(262d) Phase Transition of Acid Clathrate Hydrate Related to the Proton Conduction: Structural Analysis of Perchloric Acid Hydrate

Clathrate hydrates are nonstoichiometic crystalline compounds which are stabilized by van der Waals interaction between the hydrophobic guest molecules and the hydrogen-bonded water host framework. Unlike classical clathrate hydrates, ionic clathrate hydrates additionally show an ionic interaction between cationic or anionic guest species and counterion-doped host network. A few strong acids form clathrate structures in which anions are encaged into the protonated water cavities. There are two types of protonated host framework. One is hydrogen fluoride-contained host framework such as hexahydrates of HPF₆, HAsF₆, or HSbF₆ and another is hydrogen fluoride-free host framework such as hydrates of HBF₄ or HClO₄. The latter is believed to contain several vacancy sites in the hydrogen bonding lattice for protonation of the host framework. In this work, the neutron powder diffraction patterns of a DClO₄·5.5D₂O clathrate were obtained at various temperatures and refined by Rietveld analysis with the direct space method. A position change of vacancy sites was predicted at about 180 K, and this phenomenon is expected to induce a phase transition related to the proton conduction behavior of the perchlorate acid clathrate. The present work provides a better understanding of the proton dynamics of the hydrogen fluoride-free acid clathrate hydrates.