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

TL;DR

This study uses temperature-dependent EXAFS and XRD to analyze the anharmonicity of lead-halide bonds in chlorine-substituted methylammonium lead triiodide, revealing phase-dependent changes and subtle effects of chlorine substitution.

Contribution

It provides a detailed analysis of anharmonicity in lead-halide bonds across phases and chlorine substitutions using combined EXAFS and XRD techniques, linking structural parameters to potential energy models.

Findings

Orthorhombic phase shows lower asymmetry in MAPbCl3 compared to MAPbI3.

Anharmonicity increases in MAPbCl3 after phase transition, decreases in MAPbI3.

Minor effects of 2% chlorine substitution on structural parameters across phases.

Abstract

This article reports on studies of chlorine-substituted MAPbI3 using combined temperature-dependent XRD synchrotron and Pb-L3 edge 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 {\alpha} and D, we could establish that our results agree with other experimental findings. Moreover, by using XRD we found that the [PbX6] octahedra shrink slightly in the tetragonal phase of MAPbI3 and MAPbI2.94Cl0.06, towards increasing temperatures. This behavior in the tetragonal phase is related to the dominant negative tension effects observed by EXAFS.