Disordered configuration of methylammonium of CH3NH3PbBr3 determined by single crystal neutron diffractometry

In order to understand the mechanism of the structural phase transition in CH3NH3PbBr3, neutron diffraction data from the single crystal is analyzed by the least-squares method and the maximum entropy method. The disordered configuration of a methylammonium ion in a cage surrounded by eight PbBr6 octahedrons is identified unambiguously. With comparing the low-temperature structure, the transition is characterized by the condensation of the rotational motion of the PbBr6 octahedron around the Pb-Br axis, which accompanies the partial ordering of the methylammonium ion to reduce the site symmetry.