Neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide

Journal article

Abstract We report the results of a neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide, with a focus on the system of orientational distortions of the framework of PbI6 octahedra. The results are analysed in terms of symmetry-adapted lattice strains and normal mode distortions. The higher-temperature cubic–tetragonal phase transition at 327 K is weakly discontinuous and nearly tricritical. The variations of rotation angles and spontaneous strains with temperature are consistent with a standard Landau theory treatment. The lower-temperature transition to the orthorhombic phase at 165 K is discontinuous, with two systems of octahedral rotations and internal distortions that together can be described by 5 order parameters of different symmetry. In this paper we quantify the various symmetry-breaking distortions and their variation with temperature, together with their relationship to the spontaneous strains, within the formalism of Landau theory. A number of curious results in the low-temperature phase are identified, particularly regarding distortion amplitudes that decrease rather than increase with lowering temperature.

Authors: Liu, J; Du, J; Phillips, AE; Wyatt, PB; Keen, DA

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Journal of Physics: Condensed Matter
• Volume: 34
• Issue: 14
• Pages: 145401-145401
• Publication date: 2022-01-12
• DOI: 10.1088/1361-648x/ac4aa9
• EISSN: 1361-648X
• ISSN: 0953-8984
• Language: English
• Keywords: Condensed matter physics; Ferroelectricity; Physics; Materials science; Deuterium; Phase transition; Chemistry; Orthorhombic crystal system; Landau theory; Dielectric; Diffraction; Tetragonal crystal system; Quantum mechanics; Crystallography; Optics; Neutron diffraction; Lattice (music); Powder diffraction; Crystal structure