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 Fm3m) and in a hexagonal unit cell (h-Li3Sb; Na3As structure type; space group P6(3)/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.