A DFT study of Mg/Ti Fluorites-Structure Materials: Optoelectronic, Photocatalysis, and Hydrogen storage applications

This study employs density functional theory (DFT) to investigate the fluorite-derived tetragonal hydride MgTiH4, highlighting its multifunctionality for energy-related applications. Structural optimization reveals lattice constants of a = b = 3.30 Å and c = 5.24 Å, with a unit cell volume of 56.88 Å3 and a high bulk modulus of 114.21 GPa, indicating mechanical robustness. Compared to its CaTiH4 and SrTiH4 analogs, MgTiH4 exhibits the highest phonon frequency (1252.60 cm-1) and superior thermodynamic stability (formation enthalpy: –2.66 eV). Electronic band structure analysis reveals a narrow indirect band gap of 0.089 eV, suggesting semiconducting behavior with potential for low-light optoelectronics. Photocatalytic assessment shows that the conduction and valence band edges of MgTiH4 (1.185 V and 1.275 V vs. NHE) lie above water redox potentials, precluding spontaneous water splitting, though visible-light activation remains viable. Optical studies highlight strong UV absorption (up to 2.92 × 105 cm-1), high refractive index (7.81), and a pronounced dielectric constant (ε1 = 59.25), indicating excellent light-matter interaction. Gravimetric hydrogen storage capacity reaches 5.291 wt%, approaching DOE targets, while post-desorption studies confirm good structural reversibility and reduced bulk modulus (49.24 GPa), ensuring feasible hydrogen cycling. In conclusion, MgTiH4 outperforms CaTiH4 and SrTiH4 in multiple metrics, establishing it as a multifunctional candidate for future energy and photonic technologies