Electronic Structure and Electron-Phonon Coupling Constant of Some Metal Hydrides

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With the aim of explaining on theoretical grounds some experimental results on the superconducting transition temperatures, Tc, of metal hydrides and possibly make predictions on the values of Tc, the electron-phonon coupling parameter λ is evaluated for several simple, transition and non-magnetic rare earth metal hydrides. The electronic parameter η defined by Mc Millan is obtained within the rigid muffin-tin model from our augmented plane wave band structure results, while available experimental data are used to evaluate the ‘phonon contribution’. The following general trends in the variation of H emerge from these calculations: (1) The magnitude of the electron-optical phonon matrix element which is dominated by the s-p scattering is found to be small for the early transition metal (TM) dihydrides and LaH2; it is somewhat larger for the other TM dihydrides of the 4d series. This is in contrast to PdH which has a significantly larger value of ηH. Large values of ηH are also obtained for A1H2 and A1H (which can be prepared by ion implantation) making these compounds good candidates for superconductivity. (2) As the metal ηmetal is essentially determined by the d-f scattering for the late members of the TM series while the p-d mechanism is important for the early members of the TM series, for LaH2 and particularly for the AlHx system. In most cases, a reduction of ηmetal from its value in the pure metal is obtained for the cubic metal hydrides studied here. The essential features of the electronic structure of the stoichiometric metal hydrides under study are also underlined.