Publication date: 14th January 2021
We report the results of a neutron total scatting study of deuterated methylammonium lead iodide in each of the three phases at ambient pressure. The data have then been used as the basis of a reverse Monte Carlo (RMC) study. Total scattering + RMC has an advantage over analysing Bragg scattering with traditional crystal structure refinement tools (such as Rietveld refinement) in the study of disordered materials in that the latter methods require identification of some specific crystallographic sites. The headline results from our work are a) in the high-temperature cubic phase there is nearly completely random orientations of the methylammonium ions, without strong preferences for particular orientations; b) in the intermediate-temperature tetragonal phase there is a gradual change in the degree of orientational disorder on cooling; c) there is a high degree of orientational order in the low-temperature orthorhombic phase; d) analysis of the PbI6 orientations and spontaneous strains show that the cubic–tetragonal phase transition is continuous and consistent with phase transitions in other perovskites, but there are is unusual temperature dependence. We compare our results from recent total scattering + RMC studies of KCN [1] and CaPbI3 [2]; in the case of KCN we can compare the orientational disorder of the C–N bonds, and in the case of CsPbI3 we compare an extraordinarily high degree of anharmonicity of the Pb–I bonds.
ISIS facility for neutron beam time. China Scholarship Council for funding for JL
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