Positron Annihilation in Non-Dilute AgPd and CuPd Alloys. An Angular Correlation Study of Their Momentum Density and Fermi Surface

摘要

The present work has been devoted to the investigation of the electronic structure of non-dilute disordered alloys by means of the technique of angular correlation of slow-positron annihilation radiation. Attention is paid to the theoretical aspects of positron annihilation in a metal. A selection rule based on group theory is reformulated and a table is given of the observability of energy bands in the extended zone scheme for electrons in an fcc lattice. The approximation scheme of Hubbard for the calculation of the band structure and the electronic wave functions is reviewed. With this scheme, the band structures of palladium and silver have been calculated and a good agreement with the results of other calculation methods has been found. Furthermore, the two-photon momentum densities in palladium and silver have been calculated. The spherically averaged angular correlation profiles have been constructed from the two-photon momentum densities and are compared with experimental results obtained from polycrystalline samples. Experiments were performed on single crystals of Ag, Pd and Ag/sub 0.68/Pd/sub 0.32/ each oriented in five different lattice directions. The two-photon momentum densities for these three materials are given and the Fermi surfaces of Ag and Ag/sub 0.68/Pd/sub 0.32/ are derived from the momentum densities. The Fermi surface of silver was found to be in rather good agreement with De Haas-Van Alphen data. The Fermi surface of Ag/sub 0.68/Pd/sub 0.32/ encloses 0.75 +- 0.03 states/atom. Experiments on polycrystalline samples of two series of AgPd and CuPd alloys were also performed. The volumes of the Fermi spheres have been derived from the spherically averaged momentum densities. (Atomindex citation 09:364873)