Neutron Total Scattering and Reverse Monte Carlo Study of orientational disorder and phase transitions in methylammonium lead iodide
Martin Dove a b, Anthony Phillips b, Jiaxun Liu b, David Keen c
a Sichuan University
b Queen Mary University of London, Mile End Road, London, United Kingdom
c ISIS Neutron and Muon Facility
Proceedings of Atomic-level characterization of hybrid perovskites (HPATOM)
Online, Spain, 2021 January 26th - 28th
Organizers: Dominik Kubicki and Amita Ummadisingu
Invited Speaker, Martin Dove, presentation 014
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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