The phase diagram of the Co–H system was determined by in situ X-ray diffraction up to 1300C and H pressure of pH = 7.4GPa, encompassing ε (hexagonal close-packed), γ(face-centered cubic) and liquid phases. A steep drop of the ε–γ phase boundary at pH ≥ 6GPa was shown to be concomitant with an abrupt increase of the solubility in the γ phase at pH  ~5GPa, one of the characteristic features of supercritical anomaly. Prolonged heat treatments in the γ phase caused gradual lattice contraction, indicating that super-abundant vacancies were formed. From the systematic variation of observed lattice parameters at various pH, T conditions, it was inferred that, irrespective of initial hydride compositions, the same defect hydride of composition Co3VacH4 was formed after the heat treatments. Vacancy-H binding energies were deduced from thermal desorption data measured on heat-treated samples, and on this basis the energetics of super-abundant vacancy formation was considered. The migration volume of a vacancy-H cluster was estimated from the pressure dependence of the rate of lattice contraction.

The Phase Diagram and Superabundant Vacancy Formation in Co–H Alloys. Y.Fukai, S.Yokota, J.Yanagawa: Journal of Alloys and Compounds, 2006, 407[1-2], 16-24