First-principles analysis of X2InBiF6 (X = Na, K and Rb) fluoroperovskites for optoelectronic and photovoltaic applications

This study investigates the potential of fluoroperovskite compounds X 2 InBiF 6 , where X represents sodium, potassium, or rubidium, for renewable energy applications. Using density functional theory within the CASTEP software, we examined their structural, mechanical, electronic, and optical properties. Structural evaluations yield Goldschmidt tolerance factors of 0.856, 0.935, and 0.960, indicating geometric stability. Mechanical investigations confirm this stability through the calculated elastic constants, revealing the sodium-based Na 2 InBiF 6 as the stiffest compound (Young’s modulus E = 29.41 GPa), while the rubidium-based Rb 2 InBiF 6 is the most flexible (E = 24.47 GPa). Despite these differences, all three compounds exhibit good ductility with high Pugh’s ratios (B/G = 3.16, 2.39, and 3.94, respectively), ionic bonding, and significant anisotropy. Electronic band structure analysis classifies these materials as direct band gap semiconductors, with energy gaps of 1.533 eV for the sodium, 2.308 eV for the potassium, and 2.496 eV for the rubidium variants. Optical analyses, including the complex dielectric function, show strong absorption, with absorption coefficients reaching exceptionally high values between 104.15 × 10 3 cm −1 and 193.68 × 10 3 cm −1 in the visible to ultraviolet regions. This strong optical conductivity is a fundamental requirement for efficient light-harvesting in photocatalytic systems. Taken together, their demonstrated mechanical robustness, favorable direct-gap electronic structure, and strong optical absorption establish the X 2 InBiF 6 compounds as promising foundational materials for renewable energy technologies, specifically targeting photocatalytic water splitting.