Heats of solution and lattice-expansion and trapping energies of hydrogen in transition metals

Abstract

The heat of hydrogen solution in a metal at infinite dilution ΔH${¯}_{\infty }$ is shown to depend on (1) the distance R between a hydrogen atom and its metallic nearest neighbors, (2) the characteristic band-structure energy ΔE=$E_F$-$E_s$, where $E_F$ is the Fermi energy and $E_s$ basically the center of the lowest conduction band of the host metal, and (3) the width $W_d$ of the d band of the host metal. The semiempirical relation ΔH${\mathrm{¯}}_{\mathrm{\infty }}$=αΔE ${\mathrm{W}}_{\mathit{d}}^{1/2}$ ? [with α=18.6 (kJ/mol H)(A${\mathrm{̊}}^4$ ${\mathrm{eV}}^{\mathrm{-}3/2}$) and β=-90 kJ/mol H if ΔE, $W_d$, and R are given in units of eV and Å, respectively] reproduces the experimental values of ΔH${¯}_{\infty }$ remarkably well. It also reproduces the volume expansion accompanying hydrogen absorption and predicts the correct interstitial site occupancy of hydrogen in a transition metal. Furthermore, it makes it possible to estimate the binding energy of hydrogen to a vacancy.