Relativity-Induced Ordering and Phase Separation in Intermetallic Compounds

The formation enthalpies of ordered compounds and the mixing enthalpies of random alloys were calculated for Ni-Au, Ni-Pt, and Au-Pt using an Ising-like cluster expansion based on the local-density formalism. We show that relativity i) induces long-range order in Ni-Pt due to a reduction in packing strain and enhancement of s-d coupling, but ii) it leads to phase separation in Au-Pt due to suppression of the Au( s,p ) → Pt( d ) charge transfer.