Pressure-Induced Indirect-to-Direct Band Gap Transition and Tunable Deep-UV Response in CsCaF3 Perovskite

This study presents a comprehensive first-principles investigation of the structural, elastic, electronic, and optical behavior of cubic CsCaF3 under hydrostatic pressure. The material is confirmed to be a stable Pm-3m fluoride perovskite, with a lattice constant of 4.496 Å and a tolerance factor of 0.902. At ambient conditions, CsCaF3 exhibits high intrinsic stiffness (C11=107.88 GPa, B=53.07 GPa, G=29.16 GPa, E=73.94 GPa) and maintains mechanical stability while becoming progressively stiffer under compression. The electronic structure reveals a wide indirect band gap of 7.1 eV that broadens to 8.43 eV and transforms into a direct gap at elevated pressures. Optical calculations show strong transparency in the visible range, with a low refractive index (1.58) and reflectivity (~5%), and a deep-UV absorption edge near 6 eV. Pressure enhances these features, increasing the refractive index to 1.66 and the maximum reflectivity to 45.87% at 24 GPa. The plasmon resonance also displays pronounced tunability, blue-shifting from 29.56 to 30.79 eV with a fourfold rise in intensity. Analysis of the effective-electron number further indicates pressure-driven redistribution of spectral weight within the UV region. Together, these findings demonstrate that CsCaF3 combines robust structural stability with highly pressure-tunable optical and plasmonic responses, positioning it as a promising candidate for deep-UV optoelectronics, photonic coatings, and pressure-responsive optical technologies.