Methylammonium Rotational Dynamics in Lead Halide Perovskite by Classical Molecular Dynamics: The Role of Temperature

Abstract

An interatomic model potential for molecular dynamics is derived from first-principles and used to study the molecular rotations and relaxation times in methylammonium lead halide, here considered the prototypical example of a hybrid crystal with a strong reorientational dynamics. Within the limits of a simple ionic scheme, the potential is able to catch the main qualitative features of the material at zero and finite temperature and opens the way to the development of classical potentials for hybrid perovskites. In agreement with experiments and previous theoretical findings, the molecule trajectories exhibit a transition from a dynamics dominated by high symmetry directions at low temperature to a fast dynamics at room temperature in which the molecule can reorient quasi-randomly. By computing the angular time correlation function we discuss the reorientational time as a function of temperature in comparison with existing literature, providing a simple model and a clear attribution of the relaxation times in terms of their temperature dependence. This work clarifies the temperature dependence of the relaxation times and the interpretation of the experimental data in terms of the different mechanisms contributing to the molecule dynamics.

A new potential for methylammonium lead iodide

“…This does not suggest any phase change from orthorhombic to tetragonal, as reported by Mattoni et al 45 . However, our room temperature lattice parameters are in good agreement with experiment 56 and produce a similar cell volume.…”

“…The autocorrelation dies away quickly as temperature increases, and there is more dramatic change as the material is simulated at temperatures above 150 K, which is a temperature that coincides with an experimentally observed transition state. Compared to the work of Mattoni et al the autocorrelation function is not as strongly influenced by increasing temperature 45 .…”

“…The only example of a classical potential for MALI that we have found is the MYP potential of Mattoni etl al. 45 Within this potential the interactions between the atoms that make up the MA ion are governed by the potentials and parameters taken from the AMBER force field. A Buckingham potential is used to describe the interactions between the lead and iodide ions.…”

Energy Landscape of Molecular Motion in Cubic Methylammonium Lead Iodide from First-Principles

“…11,[20][21][22][23] CH 3 NH 3 PbI 3 undergoes a transition from the high-temperature pseudo-cubic aristotype P m3m structure to the tetragonal I4/mcm phase (a 0 a 0 c − tilt system in Glazer notation) 24 at 330 K and then to the orthorhombic P nma phase (a + b − c − tilt system) below 160 K. [25][26][27][28] The transitions associated with A-cation ordering have been the subject of some debate. [11][12][13]20,[29][30][31] Nevertheless, recent quasi-elastic neutron scattering experiments indicate that the CH 3 NH 3 + molecules in the cubic phase dynamically disorder and undergo both fast reorientations of the C−N bond axis (≈5 ps at 300 K) and faster on-axis rotations about the C−N bond axis (≈1 ps at 300 K). 11 Some degrees of freedom freeze out upon cooling and only on-axis rotations are observed in the low temperature orthorhombic phase (≈4 ns at 70 K) accompanied by a dramatic loss in dielectric permittivity.…”

Interplay between organic cations and inorganic framework and incommensurability in hybrid lead-halide perovskite CH3NH3PbBr3

“…[21] These internal CH 3 NH 3 PbBr 3 modes show an abrupt broadening and intensity decrease at the same temperature ( ≈ 150 K) where the abrupt orthorhombic-tetragonal phase transition occurs in the low-frequency Raman spectra, indicating that the phase transition is coupled to the disordered motion of the CH 3 NH 3 PbBr 3 . [22] We now focus on the temperature range across the abrupt phase transition in CH 3 NH 3 PbBr 3 between the low temperature ordered phase and high temperature disordered phase. As shown in Figure 2, a sharp low frequency mode around 13 cm −1 appears in the low frequency spectra below 146 K and disappears at the orthorhombic phase, below 140 K. This result shows the existence of an intermediate phase within a temperature window of about 5 K, between the orthorhombic phase and tetragonal phase.…”