Vibrational properties of Nix Pt1−x alloys: An understanding from ab initio calculations

The experimental phonon dispersion relations in Nix Pt1−x alloys reveal anomalous features in the phonon branches which are dependent upon their composition and phonon propagation directions. We investigate the origin of the anomalous features by computing the lattice dynamics for the alloy system at various compositions with the newly developed first-principles based methodology which takes into account the disorders in the mass, the force-constant, and the environment. The physics behind the anomalous features is discussed using the coherent scattering structure factors, the disorder-induced widths, and the vibrational densities of states. Our results agree well with the experimental observations both qualitatively and quantitatively for most of the compositions. We demonstrate that the anomalous features arise due to the correlated vibrations of the unlike pairs of species. For Ni0.25 Pt0.75 alloy, our calculations do not find a resonance like behavior in the phonon branches, as observed in the experim...