Modulated structure in phase II of CH3NH3PbCl3

Dielectric crystal of CH3NH3PbCl3 takes a cubic perovskite structure at high temperature phase, and performs successive transitions at low temperatures. Although the high temperature structure is isomorphous to the compounds in which halogen atoms are replaced by bromine or iodine, the low temperature structure has been reported to be different from others. In order to confirm the transition sequence and to investigate the transition mechanism, X-ray diffraction from the single crystal was observed by a four-circle diffractometer. In phase II, superstructure reflections were found newly, which were missed in the previous diffraction study on powder samples. One type of superstructure reflections indicates-cell doubling, while another one incommensurate structure of phase II. The ordering of methylammonium ion is discussed.