An IntricateBalance of Ionicity and Covalency: Metal-LikeConduction in All-Inorganic Halide Double Perovskite Cs₂AgSbCl₆

Halide perovskites have recently evolved as attractive materials with enormous technological significance due to synthetic control over the structure–property relationship. Halide perovskites are often realized to be either electrical insulators or semiconductors. We present an unusual metal-like conduction (thermally deactivated) in a Pb-free all-inorganic halide double perovskite, Cs₂AgSbCl₆. The experimental results were understood using density functional theory studies, combined with molecular dynamics simulations and electron localization function calculations, revealing retention of the predominant ionicity of the Ag–Cl bond and an increase in the covalency of the Sb–Cl bond at an elevated temperature, which resulted in a significant change of the electronic band structure, including the density of states, thereby exhibiting an intricate balance of ionicity and covalency. A significant modulation of the electrical conductivity (more than 3 orders of magnitude) without any noticeable structural change will stimulate the investigation of hitherto unknown electronic phase transitions in halide double perovskites. Additionally, light-induced unidirectional rectification of current in Cs₂AgSbCl₆ was ascribed to a dynamic internal polarization effect.