Ideal strength and deformation mechanism in inorganic halide double perovskite Cs₂AgBiBr₆

Inorganic halide double perovskites have been successfully applied in various optoelectronic devices. Due to their soft metal-halide bonds and intrinsically brittle nature, these devices are prone to mechanical damage. However, studies on the impact of shear strain on their structure and properties are currently scarce. Here, we investigated the intrinsic mechanical and electronic properties of Fm3̄m phase Cs₂AgBiBr₆ under shear deformations. The results reveal that under different shear loads, the primary structural deformation occurs in the [AgBr₆] octahedra due to the very weak ionic Ag-Br bonds. The lowest ideal strength of Cs₂AgBiBr₆ under the (111)[11̄0] shear load is comparable to that of highly ductile metals (such as Ag) and layered semiconductors (such as InSe). Additionally, this shear strain leads to elongation of the Ag-Br bond lengths in Cs₂AgBiBr₆, thereby resulting in a slight increase in the material's band gap. More interestingly, shear strain along the (111)[11̄0] slip system induces the rotation of [AgBr₆] and [BiBr₆] octahedra, resulting in a phase transition from Fm3̄m to I4/m. The I4/m phase exhibits an increased band gap value and a reduced Fröhlich polaron coupling constant compared to the Fm3̄m phase. Our study provides significant insights into the atomic structure and optoelectronic properties of halide double perovskites under shear loading, which are critical for their practical applications.