Isotope Effects in the PdH System: Lattice Dynamics of Pd${\mathrm{T}}_{0.7}$

The quasiharmonic phonon dispersion relation of Pd${\mathrm{T}}_{0.7}$ has been determined by coherent neutron scattering. Calculation of the phonon terms entering the current models for superconductivity in PdH, PdD, and PdT shows conclusively that the magnitude of the measured anharmonic frequency shifts is not sufficient to explain the reverse isotope effect in these systems. The effect of the large zero-point motion of the hydrogen isotopes on both the electronic structure and the electron-phonon interaction is estimated, and shown to be necessary to explain the experimental data.