Disordered Configuration of Methylammonium of CH3NH3PbBr3 Determined by Single Crystal Neutron Diffractometry

In order to understand the mechanism of the structural phase transition in CH 3 NH 3 PbBr 3 , 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 PbBr 6 octahedrons is identified unambiguously. With comparing the low-temperature structure, the transition is characterized by the condensation of the rotational motion of the PbBr 6 octahedron around the Pb-Br axis, which accompanies the partial ordering of the methylammonium ion to reduce the site symmetry.