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Migration energies in L12 intermetallic compounds

The migration energies EM for the jumps into nearest-neighbour vacancies in L12 intermetallic compounds are related to the static lattice Green functions that can be calculated from the measured phonon dispersions. The present approach is an extension of a similar approach used earlier for BCC and FCC pure metals (Schober H R, Petry W and Trampenau J 1992 J. Phys.: Condens. Matter 4 9321). In A3B compounds with the L12 structure, three first-nearest-neighbour jumps into vacancies have to be distinguished and in some cases there is a bias, ΔE, between final and initial configurations. As was done earlier for monatomic lattices by Schober et al, the migration energy is split into two terms, one depending only on structure and a material-dependent term, given by the Green function elements. The difference in size between A and B atoms has to be taken into account and the compounds have to be separated into two groups depending on the size of the majority atoms relative to the minority ones. The formulae have been checked by computer simulations. The values of EM or EM-ΔE/2 are calculated for those L12 compounds where the phonon dispersions have been measured. In the case of the Ni3Al system, other theoretical and experimental determinations compare well with our model. Comparing these energies with the critical temperatures of stability of the L12 structure, we note a significant contribution of the ordering energy to the migration energy for all three jump types.