Vibrational, optical and elastic properties of Cs2AgBiX6 (X= Br, Cl) double perovskite materials: DFT, PL, Raman and Brillouin scattering studies

In this study, we explore the temperature-dependent Raman, photoluminescence (PL) and Brillouin spectroscopy, as well as Density Functional Theory (DFT) calculation results of Cs ₂AgBiBr ₆ and Cs ₂AgBiCl ₆ double perovskites. Single crystals of Cs ₂AgBiBr ₆ and Cs ₂AgBiCl ₆ were synthesized via a hydrothermal method. The lattice structure has been clearly defined, and the electronic band structures, validated by DFT calculations, are consistent with previously reported data. Optical properties calculated via DFT approach indicate superior energy storage capacity for Cs ₂AgBiBr ₆. Changes in optical properties at room temperature suggest potential suitability for specific applications, ranging from photovoltaics to lasers. Raman and Brillouin spectroscopy have revealed significant insights regarding the vibrational properties and atomic interactions within the materials, thereby contributing to a better understanding of the atomic interactions within the perovskite structures. Experimental data has highlighted anomalies in the Raman shifts, indicating the presence of a phase transition in Cs ₂AgBiBr ₆ at 115 K. However, no such phase transition was observed in Cs ₂AgBiCl ₆. This observation is further supported by Brillouin spectroscopy. PL measurements on Cs ₂AgBiBr ₆ revealed the presence of a low-temperature phase transition at 35 K. DFT calculations also provide detailed insights into the elastic properties of these perovskites, notably indicating superior mechanical strength for Cs ₂AgBiCl ₆. These findings will enhance the understanding of perovskite materials and expand their potential technological applications, especially in environments with varying thermal conditions