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(H 2 ) = 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).

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

“…Then we obtain e H m (hcp) + e b 2 = 0.65 eV, hence e b 2 = 0.27 eV. Similar analysis of the fcc peaks at T P 2 = 700 K, T P 3 = 840 K and T P 4 Binding energies thus deduced are listed in Table 1, together with those determined previously in Ni and Cu. Samples prepared by three different methods -ion implantation, high p H , T treatment, and electrolysis -yield nearly the same result.…”

“…Regarding the axial ratio of the phase, its very weak dependence on p H , T and x was also noted in the hcp(dhcp) solid-solution phase of Cr-H [3], Mn-H [5], Fe-H [7] and Mo-H [4]. The implication is that the lattice dilatation caused by interstitial H atoms is nearly isotropic, and this isotropic distortion persists under different conditions of p H , T. This feature may be correlated with the fact that the axial ratio of the phase of these metals (1.618-1.637) is rather close to the ideal value of close packing (1.633).…”

“…By developing a technique for in situ measurements of XRD under high H 2 pressures (p H ≤ 10 GPa) and high temperatures (T ≤ 1300 • C), we have determined the phase diagram of a series of transition-metal -hydrogen systems including Ti-H [1], V-H [2], Cr-H [3], Mo-H [4], Mn-H [5], Fe-H [6,7] and Ni-H [8,9] alloys over wide ranges of temperature and pressure, and discovered in this process the formation of superabundant vacancies (SAVs) -more precisely, the vacancy-H clusters [10,11]. The process of SAV formation and its implication for various physical properties of M-H alloys have also been investigated [12][13][14][15][16][17][18][19][20], including in particular the enhanced diffusion of M-atoms [21][22][23].…”

T–Pphase diagrams and isotope effects in the Mo–H/D systems

“…The pressures of formation and decomposition of molybdenum deuteride determined from the midpoints of the steps in the isotherms measured at increasing and decreasing pressure, respectively, are shown in figure 2(a). [6], dashed curves of [5]). α is the dilute deuterium or hydrogen solid solution in bcc Mo; ε is the approximately stoichiometric ε-MoD or ε-MoH compound with an hcp metal lattice; γ is the molybdenum hydride with a fcc metal lattice.…”

“…At present, T -P diagrams are constructed for most binary metal-hydrogen systems that form hydrides at hydrogen pressures up to 9 GPa (see review [1] and recent papers [2][3][4][5]). In particular, phase transformations in the Mo-H system were studied at pressures of up to 5 GPa and temperatures up to 500 • C using in situ resistometry [6] and later up to 1200 • C using in situ x-ray diffraction [5]. Phase transformations in metal-deuterium systems at high deuterium pressures are studied to a much lesser extent.…”