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Influence of Chloride Substitution on the Rotational Dynamics of Methylammonium in MAPbI$_{3-x}$Cl$_{x}$ Perovskites

The hybrid halide perovskites MAPbI$_{3}$, MAPbI$_{2.94}$Cl$_{0.06}$, and MAPbCl$_{3}$ (MA - methylammonium) have been investigated using inelastic and quasielastic neutron scattering (QENS) with the aim of elucidating the impact of chloride substitution on the rotational dynamics of MA. In this context, we discuss the influence of the inelastic neutron scattering caused by low-energy phonons on the QENS resulting from the MA rotational dynamics in MAPbI$_{3-x}$Cl$_{x}$. Through a comparative temperature-dependent QENS investigation with different energy resolutions, which allow a wide Fourier time window, we achieved a consistent description of the influence of chlorine substitution in MAPbI$_{3}$ on to the MA dynamics. Our results show that chlorine substitution in the low temperature orthorhombic phase leads to a weakening of the hydrogen bridge bonds since the characteristic relaxation times of C$_{3}$ rotation at 70 K in MAPbCl$_{3}$ (135 ps) and MAPbI$_{2.94}$Cl$_{0.06}$ (485 ps) are much shorter than in MAPbI$_{3}$ (1635 ps). For the orthorhombic phase, we obtained the activation energies from the temperature-dependent characteristic relaxation times $τ$C$_{3}$ by Arrhenius fits indicating lower values of E$_{a}$ for MAPbCl$_{3}$ and MAPbI$_{2.94}$Cl$_{0.06}$ compared to MAPbI$_{3}$. We also performed QENS analyses at 190 K for all three samples. Here we observed that MAPbCl$_{3}$ shows slower MA rotational dynamics than MAPbI$_{3}$ in the disordered structure.