The ordered phase of methylammonium lead chloride CH3ND3PbCl3

The ordered phase of methylammonium lead chloride crystallizes in space group Pnma, with cell parameters a=11.1747(2)Å, b=11.3552(1)Å and c=11.2820(1)Å each is doubled with respect to the room temperature disordered cubic phase (a=5.669Å). This unusual cell basis for space group Pnma is not that of a standard tilt system. The structure was solved by ab initio methods using the programs EXPO and FOX, and refined from both powder neutron and synchrotron data, revealing that the most rigid unit is actually the methylammonium cation, rather than the PbCl6 octahedra, in agreement with existing spectroscopic data.