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

In understanding the emerging photovoltaic materials lead-halide perovskites, orientational dynamics of the organic cations has garnered much attention, whereas the lead-halide framework is the actual optoelectronically active component. The interplay between the organic and inorganic moieties is thus key to a complete picture linking structural dynamics to electronic properties. Yet the mechanism and consequences of this organic-inorganic coupling have largely been obscured. This work elucidates the unique structural role of the organic dipolar cations. Frustrated competition between the organic and inorganic structural ordering results in an incommensurate phase. The authors uncover a new hybrid amplitudon phonon showing soft mode behavior.