Scattering toward Local Order in Hybrid Metal Halide Perovskites

Research activity in hybrid metal halide perovskites for photvoltaics materials has brought a surge of interest in their “2D” layered and colloidal nanocrystal analogues. Countless organic molecules may serve to separate slabs of metal halide octahedra or terminate nanocrystal surfaces, and their chemical structure has significant impact on orientation and distortion of the corner sharing lead halide octahedra. The design freedom here has produced MHP systems with emergent properties of interest for spintronics, lasing, and beyond.^1,2 To date however, lattice dynamics of the inorganic constituents, structural constraints imposed by ammonium cations, and correlations between the two have not been well established. To achieve a better understanding of MHP systems overall, characterization methods that probe the structural <u>local order</u> are required.<br/><br/>Recent advances in X-ray beam probes now enable study of the local order to reveal static and dynamic lattice distortions, defects, and critical information otherwise hidden under the average crystal structure assigned by conventional crystallography. Preceding studies of local order by our group have established the presence of a dynamic lower symmetry phase in the globally higher symmetry cubic structure of the bulk 3D MHP and raise the question whether similar dynamics are driving optoelectronic properties in other MHP systems.³ Here we present a study of the local order in hybrid MHP using synchrotron X-ray diffuse scattering which reveals significant deviation from a perfectly periodic crystal structure. Analysis of oriented three-dimensional reciprocal space from scattering of single crystals provides spatial understanding of dimensionally correlated disorder in the inorganic sublattice. Ready comparison across systems highlights the dependence of octahedral distortions on cation species. The unique local order revealed in MHP provides unique structural information to toward explaining emergent properties otherwise ill-explained by the crystallographic space group. Establishing structure-property relationships in this material class helps enable the intentional design of new MHP systems for application in topological materials, ferroelectrics, phase-change memory, and engineered heterostructures for improved environmental stability of photovoltaics.<br/><br/>References<br/>1. Blancon, J.-C., Even, J., Stoumpos, Costas. C., Kanatzidis, Mercouri. G. & Mohite, A. D. Semiconductor physics of organic–inorganic 2D halide perovskites. <i>Nat. Nanotechnol.</i> <b>15</b>, 969–985 (2020).<br/>2. Straus, D. B. & Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. <i>J. Phys. Chem. Lett.</i> <b>9</b>, 1434–1447 (2018).<br/>3. Weadock, N. J., Sterling, T. C. & Vigil, J. A. The Nature of Dynamic Local Order in CH3NH3PbI3 and CH3NH3PbBr3. <i>Accepted to Joule </i> (2023).