Formation of the ABC6-type ordered structure in fcc alloys

Abstract

Abstract A new class of ordered structures designated as of the ABC6-type has been found in the ternary CuMnPt6 alloy. It has a cubic symmetry with the unit cell consisting of 2 ⨯ 2 ⨯ 2 fcc unit cells, whose space group is Fm3m. A closely related structure with Cu replaced by Pt has also been found in the binary MnPt7 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. MnPt7 is isostructural to CuPt7 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 double-step transition and the ABC6-type phase forms directly from the disordered phase.