Insights on structural, elastic, electronic and optical properties under pressure of Cs-based fluoroperovskite CsMF3 (M=Ge, Sn, and Pb) compounds

This paper examines the structural, elastic electronic, and optical properties of CsMF3 ( M = Ge, Sn, and Pb ) perovskites under pressure using density functional theory. The elastic constants for CsGeF3, and CsPbF3 at zero pressure align well with literature, while this study offers the first estimation of elastic properties of CsSnF3. At zero pressure, these compounds are mechanically stable and ductile, exhibiting direct band gap of 2.86 eV for CsGeF3, 2.09 eV for CsSnF3, and 4.61 eV for CsPbF3. Increasing pressure reduces unit cell volumes and reveals mechanical instability, with potential phase transitions occurring at 28.6 GPa, 11.42 GPa and 8.68 GPa for CsGeF3, CsSnF3, and CsPbF3, respectively. CsMF3 show increased ductility and reduced hardness with pressure. As pressure increases, the band gap decreases, leading to a shift in the absorption threshold to lower energies. For both CsGeF3 and CsSnF3, this shift results in a significant increase in absorption within the visible radiation range. Using quadratic fitting function, we predict electronic transitions to metallic states at 37.8 GPa, 24.6 GPa and 57.9 GPa for CsGeF3, CsSnF3, and CsPbF3, respectively. However, these metallisation pressures indicate mechanical instability, necessitating cautious interpretation and re-evaluation of previous studies considering metallisation and material stability.