Trends in the electron-phonon coupling parameter in some metallic hydrides

We present an evaluation of the electron-phonon coupling parameter $\lambda$, using the McMillan formalism, for several classes of stoichiometric mono- and dihydrides with a metallic underlying fcc structure. We calculate the electronic term $\eta$ and use experimental estimates for the phonon energies when available. We derive systematic trends concerning both contributions to $\eta$ stemming, respectively, from the metallic site $M$ and the hydrogen site H. We show that ${\eta }_{\mathrm{H}}$ is generally small, but it may become large if the Fermi energy is in the metal $s-p$ band as in the filled $d$-band transition-metal (TM) hydrides such as PdH; ${\eta }_{\mathrm{H}}$ may also be large when a metal-hydrogen antibonding band crosses the Fermi level, a case which happens in AlH and may happen for some unstable dihydrides. The metallic contribution ${\eta }_M$ is calculated to be small for all stable mono- and dihydrides like PdH, NiH, Zr${\mathrm{H}}_2$, Nb${\mathrm{H}}_2$, etc., but nothing in principle prevents this contribution from becoming as large as in some pure TM, if one sweeps the Fermi level through the whole metallic $d$ band. Good agreement with the available experimental data is obtained concerning the occurrence of superconductivity in the compounds considered.