Stress effects on vibrational spectra of orthorhombic and tetragonal hybrid perovskites.*

Strain plays an important role in semiconductor performance and stability. Strains may develop in organic metal-halide perovskites which affect carrier mobility, non-radiative recombination, degradation and other optoelectronic properties. Measuring spatially varying strains is difficult but imperative for understanding these effects. We have used DFT to investigate effects of applied strain on the vibrations of tetragonal and orthorhombic methylammonium lead iodides (MAPI), building on our previous study of the high-temperature pseudo-cubic phase [arXiV:1907.03673]. Applying small uniaxial strains along three crystal axes, we have analyzed changes in frequency and phonon displacement patterns. We identified favorable modes for experimental measurements of local strain by Raman microscopy, and we calculated mode Grüneisen and Grüneisen parameters in different directions to connect with reports of negative thermal expansion in c-direction. Our study gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand degradation