Phase Diagram and Superabundant Vacancy Formation in Cr-H Alloys

X-ray diffraction measurements on the Cr–H system were made using synchrotron radiation at high hydrogen pressures and high temperatures, and the phase diagram was determined up to p(H2)=5.5 GPa and T\1400 K. Three solid phases were found to exist; a bcc phase (α) of low hydrogen concentrations, x=[H]⁄[Cr]\0.03 existing at low hydrogen pressures (\4.4 GPa), and two high-pressure phases, an hcp (ε) phase at lower temperatures and an fcc (γ) phase at higher temperatures, both having high hydrogen concentrations x∼1. A drastic reduction of the melting point is caused by dissolution of hydrogen. A gradual lattice contraction observed in the fcc phase indicates the formation of superabundant Cr-atom vacancies (vacancy-hydrogen clusters). Thermal desorption measurements after recovery from high p(H2), T treatments revealed several desorption stages including those due to the release from vacancy-hydrogen clusters and from hydrogen-gas bubbles, and allowed determination of relevant trapping energies.