Lack of support for adaptive superstructure NiPt7: Experiment and first-principles calculations

Order and effective interaction parameters on the Pt-rich side of solid Ni-Pt alloys have been investigated by experimental and first-principles theoretical techniques. Diffuse x-ray scattering was taken from single-crystalline Ni-87.8 at. % Pt aged at 603 K to set up a state of thermal equilibrium. From the separated short-range order scattering, effective pair interaction parameters were determined. These experimentally deduced values do not produce the suggested NiPt7 superstructure at lower temperatures. Instead of that, phase separation into NiPt3 regions with L1(2) structure and a Pt-rich matrix is observed in Monte Carlo simulations and supported by x-ray scattering of Ni-75.2 at. % Pt. First-principles calculations at 0 K also show that the suggested NiPt7 phase is unstable against decomposition into NiPt3 and Pt.