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{TiSb}$ solid solutions have been investigated using x-ray diffraction, ${}^{57}\mathrm{Fe}$ M\ossbauer spectroscopy, resistivity, and thermopower measurements. The electronic structure of the compounds was studied using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA). The resistivity curves measured over the 80--800 K temperature range show a remarkable modification of $\ensuremath{\rho}(T)$ versus x in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{TiSb},$ i.e., a metalliclike character both in Fe-rich $(x=0.25)$ and Ni-rich $(x=0.75)$ samples and a semiconductinglike behavior near $x=0.5.$ Moreover, the room-temperature Seebeck coefficient S changes from highly positive in ${\mathrm{Fe}}_{0.6}{\mathrm{Ni}}_{0.4}\mathrm{TiSb}$ to highly negative in ${\mathrm{Fe}}_{0.4}{\mathrm{Ni}}_{0.6}\mathrm{TiSb},$ while S is near zero in the $x=0.25$ and $x=0.75$ compositions. Based on the KKR-CPA results, the electrical transport properties of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{TiSb}$ arise from the appearance of an energy gap at the Fermi level for the $x=0.5$ content. This electronic structure feature indicates that FeTiSb and NiTiSb can be seen as one-hole and one-electron compounds, respectively, explaining a change in sign of the Seebeck coefficient in disordered phases near $x=0.5.$