Measuring Phonon Lifetimes and Lattice Dynamics of Single-Crystal Methylammonium Lead Iodide with Neutron Triple-Axis Spectroscopy

There is growing interest in the fundamental physical properties of the photovoltaic absorber methylammonium lead iodide (CH₃NH₃PbI₃). In particular, the phonons provide information about the dynamical coupling of electronic states to the lattice and the vibrational coupling between PbI₆ octahedra and the MA⁺ cations, both of which may influence the opto-electronic properties. We report results from neutron inelastic scattering measurements on fully-deuterated single crystals of CD₃ND₃PbI₃ to characterize the low-energy lattice dynamics.

We have measured the momentum-resolved transverse acoustic (TA) phonon dispersion and lifetimes in the orthorhombic, tetragonal, and cubic phases. The TA phonon becomes strongly damped with increasing wave vector, and on heating into the tetragonal phase the TA phonon lifetimes decrease by roughly a factor of two for wave vectors near the Brillouin zone boundary. This has major implications for the carrier scattering and cooling processes.

In addition, we observe quasi-elastic scattering (QES) that increases in intensity upon heating into the tetragonal phase. This QES varies strongly with reduced wave vector and corresponds to the coupled dynamics of the MA⁺ cation and PbI₆ octahedral distortions over nm-scale domains and a few-ps timescale.