title: Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites.
abstract: We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.
authors.affiliation: Physics department, Imperial College London, SW7 2AZ, UK. piers.barnes@imperial.ac.uk.
authors.forename: Aurélien M A
authors.initials: AM
authors.lastname: Leguy
authors.forename: Alejandro R
authors.initials: AR
authors.lastname: Goñi
authors.forename: Jarvist M
authors.initials: JM
authors.lastname: Frost
authors.forename: Jonathan
authors.initials: J
authors.lastname: Skelton
authors.forename: Federico
authors.initials: F
authors.lastname: Brivio
authors.forename: Xabier
authors.initials: X
authors.lastname: Rodríguez-Martínez
authors.forename: Oliver J
authors.initials: OJ
authors.lastname: Weber
authors.forename: Anuradha
authors.initials: A
authors.lastname: Pallipurath
authors.forename: M Isabel
authors.initials: MI
authors.lastname: Alonso
authors.forename: Mariano
authors.initials: M
authors.lastname: Campoy-Quiles
authors.forename: Mark T
authors.initials: MT
authors.lastname: Weller
authors.forename: Jenny
authors.initials: J
authors.lastname: Nelson
authors.forename: Aron
authors.initials: A
authors.lastname: Walsh
authors.forename: Piers R F
authors.initials: PR
authors.lastname: Barnes