Migration energies in L12intermetallic compounds

Abstract

The migration energies EM for the jumps into nearest-neighbourvacancies in L12 intermetallic compounds are related to thestatic lattice Green functions that can be calculated from themeasured phonon dispersions. The present approach is an extensionof a similar approach used earlier for BCC and FCC puremetals (Schober H R, Petry W and Trampenau J 1992 J. Phys.: Condens. Matter 4 9321). In A3B compounds with the L12structure, three first-nearest-neighbour jumps into vacancies haveto be distinguished and in some cases there is a bias, ΔE, between final and initial configurations. As was done earlier formonatomic lattices by Schober et al, the migration energy is splitinto two terms, one depending only on structure and a material-dependent term, given by the Green function elements. Thedifference in size between A and B atoms has to be taken intoaccount and the compounds have to be separated into two groupsdepending on the size of the majority atoms relative to the minorityones. The formulae have been checked by computer simulations.The values of EM or EM-ΔE/2 are calculatedfor those L12 compounds where the phonon dispersions have beenmeasured. In the case of the Ni3Al system, other theoretical andexperimental determinations compare well with our model. Comparingthese energies with the critical temperatures of stability of theL12 structure, we note a significant contribution of the orderingenergy to the migration energy for all three jump types.