Interplay of structural changes, symmetry and local interactions in orthorhombic and tetragonal hybrid perovskites under stress

Local stress may develop in organic metal-halide perovskites which affects different optoelectronic properties and can play a major role in their performance and stability. Previously we have studied the high-temperature pseudo-cubic phase of CH₃NH₃PbI₃ [arXiV:1907.03673]. In this work, we analyzed the vibrations of low-temperature orthorhombic and room-temperature tetragonal hybrid perovskite phases under uniaxial strain using density functional perturbation theory. We have identified specific IR- and Raman-active modes with large linear shifts that may be useful to measure strain within these perovskite materials using spectroscopy. Considering exact and approximate symmetries, we have analyzed changes in the structural parameters, phonon displacement patterns, and dynamical matrices, to understand changes in frequency from a perturbative approach. We also identify which atomic interactions play a significant role in change of vibrational modes under strain. Additionally, we calculate the full stiffness tensor for both the phases to understand their mechanical properties. Our study gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand photovoltaic performance and degradation.