Stress/pressure-stabilized cubic polymorph of Li3Sb with improved thermoelectric performance

Abstract

Li₃Sb has two polymorphs crystallizing in a face-centered cubic cell (c-Li₃Sb; BiF₃ structure type; space group Fmm) and in a hexagonal unit cell (h-Li₃Sb; Na₃As structure type; space group P6₃/mmc). c-Li₃Sb was predicted to be a promising thermoelectric material based on recent first-principles studies; however, the experimental transport characteristics have remained unknown so far. Herein, successful preparation of c-Li₃Sb is reported by stress-induced mechanochemical synthesis (high-energy ball milling) along with its high-temperature thermoelectric properties. Hexagonal Li₃Sb (h-Li₃Sb) was revealed to be the stable phase at ambient conditions, while it starts unexpectedly transforming to c-Li₃Sb by ball milling or under 60 MPa applied pressure at room temperature. The transport properties measurements performed on two polycrystalline specimens evidence that c-Li₃Sb behaves as a p-type degenerate semiconductor due to the formation of Li vacancies. In agreement with lattice dynamics calculations, c-Li₃Sb exhibits very low lattice thermal conductivity despite the lightweight of Li. A zT value of around 0.3 was obtained at 550 K. Modelling suggests that the hole concentration should be reduced through aliovalent substitutions or under Li-rich conditions for further optimization. Although the strong air sensitivity of Li₃Sb makes its use in thermoelectric applications challenging, this simple superionic binary provides an attractive experimental platform to elucidate the effect of stress/pressure on phase transitions as well as that of Fermi surface complexity on thermoelectric properties.