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 degrees C and hydrogen pressure of p(H) = 7.4 GPa, encompassing epsilon(hcp), gamma(fcc) and liquid phases. A steep drop of the epsilon-gamma phase boundary at p(H) >= 6 GPa is shown to be concomitant with an abrupt increase of the solubility in the gamma phase at p(H) similar to 5 GPa, one of the characteristic features of supercritical anomaly. Prolonged heat treatments in the gamma phase caused gradual lattice contraction, indicating that superabundant vacancies (SAVs) are formed. From the systematic variation of observed lattice parameters at various p(H), T conditions, it is inferred that, irrespective of initial hydride compositions, the same defect hydride of composition Co(3)VacH(4) 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. (c) 2005 Elsevier B.V. All rights reserved.