Universal features of hydrogen absorption in amorphous transition-metal alloys

We propose that all $A_{1\mathrm{-}x}$ $B_x$ glasses [where A (B) is a late (early) transition metal] are structurally isomorphic, chemically random alloys which store hydrogen in tetrahedral interstitial sites $A_{4\mathrm{-}n}$ $B_n$ (in decreasing order n=4,3,2,. . . ). The maximum absorbed hydrogen-to-metal atomic ratio within each type of interstitial site is ${1.9(}_n^4$)$x^n$(1-x${)}^{4\mathrm{-}n}$ [${(}_n^4$)=4!/n!(4-n)!] independent of alloy and temperature. The chemical potential as a function of hydrogen concentration within a single site type n is also independent of composition and temperature. The only nonuniversal feature is the dependence of the typical site energies $E_n$ of the type-n sites on the A and B atoms, which may, however, be estimated from crystalline hydride properties. This model agrees with our electrochemical measurements of hydrogen in Ni-Zr, Pd-Ti, and Ni-Ti, predicts total H/M ratios for Ni-Zr and Cu-Ti alloys in excellent agreement with literature gas-phase data over a wide range of compositions and thermodynamic conditions, and is consistent with literature H/M data on other alloys at isolated compositions. We show theoretically that infinite near-neighbor hydrogen-hydrogen interactions (blocking) in a glass dominated by fivefold rings of tetrahedral units predicts the observed x dependence of H/M with a prefactor of 1.9–2.1, in excellent agreement with the observed factor of 1.9. This result supports theoretical models of icosahedral ordering in glasses.