New Ternary Ordered Structures in CuMPt₆ (M = 3d elements) Alloys

Abstract

X-ray and electron diffraction measurements were performed to investigate the structure and ordering behaviour of the ternary alloys CuMPt₆ (M=Ti, V, Cr, Mn, Fe, Co, and Ni). X-ray polycrystalline diffraction patterns of all the specimens quenched from 1000°C have shown that a single phase is formed at this stoichiometric composition. The alloys with M=Cr, Mn, Co, and Ni have the face-centred cubic (fcc) structure, while in the alloys with M=Ti, V, and Fe ordering has occurred and the structure is of the Cu₃Au type. On annealing at lower temperatures ordering has been induced in the alloys with M=Cr, Mn, and Co and the structure is of the Cu₃Au type, though the ordering in the last alloy has remained incomplete. Detailed X-ray diffraction measurements on single crystals of the CuMnPt₆ alloy have revealed that further ordering takes place and structure changes from the Cu₃Au type into the cubic ABC₆ type with the unit cell as large 2×2×2 as the fcc unit cell, a new observation of the double-step ordering in the ternary fcc alloy. The corresponding transition temperatures are T_c=970(±5)°C and T_cl=750(±5)°C.