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

Abstract

This article reports on studies of chlorine-substituted MAPbI 3 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 T 2 -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 C 3 ) in MAPbCl 3 is much lower than in MAPbI 3 . Compared with the behavior in the orthorhombic phase, the anharmonicity changed after the phase transition to the roomtemperature phase, with MAPbCl 3 showing an increased anharmonicity and MAPbI 3 a decrease. The differences between MAPbI 3 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 k 0 and k 3 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 [PbX 6 ] octahedra appear to shrink slightly in the tetragonal phase of MAPbI 3 and MAPbI 2.94 Cl 0.06 , toward increasing temperatures. This behavior in the tetragonal phase is related to the dominant negative tension effects observed by EXAFS.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

“…In contrast to mixing with iodine, no evidence of the miscibility gap was found for the mixed MAPb(Br 1−x Cl x ) 3 perovskites due to more similar radii of Br − and Cl − anions (196 pm vs 181 pm). 362364,368,369,377−382 Several studies observed cubic symmetry at room temperature for different compositions covering the whole mixing interval 364,380,381,383 in agreement with the symmetry of the end members (see Figure 3).…”

Effect of Residual Chloride in FAPbI3 Film on Photovoltaic Performance and Stability of Perovskite Solar Cell

“…The change in the Pb–I bond distance observed from EXAFS is, however, inconsistent with almost no change in lattice constant observed from XRD. The local Pb–I bond distance was reported to be different from the XRD results depending on the composition of materials due to the different rigidities of Pb–halide octahedra . Thus, the local structure and the Pb–I bond distance obtained from EXAFS might provide different information, although it is incommensurable with the bulk lattice parameters.…”

“…The local Pb−I bond distance was reported to be different from the XRD results depending on the composition of materials due to the different rigidities of Pb−halide octahedra. 31 Thus, the local structure and the Pb−I bond distance obtained from EXAFS might provide different information, although it is incommensurable with the bulk lattice parameters. The origin of the elongated Pb−I bond distance might be related to the thermally induced PbI 2 with a longer Pb−I bond distance of ∼3.28 Å than that in FAPbI 3 (3.18 Å).…”