Pressure‐Induced Electronic and Dynamical Instability IncubicAgMgX 3 (X = F, Cl, Br) Halide Perovskites at High Pressure: A First‐Principles Study
Yu. N. Zhuravlev, Z.Y. KhattariABSTRACT
A systematic study of the structural, electronic, elastic, and vibrational properties of cubic halide perovskites AgMgX 3 (X = F, Cl, Br) under hydrostatic pressure (0–50 GPa) was performed using Density functional theory (DFT) with two codes: the all‐electron Gaussian‐basis‐set code CRYSTAL for the electronic and spectroscopic properties, and the plane‐wave pseudopotential code CASTEP for the vibrational and thermodynamic properties. Lattice constants increase from F to Br, while bulk moduli decrease from 92.31 to 34.26 GPa. The bandgap of AgMgF 3 increases with pressure, whereas it decreases for AgMgCl 3 and AgMgBr 3 . This opposite behavior arises from different valence‐band characters: Ag d‐states in AgMgF 3 vs. halogen p‐states in the chloride and bromide. Full Brillouin‐zone phonon dispersions reveal that the cubic phase of all three compounds is dynamically unstable, with an R‐point soft mode corresponding to out‐of‐phase rotations of the MgX 6 octahedra. For AgMgF 3 , the rhombohedral phase is lower in enthalpy than the cubic phase by 54–118 meV per formula unit, confirming it as the ground state. For AgMgCl 3 and AgMgBr 3 , the relaxation does not converge to a well‐defined minimum, so their ground states remain undetermined. AgMgF 3 is promising for deep UV photodetectors, provided its rhombohedral polymorph is stabilized.