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We present Raman and terahertz absorbance spectra of methylammonium leadhalide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and370 K. These results show good agreement with density-functional-theory phononcalculations.1 Comparison of experimental spectra and calculated vibrationalmodes enables confident assignment of most of the vibrational features between50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked intheir cavities at the orthorhombic-to-tetragonal phase transition, plays a keyrole in shaping the vibrational spectra of the different compounds.Calculations show that these dynamics effects split Raman peaks and create morestructure than predicted from the independent harmonic modes. This explains thepresence of extra peaks in the experimental spectra that have been a source ofconfusion in earlier studies. We discuss singular features, in particular thetorsional vibration of the C-N axis, which is the only molecular mode that isstrongly influenced by the size of the lattice. From analysis of the spectrallinewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes,which can be linked to low lattice thermal conductivity. We show that opticalrather than acoustic phonon scattering is likely to prevail at room temperaturein these materials.

Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites