The phase diagram and superabundant vacancy formation in Co–H alloys

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