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

## TL;DR

This study investigates the structural dynamics of hybrid CH3NH3PbBr3 and inorganic CsPbBr3 perovskites, revealing an incommensurate phase in the hybrid and highlighting the interaction between organic cations and the inorganic framework.

## Contribution

It uncovers the incommensurate phase in hybrid perovskite and links organic-inorganic interactions to structural dynamics, advancing understanding of their phase transitions.

## Key findings

- Incommensurate phase observed in hybrid CH3NH3PbBr3.
- Sharp soft mode identified in Raman spectra during incommensurate phase.
- Contrasting phase transition behaviors between hybrid and inorganic perovskites.

## Abstract

Organic-inorganic coupling in the hybrid lead-halide perovskite is a central issue in rationalizing the outstanding photovoltaic performance of these emerging materials. Here we compare and contrast the evolution of structure and dynamics of the hybrid CH3NH3PbBr3 and the inorganic CsPbBr3 lead-halide perovskites with temperature, using Raman spectroscopy and single-crystal X-ray diffraction. Results reveal a stark contrast between their order-disorder transitions, abrupt for the hybrid whereas smooth for the inorganic perovskite. X-ray diffraction observes an intermediate incommensurate phase between the ordered and the disordered phases in CH3NH3PbBr3. Low-frequency Raman scattering captures the appearance of a sharp soft mode in the incommensurate phase, ascribed to the theoretically predicted amplitudon mode. Our work highlights the interaction between the structural dynamics of organic cation CH3NH3+ and the lead-halide framework, and unravels the competition between tendencies of the organic and inorganic moieties to minimize energy in the incommensurate phase of the hybrid perovskite structure.