Molecular orientational melting within a lead-halide octahedra framework - the order-disorder transition in CH3NH3PbBr3

TL;DR

This study investigates the temperature-dependent molecular dynamics in CH3NH3PbBr3 perovskites, revealing a transition from static to dynamic molecular states linked to structural changes, using neutron scattering and Raman spectroscopy.

Contribution

It provides new insights into the molecular melting process and the coupling between molecular and framework dynamics in lead-halide perovskites.

Findings

Molecules are static below 150 K and become dynamic above this temperature.

Molecular melting correlates with structural phase transitions.

Hydrogen bonding influences molecular and framework dynamics.

Abstract

Lead-halide organic-inorganic perovskites consist of an inorganic host framework with an organic molecule occupying the interstitial space. The structure and dynamics of these materials have been heavily studied recently due to interest in their exceptional photovoltaic properties. We combine inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron scattering to study the temperature dependent dynamics of the molecular cation in CH3NH3PbBr3. By applying high resolution quasielastic neutron scattering, we confirm the [CH3NH3]+ ions are static in the low temperature orthorhombic phase yet become dynamic above 150 K where a series of structural transitions occur. This molecular melting is accompanied by a temporal broadening in the intra-molecular modes probed through high energy inelastic spectroscopy. Simultaneous Raman measurements, a strictly |Q|=0 probe, are suggestive that this broadening is due to local variations in the crystal field environment around the hydrogen atoms. These results confirm the strong role of hydrogen bonding and also a coupling between molecular and framework dynamics.