Elastic properties and thermal expansion of lead-free halide double perovskite Cs2AgBiBr6

Abstract

The elastic properties and thermal expansion behavior of lead-free halide double perovskite, Cs2AgBiBr6, were studied first with the aid of first principles calculations, which were followed by experimental characterization. The calculated full elastic constants enable the complete mapping of Young’s and shear moduli, and Poisson’s ratios of Cs2AgBiBr6 along all crystallographic orientations. Results show that significant anisotropy in them despite the fact that Cs2AgBiBr6 possesses cubic symmetry. Detailed structural analysis and a spring model were utilized to rationalize the observed anisotropy. Further, the theoretical results were validated by recourse to nanoindentation and high-pressure synchrotron powder X-ray diffraction experiments. The superior mechanical behavior of Cs2AgBiBr6 over the hybrid bromide analogues, MAPbBr3 (MA=CH3NH3+), were attributed to the higher framework stiffness of the former, which is a result of the presence of relatively stronger Ag-Br and Bi-Br bonds in it. Variable temperature single crystal X-ray diffraction reveal linear and volumetric thermal expansion coefficients to be about 20% lower than those of MAPbBr3, again suggesting its higher framework rigidity. The superior stiffness, thermal expansion behavior, moisture stability, and nontoxicity of Cs2AgBiBr6 make it a strong candidate for potential applications in optoelectronics and photovoltaics.