Fig. 4: Schematic illustration of the structural dynamics of melting and vitrification in the two-dimensional hybrid organic-inorganic perovskite (S-NEA)₂PbBr₄.

This figure depicts the transition from the crystalline to the liquid and glassy phases, highlighting key structural and dynamic changes. In the crystalline phase, [PbBr₆]⁴⁻ octahedra form a well-ordered corner-sharing network, stabilised by hydrogen bonding with (S-NEA)⁺ cations. Upon melting, long-range periodicity is lost, but short-range order persists with partial retention of Pb-Br coordination and connectivity between neighbouring octahedra. Increased molecular motion in the liquid phase leads to further structural disorder, though weak extended correlations remain. Vitrification kinetically traps the disordered liquid structure, resulting in a glass that retains key motifs of the melt. The labelled structural correlations indicate (i) within individual octahedra, (ii) between neighbouring octahedra, and (iii) between second-nearest-neighbour octahedra (either diagonal or linear).