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

The heat of hydrogen solution in a metal at infinite dilution _ is shown to depend on (1) the distance R between a hydrogen atom and its metallic nearest neighbors, (2) the characteristic band-structure energy =E₅-Eₒ, where E₅ is the Fermi energy and Eₒ basically the center of the lowest conduction band of the host metal, and (3) the width W₃ of the d band of the host metal. The semiempirical relation _= W₃^1/2? with =18. 6 (kJ/mol H) (A^4 eV^-3/2) and =-90 kJ/mol H if, W₃, and R are given in units of eV and A, respectively reproduces the experimental values of _ 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.