Hybrid perovskite materials CH₃NH₃PbI₃ (MAPI) and CH₃NH₃PbI_3-xCl_x (MAPIC) are used as optically active components in high efficiency solution processed solar cells. Within the perovskite crystal structure the methyl ammonium (MA) ions are caged between lead halide octahedra. The MA ions are electrical dipoles which have the potential to contribute to ferroelectric properties of the material. These ions are also speculated to play a critical role in the stability and hysteresis of MAPI and MAPIC photovoltaic devices. 

We present neutron diffraction and quasi-inelastic neutron scattering (QENS) data to examine the structure and behaviour of the MA ions in MAPI and MAPIC for the temperature range 7 – 380 K. Neutron diffraction allows the crystal positions of hydrogen nuclei to be determined which cannot be easily achieved using X-ray diffraction. Two crystalline phase transitions are observed at 150-170 K and ~ 330-350 K. QENS focusses on the dynamic motion of hydrogen nuclei within the structure. 

The data could be consistent with the rotation of the hydrogen ions around the C-N axis. A second rotational mode is observed above 140 K which can be attributed to reorientation of the C-N axis with respect to the crystal. Activation energies for these rotational movements are estimated, and the residence times in the possible orientations are obtained.

The inferred active fraction of rotating MA is analysed. The proportion of CH₃-rotors undergoing reorientation around the C-N axis increases linearly with temperature, which could be consistent with the reported H-bonds between MA and the halides of the inorganic moiety. The fraction experiencing reorientations of the C-N axis itself is independent of temperature, thus pointing at steric hindrance due to the extreme softness of the material at atomic level. 

Different geometries of reorientation are compared corresponding to hops between different possible lowest energy configurations. Molecular dynamics simulations are used to determine the most likely geometry.

Finally we speculate on the possible consequences of these reorientations for the material properties. In particular, we use Ising-type simulations to show how the different possible MA arrangements could contribute to ferro- or antiferro-electric properties, and the possible consequences for the charge transport characteristics of the material. We examine whether realignment of the MA ion domains under varying electric fields could contribute to the hysteresis observed in the current-voltage curves of MAPI and MAPIC solar cells.