Why the knowledge of the anharmonicity is important for the structural processes that govern the orthorhombic/tetragonal phase transformation in chlorine-substituted MAPbI3

Götz Schuck^a, Daniel M. Többens^a, Dirk Wallacher^a, Nico Grimm^a, Tong Sy Tien^b, Susan Schorr^a^,^c

^aHelmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany

^bDepartment of Basic Sciences, University of Fire Prevention and Fighting, 120602 Hanoi, Vietnam

^cInstitut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)

València, Spain, 2022 May 19th - 25th

Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson

, Götz Schuck, presentation 267
DOI: https://doi.org/10.29363/nanoge.hopv.2022.267
Publication date: 20th April 2022

We report on studies of chlorine-substituted MAPbI₃ using combined temperature-dependent X-ray diffraction (XRD) synchrotron and Pb L3-edge extended X-ray absorption fine structure (EXAFS) to analyze the anharmonicity of the lead−halide bond.[1]

The EXAFS parameters were described in the orthorhombic phase by an Einstein or T²-type behavior, which was then compared with the experimental EXAFS parameters of the tetragonal/cubic phase. In the orthorhombic phase, it was observed that the asymmetry of the pair distribution function (cumulant C3) in MAPbCl₃ is much lower than in MAPbI₃. Compared with the behavior in the orthorhombic phase, the anharmonicity changed after the phase transition to the room temperature phase, with MAPbCl₃ showing an increased anharmonicity and MAPbI₃ a decrease. The differences between MAPbI₃ and 2% chlorine substitution were small, both in the orthorhombic and tetragonal phases.

By determining the structural parameters required to convert the effective force constants k₀ and k₃ resulting from the EXAFS analysis into the Morse potential parameters α and D, we could establish that our results agree with other experimental findings. The Morse potential parameters can then also be directly compared with results from theoretical calculations. Moreover, using XRD, we found that the [PbX₆] octahedra appear to shrink slightly in the tetragonal phase of MAPbI₃ and MAPbI_2.94Cl_0.06, toward increasing temperatures. This behavior in the tetragonal phase is related to the dominant negative tension effects observed by EXAFS.

References:

[1] Schuck, G.; Többens, D. M.; Wallacher, D.; Grimm, N.; Tien, T.S.; Schorr, S. Temperature-Dependent EXAFS Measurements of the Pb L3-Edge Allow Quantification of the Anharmonicity of the Lead–Halide Bond of Chlorine-Substituted Methylammonium (MA) Lead Triiodide. J. Phys. Chem. C 2022, 126, 12, 5388–5402

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