Structural, vibrational, and thermodynamic properties of ordered and disordered Ni_1-xPt_x alloys from first-principles calculations

In terms of first-principles phonon calculations and the quasiharmonic approach, the structural, vibrational, and thermodynamic properties have been investigated for the ordered and disordered Ni_1-xPt_x alloys, with the main focus being on disordered Ni_0.5Pt _0.5. To gain insight into the disordered alloys, we use special quasirandom structures (SQSs) and demonstrate their capabilities in predicting (i) the bond-length distributions, (ii) the phonon spectra, and (iii) the elastic stiffness constants of the disordered alloys. It is found that the Pt-Pt atomic pairs possess the longest bond lengths relative to the Ni-Pt and Ni-Ni ones in the disordered alloys, the predicted force constants indicate that the Pt-Pt bond is stiffer when compared to the Ni-Pt and the Ni-Ni ones for both the ordered and disordered alloys, and the phonon density of states of the disordered alloys are similar to the broadened versions of the ordered cases. Based on the results of the ordered and disordered alloys, a slightly positive deviation from Vegard's law is found for the volume variation of Ni _1-xPt_x, and correspondingly, a negative deviation is predicted for the change of bulk modulus. With increasing Pt content, the bulk modulus derivative relative to pressure increases approximately linearly, whereas the magnetic moment decreases. In addition, the SQS-predicted relative energies (enthalpies of formation) for the disordered Ni_1-xPt _x are also compared to cluster expansion predictions. As an application of the finite temperature thermodynamic properties, the phase transition between the ordered L1₀ and the disordered Ni _0.5Pt_0.5 is predicted to be 755 ± 128 K, which agrees reasonably well with the measurement ∼900 K, demonstrating that the driving force of the phase transition stems mainly from the configurational entropy rather than the vibrational entropy.