COMPOSITIONAL SHORT-RANGE ORDERING IN METALLIC ALLOYS - BAND-FILLING,CHARGE-TRANSFER, AND SIZE EFFECTS FROM A 1ST-PRINCIPLES ALL-ELECTRON LANDAU-TYPE THEORY

Using a mean-field statistical description, we derive a general formal ism to investigate atomic short-range order in alloys based on a densi ty-functional description of the finite-temperature, grand potential o f the random alloy. This ''first-principles,'' Landau-type approach at tempts to treat several contributions (electronic structure, Fermi sur face, electrostatics, magnetism, etc.) to the electronic energy on an equal footing. An important ingredient for the statistical averaging i s the replacement of the molecular mean fields (Weiss fields) with Ons ager cavity fields, which forces the diagonal part of the fluctuation- dissipation theorem to be obeyed. To show its general applicability an d usefulness, we apply the theory to three fcc alloy systems. In Cu0.7 5Pd0.25, the incommensurate atomic short-range order is driven by a Fe rmi-surface effect, in agreement with earlier work. In contrast, Pd0.5 Rh0.5 exhibits clustering tendencies, with both band-filling and charg e-rearrangement effects being important in setting the spinodal temper ature at 1150 K, in good agreement with experiment. In the final examp les of three nickel-rich NiCr alloys, previously ignored electrostatic effects are found to play a significant role in determining the atomi c short-range order.