Lattice dynamics and superconducting Tc for dilute H and D interstitials in Al

The lattice dynamics of isolated H and D interstitial ions in A1 have been calculated with the initial aim of establishing the local mode frequencies. Kohn-Sham nonlinear screening was used for the H,D potentials, anharmonic effects due to the large interstitial motion were accounted for within a quasiharmonic approximation, and the relaxations of the A1 neighbors of an interstitial were also included. The calculations were carried out for both the octahedral and tetrahedral interstitital sites, and they predict a local mode frequency well separated from the Al phonon frequency, with the possible exception of octahedral D. Small, but not negligible, anharmonic effects are predicted. These show up very strongly in the calculation of the changes in Tc due to low concentrations of interstitials (≤ 1%). A definite inverse isotope effect is predicted if it is the octahedral site that is occupied, whereas only a very weak isotope effect is predicted for the tetrahedral case.