Thermally Driven Layered Phase Transition and Decomposition Kinetics of γ-AlH₃: A Multiscale Study Integrating Core-Shell Dynamics and Fluorescence-Guided Analysis

In this study, the γ → α phase transition and decomposition of AlH₃ were probed using integrated hot-stage polarized microscopy, in situ XRD, DSC, and fluorescence analysis. Phase coexistence at 100 °C and complete transition at 140 °C were demonstrated by in situ XRD. Meanwhile, synchronized fluorescence decay (ImageJ-quantified) and XRD evolution analysis confirmed the temperature-dependent kinetics, with the isothermal γ → α durations decreasing from 225 min (100 °C) to 5 min (180 °C). The transition involved competing surface nucleation and bulk diffusion, which was accelerated by the reduced diffusion resistance at elevated temperatures. Above 160 °C, α → Al decomposition dominated via interfacial reactions and H₂ release, accompanied by gas-induced crystalline fracturing. DSC analysis revealed heating-rate-dependent core–shell thermal gradients, which caused hysteresis. At the same time, the experiment also shows that the surface oxidation of γ-AlH₃ may have hindered transitions through passivation layer formation. This work validates Gao et al.’s core–shell model, demonstrating that combined fluorescence and conventional techniques elucidate kinetic laws in metastable systems.