Investigation of the properties of lead-free Cs₂NaBiX₆ (X = I and Br) double perovskites using density functional theory (DFT)

In this comprehensive study, we investigate the properties of Cs₂NaBiX₆ (X = I and Br) lead-free double perovskites using density functional theory (DFT). This is the first in-depth DFT-based analysis that delves into the substantial characteristics of these materials, addressing a significant gap in the existing literature. Our analysis focuses on the optical and electronic properties of these materials. Additionally, we examine the impact of strain (tensile and compressive) on their characteristics. The negative values of formation energy and lack of negative frequencies in phonon dispersion band structures provide evidence for the thermodynamic and dynamic stability of considered compounds, respectively. The results reveal that Cs₂NaBiBr₆ is the most thermodynamically and moisture-stable variant, as corroborated by the narrower energy fluctuation range, indicating superior thermal stability. The adsorption energy of water molecule on Cs₂NaBiBr₆ and Cs₂NaBiI₆ are − 0.109 and − 0.114, respectively.Using PBE approximent, bandgap values for Cs₂NaBiBr₆ and Cs₂NaBiI₆, are 2.09 and 3.14 eV, respectively. Furthermore, the DFT analysis suggests that the mechanical moduli increase as the halide changes from iodide to bromide. According to the computed Pugh's ratio, these materials are ductile. These findings highlight the promising potential of lead-free double perovskites Cs₂NaBiX₆ (X = I and Br) for applications in optoelectronic devices.