Transmission Electron Microscopy Imaging of Cluster Unit Dissolution from a Single Crystal of Tetra-n-butylammonium Fluoride Semiclathrate Hydrate for Latent-Heat Storage Material

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A nonclassical phase-transition model in which clusters act as the growth units has gained acceptance, contrasting with the classical phase-transition model where atoms or molecules attach or detach individually. However, this cluster-based model for phase transitions has not been established in phase transitions of semiclathrate hydrates, latent-heat storage materials, in which cage structures form and dissociate at the crystal surface. Using liquid-cell transmission electron microscopy, we directly observed the dissociation behavior of a single crystal of tetra- n -butylammonium fluoride semiclathrate hydrate induced by electron-beam irradiation. Early dissociation stages showed moiré patterns, indicating that a 10–30 nm region of the crystal surface had transitioned to a metastable phase. Subsequently, spherical clusters, potentially responsible for the memory effect that facilitates recrystallization after dissociation, appeared without diffraction contrast attributable to crystal. These observations provide insights into nonclassical dissociation processes of hydrates to understand the phase transition of latent-heat storage materials.