Tunable, Reversible Glass–Crystal Transitions in Single & Binary Metal–Bis(acetamide) Glasses

With large electrical or optical contrast across a continuous transition between two phases encoding the logic states of “0” and “1”, phase-change materials provide promises to address growing demand for data storage and processing. While glass–crystal transitions in chalcogenide alloys have long been studied for rewritable data storage and for neuro-inspired computing more recently, challenges remain with their high energy budget and limited structural diversity and tunability. Glassy phases of metal–organic frameworks offer great potential of using coordination chemistry and reticular synthesis to significantly reduce melting temperatures in comparison to chalcogenide glasses, as well as to predictively tune crystallization kinetics. Although recrystallization has rarely been observed in metal–organic glasses, here we report a novel series of two-dimensional networks M(eba)₃[M′Cl₄] (eba = <i>N</i>,<i>N</i>′-ethylenebis(acetamide), M/M′ = Mn, Fe; M = Mn, Fe, Co, M′ = Zn) that undergo low-temperature melting (150–190 °C) and reversible glass–crystal transitions. Their glass transition temperatures (<i>T</i>_g) are well above 298 K, leading to very stable glasses under ambient conditions. This is attributed to the short polyethylene chain in the eba ligand, which restricts the conformational flexibility of the ligand in the melt and therefore reduces the entropy of fusion. The crystallization kinetics and glass stability of these compounds are readily tunable through the judicious selection of different metal cations and by liquid-phase blending to form binary glasses. This high tunability affords exciting opportunities of forming mixed glasses with precisely tuned glass stability and crystallization behavior tailored for a specific application. Notably, a large reflectivity contrast ratio of 3.7–4.8 for a glass–crystal transition is observed in a Co-containing binary glass, which is driven by changes to the coordination environment of Co centers during crystallization and vitrification. Beyond the M(eba)₃[M′Cl₄] system, a binary metal–bis(acetamide) glass with longer polymethylene chain shows two distinct crystallization features, indicating the possibility of accessing multiple near-degenerate states with different optical properties by harnessing the compositional and structural diversity of metal–organic glasses. These results provide new insights into manipulating reversible glass–crystal transitions in metal–organic materials, which has implications for rewritable data storage at lower energy intensities and well-controlled continuous crystallizations for neuromorphic computing.