First-principles design of a lead-free Cs2LiFeCl6 double perovskite for solar-driven hydrogen evolution and CO2 reduction

Abstract

The development of lead-free, stable, and earth-abundant materials for solar energy conversion remains a critical challenge for next-generation photovoltaics and photocatalysis. Herein, we present a comprehensive first-principles investigation of the cesium-based transition-metal double perovskite Cs₂LiFeCl₆ using density functional theory with an on-site Hubbard correction. The compound crystallizes in a cubic elpasolite structure, supported by favorable Goldschmidt (1.038) and one-dimensional (2.205) tolerance factors. Thermodynamic stability is confirmed by a negative formation energy and a positive decomposition energy, while phonon dispersion calculations reveal the absence of imaginary modes, establishing dynamic stability. Mechanical analysis shows compliance with Born stability criteria and a ductile nature, with a Pugh's ratio of 1.81 and a Poisson's ratio of 0.27, indicative of predominantly ionic bonding. Spin-polarized electronic structure calculations reveal a semiconducting ground state with a Hubbard-corrected band gap of 1.578 eV, where both the valence and conduction band edges are dominated by the spin-down channel arising from Fe-3d and Cl-3p hybridization. Importantly, band-edge alignment relative to the vacuum level demonstrates that Cs₂LiFeCl₆ satisfies the energetic requirements for photocatalytic hydrogen evolution over a broad pH range and shows favorable band-edge alignment for photocatalytic CO₂ reduction half-reactions associated with CH₃OH and CH₄ production. These combined structural robustness, favorable optoelectronic characteristics, and dual photocatalytic functionality establish Cs₂LiFeCl₆ as a promising lead-free platform for integrated solar-to-fuel and photocatalytic energy-conversion applications.