| Résumé | The structure of phase IV of methylammonium lead bromide, CH3ND3PbBr3, is shown from Rietveld refinement of neutron powder diffraction data to be centrosymmetric, with space group Pnma: Z=4; a=7.9434(4)??, b=11.8499(5)??, c=8.5918(4)?? at 11?K; Rwp=2.34% Rp=1.81%. This corresponds to one of the pure tilt transitions, a-b+a-, commonly observed in perovskites. Additional distortions not required by pure tilting are found in the PbBr6 octahedra, and it appears that the structure optimizes the hydrogen bonding between the methylammonium cation and the framework. It is likely that the lowest temperature phase of the corresponding iodide also has this structure. The structure is compared to the available data for that of other Pnma perovskites. A brief comparison to the higher temperature phases in which the methylammonium ion is disordered is given. |
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