Lattice dynamics and the nature of structural transitions in organolead halide perovskites

TL;DR

This study investigates lattice dynamics in organolead halide perovskites, revealing a displacive phase transition near a tricritical point and persistent nanodomains, which impact their optoelectronic properties.

Contribution

It provides new insights into the structural phase transition mechanisms and nanodomain stability in methylammonium lead halide perovskites.

Findings

Confirmed displacive cubic-to-tetragonal transition

Identified proximity to a tricritical point

Detected stable nanodomains in high-temperature phase

Abstract

Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remarkable optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelastic X-ray scattering to study low-energy lattice excitations in single crystals of methylammonium lead iodide and bromide perovskites. Our findings confirm the displacive nature of the cubic-to-tetragonal phase transition, which is further shown, using neutron and x-ray diffraction, to be close to a tricritical point. Lastly, we detect quasistatic symmetry-breaking nanodomains persisting well into the high-temperature cubic phase, possibly stabilized by local defects. These findings reveal key structural properties of these materials, and also bear important implications for carrier dynamics across an extended temperature range relevant for photovoltaic applications.