Modulated structure in phase II of CH3NH3PbCl3

Dielectric crystal of CH3NH3PbCl3 takes a cubic perovskite structure at hig h temperature phase, and performs successive transitions at low temperature s. Although the high temperature structure is isomorphous to the compounds in which halogen atoms are replaced by bromine or iodine, the low temperatu re structure has been reported to be different from others. In order to con firm the transition sequence and to investigate the transition mechanism, X -ray diffraction from the single crystal was observed by a four-circle diff ractometer. 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 in commensurate structure of phase II. The ordering of methylammonium ion is d iscussed.