The lead-free double perovskites X2CuBiF6 (X = Na, Cs): A promising class of materials for optoelectronic applications

This paper explores the structural, mechanical, electronic, and optical properties of bismuth-based quaternary X2CuBiF6 (X = Na, Cs) double perovskite halides, aiming to identify their potential for optoelectronic applications. This study utilizes a first-principle quantum mechanical approach within WIEN2K rooted in density functional theory (DFT). Structural properties, including lattice parameters and optimization, are analyzed, meeting the Goldschmidt tolerance factor criteria for stability. The electronic properties are investigated through band structures and density of states, revealing narrow and indirect band gaps of 1.18 eV for Na2CuBiF6, and for Cs2CuBiF6, it measured 1.49 eV, signifying semiconductor behavior. The optical properties, examined through dielectric functions, show potential in ultraviolet-light detector and photo-catalysis applications. Mechanical stability is assessed using elastic constants, revealing ductile and tough materials suitable for optoelectronic applications. The calculated anisotropy index, Pugh ratio, and Young's modulus indicate promising ductility and toughness. Poisson's ratio suggests predominantly ionic with partial covalent bonding. Overall, the findings position X2CuBiF6 (X = Na, Cs) double perovskite halides as promising candidates for various optoelectronic applications.