First principles calculations to investigate structural, mechanical, electronic, optical and magnetic properties of LiGeX3 (X = Cl, Br, and I) perovskite under pressure effect

The lead-free germanium-based cubic perovskite LiGeX3 (X = Cl, Br, and I) under pressure shows potential as a promising candidate to rival traditional silicon-based perovskites for solar cell applications. This investigation utilizes pressure ranging from 0 GPa to 5 GPa and employs three different functional approximations GGA-PBE, GGA-PBESOL, and m-GGA-RSCAN within the framework of density functional theory (DFT) calculations, performed using the CASTEP module. The length and strength of the Li-X and Ge-X (Sn-X) bonds decrease with pressure. All materials exhibit increased ductility under pressure, while LiGeI3 shows lower ductility at 0 GPa. The band gap values calculated using the GGA-PBE functional exhibit a direct band gap at the R-R point, ranging from 0.849 eV to 0.242 eV for LiGeCl3, 0.617 eV to 0.154 eV for LiGeBr3, and 0.470 eV to 0.030 eV for LiGeI3. Higher pressure shows a linear downfall of the band gap which shifts the materials to a more metallic behavior and increases the density of electronic states at the Fermi level by moving up valance band electrons. Moreover, the reflectivity, dielectric constant, Loss function, and absorptivity values exhibit an upward trend with pressure, with absorption spectra demonstrating a redshift as pressure increases. These results reveal that LiGeX3 (X = Cl, Br, and I) becomes more favorable for optoelectronics, photovoltaics, thermoelectric, pressure sensors, photo- catalysis, and energy storage due to their tunable and dynamic properties.