Effect of Halide Variations on Cation Dynamics in MAPbX (X = Cl, Br, and I).

We investigate the temperature-dependent dynamics of methylammonium (MA) cations in MAPbCl across all crystallographic phases. We identify two distinct motions: i) a threefold rotation around the C-N axis in the orthorhombic phase, with an activation energy of 16 meV, and ii) a fourfold rotation of the entire-molecule in the tetragonal/cubic phase, with an activation energy of 60 meV. To explore the halide effects, we compare MAPbCl with MAPbBr and MAPbI, revealing a strong correlation between rotational motion and halide composition. The onset of the threefold rotation in MAPbCl spans a broader temperature range than in MAPbBr and MAPbI due to persistent octahedral distortions and Pb─Cl bond variations. Additionally, MAPbCl exhibits a sharp transition in elastic intensity at the tetragonal-to-cubic phase transition, absent in MAPbBr and MAPbI. A comparative analysis of activation energies and onset temperatures for the threefold rotation reveals a distinct trend: MAPbI>MAPbBr> MAPbCl. This aligns with the decreasing ionic radius from I⁻ to Cl⁻, reducing steric constraints and enhancing MA⁺ rotational freedom. The pronounced octahedral distortions in MAPbCl create a more flexible framework, enabling facile cation reorientation and multiple rotational configurations even at lower temperatures, leading to a lower onset temperature and activation energy.