Origins of compositional order in NiPt alloys

We investigate the cause of compositional ordering in fcc Ni0.50{Pt}0.50$ on the basis of a first-principles theory of electronic structure and finite-temperature concentration fluctuations. We treat the size effect, electronegativity, and band filling on an equal footing and find that the first of these dominates, leading to L1₀ ordering in agreement with experiment. An electronic origin is given for the metallurgical ``rule of thumb'' that alloys comprised of big atoms and little atoms tend to order into simple stuctures.