Origins of compositional order in NiPt alloys

We investigate the cause of compositional ordering in fcc ${\mathrm{Ni}}_{0.50}$ ${\mathrm{Pt}}_{0.50}$ 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 L$1_0$ 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.