Electronic properties and electron-phonon coupling in zirconium and niobium hydrides

The electronic structure of the two $4d$ transition-metal cubic dihydrides ZrH₂ and NbH₂ is investigated by means of an augmented-plane-wave (APW) band-structure calculation. The position of the Fermi level in a peak of the density of states (DOS) in the case of cubic ZrH₂ leads to an interpretation of the tetragonal distortion observed in all the group-IV transition-metal dihydrides in terms of a Jahn-Teller effect which lifts the degeneracy of the doubly degenerate band in the ΓL direction; these results are in agreement with magnetic susceptibility, electronic specific heat, and thermoelectric power data. The main features of the DOS of the metal $4d$ states are similar for ZrH₂ and NbH₂, while the position and width of the low-lying bands are found to depend substantially upon the compound under study. These two bands which result from metal-hydrogen and hydrogen-hydrogen interactions overlap the metal d bands located at higher energies, contrary to the case of rare-earth dihydrides for which an energy gap was obtained. Our theoretical results are in good agreement with photoemission and x-ray emission spectra. We have evaluated the electron-phonon coupling constant λ using the results of our APW calculation in conjunction with neutron scattering data. For both ZrH₂ and NbH₂, and contrary to the case of superconducting PdH, the electron-optical phonon contribution is small, due essentially to the low value of the DOS of s type at the H site at the Fermi energy and to the hardening of the optic modes. Moreover, the electronic term of the electron---acoustic-phonon contribution is reduced from its value in the pure metal. The two compounds are not found to be superconducting, in agreement with experimental results.