• 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. The Journal of Physical Chemistry C 126 (2022), p. 5388–5402

10.1021/acs.jpcc.1c05750
Open Access Version (externer Anbieter)

Abstract:
This article reports 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. 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. Moreover, using XRD, we found that the [PbX6] octahedra appear to shrink slightly in the tetragonal phase of MAPbI3 and MAPbI2.94Cl0.06, toward increasing temperatures. This behavior in the tetragonal phase is related to the dominant negative tension effects observed by EXAFS.