Exploring the properties of quaternary X₂NaTlF₆ (X = Cs, Rb) halide double perovskite materials for energy conversion, harvesting, and storage using density functional theory

Double perovskites (DPs) have attracted considerable attention for their potential in optoelectronic and thermoelectric applications. In this study, we utilize the WIEN2K code, which relies on density functional theory, to analyze the structural, electronic, mechanical, and optical properties of X2NaTlF6 (X = Cs, Rb). To assess the material's structural durability, we determine the tolerance factor, and thermodynamic stability is examined through formation energy and phonon calculations. Mechanical stability is confirmed through the relationship between elastic constants, specifically C-11 - C-12 > 0, C-11 > 0, C-11 + 2C(12) > 0, and B > 0. Furthermore, an analysis of elasticity using the IRelast package reveals their resilience, anisotropic properties, and mechanical stability. The calculated elastic moduli indicate that Rb2NaTlF6 displays ductile behavior, while Cs2NaTlF6 demonstrates brittleness, depending on the specific elastic constant under consideration. Utilizing the modified Beck and Johnson potential (TB-mBJ), we determine band gaps of 1.23 eV for Rb2NaTlF6 and 1.64 eV for Cs2NaTlF6 DPs. Additionally, we investigate optical properties, including refractive index, dielectric constant, and reflectivity, providing further insights into their behavior. Optical property calculations for both compounds suggest their potential suitability for use in optoelectronic devices, particularly LEDs, owing to their UV-Vis range absorption characteristics.