Formation of the ABC6-type ordered structure in fcc alloys

Abstract A new class of ordered structures designated as of the ABC6-type has been found in the ternary CuMnPt alloy. It has a cubic symmetry with the unit cell consisting of fcc unit cells, whose space group is Fmm. A closely related structure with Cu replaced by Pt has also been found in the binary MnPt alloy. Both alloys undergo a double-step order–disorder phase transition, ABC6-type – Cu3Au-type – fcc disorder. A partial phase diagram was constructed on the Pt-rich side of Pt–Mn. MnPt is isostructural to CuPt previously found by Schneider and Esch, though the latter does not show the double-step transition. Within the Bragg–Williams approximation, the order–disorder phenomena in the three alloy systems can be successfully reproduced. It has been found that an ordering energy of negative sign, a preference of unlike pairs, between second-nearest neighbours plays a decisive role in the formation of the ABC6-type structure. An increase in the relative magnitude of the ordering energy suppresses the doub...