Dynamic disorder and phase behavior in long alkylammonium metal halides

Résumé :

wo-dimensional hybrid organic–inorganic perovskites with general formula (CₙH₂ₙ₊₁NH₃)₂MX₄ are promising solid-solid phase change materials (SS-PCMs) for thermal energy storage (TES) due to their sharp, reversible first-order phase transitions with large latent heats. These transitions originate from cooperative order-disorder processes within the organic sublattice, a mechanism known as pseudo-melting. As reported in the literature, the transition temperature increases with alkyl chain length, providing a means to tune the operating temperature. However, long-chain compounds (n ≥ 16) have remained largely unexplored, despite being expected to display transitions in the 80-110 °C range relevant for medium-temperature TES applications such as industrial waste heat recovery in food processing, textiles and paper sectors.

While calorimetric measurements confirm the thermodynamic potential of these materials, the microscopic mechanisms driving the phase transitions remain poorly understood, particularly for long chains (n ≥ 16). Establishing a molecular-level description of the order–disorder process is essential to understand what governs the transition temperature and latent heat, and to rationally design materials with improved thermal performance. In this work, we address this gap through a comprehensive characterization combining calorimetry, vibrational spectroscopy, and neutron scattering of newly synthesized (CₙH₂ₙ₊₁NH₃)₂CuBr₄ compounds with even n = 16-22. Variable-temperature Fourier-transform infrared spectroscopy identifies conformational rearrangements of the alkyl chains across the transition, evidencing changes in intra- and intermolecular interactions consistent with enhanced chain mobility. Variable-temperature Raman spectroscopy tracks the evolution of vibrational modes associated with both the organic chains and the inorganic framework. Quasielastic neutron scattering directly probes hydrogen dynamics, revealing a progressive activation of molecular motion near of the phase transition and confirming that dynamic disorder is the microscopic origin of the large latent heat. Together, these results provide molecular-level understanding of structure–property relationships in long-chain hybrid perovskites needed to guide the design of tunable SS-PCMs for medium-temperature TES.

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Gabriel Cuello (College 6 Secretary)

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