Effect of Halide Variations on Cation Dynamics in MAPb X ₃ ( X = Cl, Br, and I)

Abstract We investigate the temperature‐dependent dynamics of methylammonium (MA) cations in MAPbCl ₃ 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 ₃ with MAPbBr ₃ and MAPbI ₃ , revealing a strong correlation between rotational motion and halide composition. The onset of the threefold rotation in MAPbCl ₃ spans a broader temperature range than in MAPbBr ₃ and MAPbI ₃ due to persistent octahedral distortions and Pb─Cl bond variations. Additionally, MAPbCl ₃ exhibits a sharp transition in elastic intensity at the tetragonal‐to‐cubic phase transition, absent in MAPbBr ₃ and MAPbI ₃ . A comparative analysis of activation energies and onset temperatures for the threefold rotation reveals a distinct trend: MAPbI ₃ > MAPbBr ₃ > MAPbCl ₃ . 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 ₃ 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.