Thermodynamic, quantum efficiency, and photocatalytic properties of Cs2MZ6 (M = Ti, Zr, Hf; Z = Cl, I) double perovskites using DFT approach

Halide double perovskites have recently received considerable attention owing to their promising properties for a range of technological demands. Compounds Cs2MZ6 (M = Ti, Zr, Hf; Z = Cl, I) have been studied individually through experimental synthesis and computational modeling. Nevertheless, comprehensive comparative analyses of their optical characteristics, quantum efficiency, and photocatalytic potential for water splitting remain limited. In this study, the structural, electronic, optical, quantum efficiency, and photocatalytic properties for water splitting of Cs2MZ6 (M = Ti, Zr, Hf; Z = Cl, I) phases are systematically investigated via the full-potential linearized augmented plane wave plus local orbitals technique. The calculated band structures indicate a gradual decrease in the bandgap from Cl to I substitution, with most variants exhibiting indirect and relatively wide gaps. “The frequency-dependent optical properties, including the real and imaginary parts of the dielectric function, absorption coefficient, and optical conductivity, are systematically calculated up to 12 eV. The results reveal pronounced absorption peaks in the ultraviolet region, highlighting the potential of these compounds for high-frequency optoelectronic applications.” Thermodynamic stability was assessed via calculations of Gibbs free energy (kJ/mol) and entropy (S, J mol−1 K−1) as functions of temperature and pressure. The results consistently exhibit negative Gibbs free energies and rising entropy, indicating robust thermal stability. Furthermore, the computed band edge positions indicate potential for photocatalytic water splitting, with Cs2ZrI6 and Cs2HfI6 showing particularly favorable alignment for both hydrogen and oxygen evolution reactions under solar irradiation.