The two-dimensional nature of dynamic disorder in hybrid metal halide perovskite semiconductors*

Hybrid metal halide perovskites are a novel class of semiconductor that require anharmonic structural and electronic calculations to explain the contradiction of a soft, defective lattice and remarkable optoelectronic performance of fabricated devices. One consequence of the anharmonicity is octahedral tilting instabilities driving structural phase transitions. These instabilities led to predictions of dynamic domains of the tetragonal phase persisting within the high temperature cubic phase. We utilize single crystal diffuse scattering to probe structural correlations hidden within the cubic phases of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃. Rods of diffuse intensity extend along the Brillouin zone edge, intersecting at the R-points, implying the cubic phase contains 2D regions of dynamically tilted PbX₆ octahedra. Furthermore, we reproduce the diffuse scattering with MD calculations and find that the orientation of organic cations is similarly correlated. Inelastic neutron scattering suggests these regions are long-lived, and we use the MD results to visualize these regions. Finally, we discuss the impact of this 2D network on optoelectronic properties.