Local Structure in\nCrystalline, Glass and Melt States\nof a Hybrid Metal Halide Perovskite

The pursuit of structure–property relationships\nin crystalline\nmetal halide perovskites (MHPs) has yielded an unprecedented combination\nof advantageous characteristics for wide-ranging optoelectronic applications.\nWhile crystalline MHP structures are readily accessible through diffraction-based\nstructure refinements, providing a clear view of associated long-range\nordering, the local structures in more recently discovered glassy\nMHP states remain unexplored. Herein, we utilize a combination of\nRaman spectroscopy, solid-state nuclear magnetic resonance (NMR),\nFourier transform infrared spectroscopy, in situ X-ray diffraction\n(XRD) and pair distribution function (PDF) analysis to investigate\nthe coordination environment in crystalline, glass and melt states\nof the 2D MHP [(S)-(−)-1-(1-naphthyl)ethylammonium]<sub>2</sub>PbBr<sub>4</sub>. While crystalline SNPB shows polarization-dependent\nRaman spectra, the glassy and melt states exhibit broad features and\nlack polarization dependence. Solid-state NMR reveals disordering\nat the organic–inorganic interface of the glass due to significant\nspatial disruption in the tethering ammonium groups and the corresponding\ndihedral bond angles connecting the naphthyl and ammonium groups,\nwhile still preserving substantial naphthyl group registry and remnants\nof the layering from the crystalline state (deduced from XRD analysis).\nMoreover, PDF analysis demonstrates the persistence of corner-sharing\nPbBr<sub>6</sub> octahedra in the inorganic framework of the melt/glass\nphases, but with a loss of structural coherence over length scales\nexceeding approximately one octahedron due to disorder in the inter-\nand intraoctahedra bond angles/lengths. These findings deepen our\nunderstanding of diverse MHP structural motifs and how structural\nalterations within the MHP glass affect properties, offering potential\nfor advancing next-generation phase change materials and devices.