Solid–Solid Phase Change Layered Hybrid Materials for Thermal Energy Storage: Impact of the Chemical Composition and Structural Topology

Solid–solid phase change materials (SS-PCMs) are attractive candidates for thermal energy storage (TES) owing to their intrinsic shape stability, yet their widespread application remains limited by the lack of design rules linking the molecular structure to phase-transition properties. Here, we present a comprehensive study of layered hybrid chlorometallates, (C n H 2n + 1 NH 3 ) 2 MCl 4 (M = Cu, Mn, Zn; n = 6–16), as tunable SS-PCMs. For that, 15 compounds (M = Cu, Mn, Zn; n = 6, 7, 12, 13, 16) were prepared, and their low-temperature (LT) forms were studied by single-crystal XRD and vibrational spectroscopies. By a multitechnique approach, involving calorimetry, temperature-dependent infrared, and Raman spectroscopies, and combined in a single synchrotron experiment temperature-dependent X-ray absorption spectroscopy (XAS), total scattering/Pair Distribution Function (PDF), and powder XRD (PXRD) analyses, we evidenced the impact of both parameters (M and n ) not only on the LT structures but also on the thermal properties and on the high-temperature (HT) structures. Especially, we evidenced that although materials based on octahedrally (here Mn and Cu) and tetrahedrallly (here Zn) coordinated cations share many common features in their LT forms (alternating organic–inorganic layered structures, alkylammonium chains parallel to each other, and supramolecular organic–inorganic interactions of the same nature and strength), their HT phases strongly differ, especially at a medium range distance. This comprehensive study is not only of fundamental interest but will also help to address questions, such as the shaping and mechanical integrity of these SS-PCMs upon thermal cycling that need to be answered prior to their integration into practical devices for next-generation TES.