Displacive character of the cubic-tetragonal transition in CH3NH3PbX3

The cubic-tetragonal phase transition in CH3NH3PbBr3 is investigated from the detailed structural analysis. The powder diffraction profiles, obtained with using a Debye-Scherrer camera installed at SPring-8, are analyzed by a Rietveld pattern fitting program. The lattice parameters change as if a second order transition takes place, with accompanying superlattice reflections. In the cubic phase, the structure could be refined either by the displacive model or by the order-disorder model. But the latter model is not fully consistent throughout the transition. The former model gives the rotating angle of the PbBr6 octhedron, which increases monotonically with decreasing temperature. The angle is considered to be an order parameter, and the displacive character of the phase transition is concluded in CH3NH3PbBr3 and CH(3)NH(3)pbI(3).