The Phase Diagram of Mo-H Alloys under High Hydrogen Pressures

Abstract

The phase diagram of the Mo–H system was determined by in situ X-ray diffraction using synchrotron radiation up to 1200 � C and the hydrogen pressure to p(H2) = 5.3 GPa. Three phases were found to exist; a bcc phase (� ) of low hydrogen concentrations at low hydrogen pressures, and two high-pressure phases, an hcp () phase at lower temperatures and an fcc (� ) phase at higher temperatures, both having a nearly stoichiometeric composition of x = [H]/[Mo] � 1. A gradual lattice contraction observed in the fcc phase indicates the formation of superabundant Mo-atom vacancies (vacancy-hydrogen clusters).