Lack of support for adaptive superstructureNiPt7: Experiment and first-principles calculations

Abstract

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 $\text{Ni-}87.8\text{ }\text{at}\text{.}\text{ }%\text{ }\text{Pt}$ 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 ${\text{NiPt}}_{7}$ superstructure at lower temperatures. Instead of that, phase separation into ${\text{NiPt}}_{3}$ regions with $\text{L}{1}_{2}$ structure and a Pt-rich matrix is observed in Monte Carlo simulations and supported by x-ray scattering of $\text{Ni-}75.2\text{ }\text{at}\text{.}\text{ }%\text{ }\text{Pt}$. First-principles calculations at 0 K also show that the suggested ${\text{NiPt}}_{7}$ phase is unstable against decomposition into ${\text{NiPt}}_{3}$ and Pt.