L1₂ - DO₂₂ Competition in the Quasi-Binary (Pt, Rh)₃V and (Pd, Rh)₃V Alloys

Abstract

The Pt₃V and Pd₃V alloys, which are ordered at low temperature on the FCC lattice, have the particular feature of exhibiting different local order in the disordered and ordered states. In the disordered state, the local order which is characterized by maxima of diffuse intensity at positions of 100 type (Fig.la,b) is that of the cubic L1₂ structure (Fig.2a). The ordered ground state has the DO₂₂ structure of quadratic symmetry, which can be obtained by introducing each L1₂ cube an antiphase boundary (APB) in the (001) planes (Fig.2b). This ordering produces superstructure reflections of 101/2 type (Fig.2c). It has been shown from neutron diffraction measurements of the diffuse intensity in Pd₃V that the energy of these APB is very low, or in other words that the atomic interactions are such that L1₂ and DO₂₂ are quasi-degenerate and competitive.