The cubic structure of Li3As stabilized by pressure or configurational entropy via the solid solution Li3As–Li2Se

Abstract

The hexagonal to cubic phase transition of Li₃As was investigated at high pressure and temperature, revealing a cubic high-pressure polymorph in the Li₃Bi structure type. This cubic structure type is preserved in the solid solution of Li₃As–Li₂Se synthesized via mechanochemical ball milling. The solid solutions were investigated via X-ray powder diffraction, showing a linear dependency of the lattice parameter a on the mole fraction of the boundary phases Li₃As and Li₂Se, according to Vegard's law. Configurational entropy is generated by mixed anion lattice occupation between arsenide and selenide and therefore stabilizes the cubic structure of the solid solution. At elevated temperatures, the solid solution of Li₃As–Li₂Se reveals an exsolution process by forming the boundary phases Li₃As and Li₂Se, proving the metastable character of the system. Impedance spectroscopy was used to determine the lithium-ion conductivities in the Li₃As–Li₂Se system, showing significantly higher conductivity values (∼10⁻⁴ to 10⁻⁶ S cm⁻¹ at 50 °C) compared to the pure end members Li₃As (∼10⁻⁷ S cm⁻¹ at 50 °C) and Li₂Se (∼10⁻⁷ S cm⁻¹ at 175 °C).