The dynamics and structure of methylammonium ions in hybrid halide perovskites; a combined neutron scattering and first-principles study

We study the influence of the methylammonium cations on the structure and dynamics of CH$_3$NH$_3$PbI$_3$ through a combined first-principles computation and neutron scattering approach. Temperature-dependent powder diffraction indicates phase transitions near 160 K and 300 K, and the lattice parameters, bond angles, and atomic positions are determined. Density-functional theory (DFT) calculations confirm that the most stable form of the orthorhombic phase is non-polar, although the energy difference compared to ferroelectric orientations is only a few meV per atom. Inelastic neutron scattering (INS) spectra are compared to first-principles computations, and significant multi-phonon scattering is observed. Computations predict that lattice dynamics are sensitive to the orientation of the dipoles, and suggest that vibrational spectroscopy may be used as a probe of the local dipole ordering. In particular, the twisting mode along the C-N bond is a single sharp peak near 39 meV for the non-polar orientation and in our INS measurements, but splits into separate peaks for certain ferroelectric orientations.