Structural, vibrational, and thermodynamic properties of ordered and disordered Ni1−xPtxalloys from first-principles calcul

Abstract

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−x Pt x alloys, with the main focus being on disordered Ni 0.5 Pt 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−x Pt 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−x Pt x are also compared to cluster expansion predictions. As an application of the finite temperature thermodynamic properties, the phase transition between the ordered L1 0 and the disordered Ni 0.5 Pt 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.

Local environment effects in the magnetic properties and electronic structure of disordered FePt

“…We model it by SQS's with an are taken from Su et al [32], the data for SQS-8, SQS-16, and SQS-32 are taken from Shang et al [33]. Note that the SQS-8 structure we use is equivalent to the SQS-8b structure of Lu et al [34].…”

“…or within the generalized gradient approximation (GGA) [33,39]. Our focus is on magnetic properties, so we used the Vosko, Wilk, and Nusair parametrization within the LDA scheme [40], because benefits of the GGA are questionable for magnetic studies [41][42][43].…”

The development of phase-based property data using the CALPHAD method and infrastructure needs

“…Even though the Additional information discrepancies on lattice stability between the classic CALPHAD modeling and DFTbased first-principles calculations still exist [123] progresses have been made to narrow the differences such as bcc Ti [124] and fcc W [125], even for liquid solution phases [44]. The efficient special quasirandom structures (SQS) approach [126][127][128] is particularly useful in predicting the enthalpy and entropy of mixing in solid solution phases using phonon or Debye models [129,130]. Consequently, the thermodynamic model parameters of all individual phases can be evaluated solely from DFT-based first-principles calculations.…”

Li–Na ternary amidoborane for hydrogen storage: experimental and first-principles study

“…The SQS method has many successful applications on alloys. 44,51,53,54 The reason why the SQS method failed to reproduce the structure of NaLi(AB) 2 − is a harder Lewis base than hydridic H from [BH 3 ], and Li + is a harder Lewis acid than Na + . When Li and Na cations co-exist, according to the general rule of "hard acids tend to bind to hard bases, soft acids tend to bind to soft bases", the donor atom N in the hard base [NH 2 ]…”