Symmetry-Dependent Mechanical and Vibrational Response of Formamidinium Lead Halide Perovskites: A DFT Study

Abstract

Formamidinium-based hybrid halide perovskites (FAPbX3, X = Cl, Br, and I) have attracted considerable attention for optoelectronic applications owing to their outstanding optical and electronic properties. However, the influence of crystal symmetry reduction on their mechanical behavior and stability has not yet been comprehensively understood. In this work, density functional theory (DFT) calculations were performed to investigate the structural, elastic, dynamical, and nonlinear mechanical properties of the cubic and ps-cubic phases of FAPbX3. The elastic constants, bulk, shear, and Young's moduli, Poisson's ratio, sound velocities, and Debye temperature were evaluated and correlated with the second Piola-Kirchhoff stress-strain response under tensile and compressive loading. The results reveal that the effect of symmetry reduction is strongly dependent on the halide composition. For FAPbCl3 and FAPbBr3, the transition from the cubic to the ps-cubic phase reduces the lattice stiffness, decreases the acoustic phonon velocities, and lowers the Debye temperature, whereas the opposite trend is observed for FAPbI3. The stress-strain analysis further reveals pronounced nonlinear, anisotropic, and asymmetric mechanical behavior, demonstrating that symmetry reduction can either activate or suppress strain-accommodation mechanisms depending on the halide species, thereby governing the mechanical stability and the onset of structural softening. These findings provide microscopic insight into the relationship between crystal symmetry, lattice dynamics, and nonlinear mechanical response in formamidinium-based halide perovskites, offering useful guidance for the design of mechanically robust optoelectronic materials.