Ab initiocalculation of the phase stability in Au-Pd and Ag-Pt alloys

Abstract

The phase stability of superstructures based on the fcc lattice in the Au-Pd and Ag-Pt alloy systems are examined from the fully relativistic electronic density functional theory. The electron-ion interaction is described by the projector augmented-wave ͑PAW͒ method and the exchange-correlation effects are treated in the generalized gradient approximation ͑GGA͒. The cluster expansion method is used to obtain effective cluster interactions on the fcc lattice and is used also to guide a systematic ground state search for both alloy systems. The ground state analysis reveals a multitude of ground states in Au-Pd, especially at the Au-rich side. Possibly long-period super-structures occur near the Au 70 Pd 30 composition. The ground state analysis indicates a uniquely stable AgPt compound with the L1 1 structure ͑CuPt prototype͒ and it also suggests a marginally stable ordered compound for Ag 3 Pt. However, our ab initio study rules out the existence of the remarkably stable Ag 3 Pt phase with L1 2 structure, reported first more than half a century ago and since then included in many assessments. We also find no indication for a stable ordered state at the AgPt 3 composition. The cluster variation method ͑CVM͒ with a large maximal cluster is used to compute the enthalpy of mixing of the disordered solid solutions and the solid portion of the Au-Pd and Ag-Pt phase diagrams. These results are critically compared with experimental data and phase diagram assessments. It is shown that cluster expansions cannot account for the high-temperature miscibility gap in the Ag-Pt system when the effective cluster interactions do not reach beyond the second nearest neighbor. Only when third nearest neighbors are included in the cluster expansion is it possible to obtain a phase diagram that agrees qualitatively with the assessed Ag-Pt phase diagram.

Ab initioprediction of vacancy properties in concentrated alloys: The case of fcc Cu-Ni

“…A cluster expansion using point, nearest neighbor pair, and nearest neighbor equilateral triangle clusters was fit to in total 71 structures; 2 pure elements, vacuum, 4 SQSs, and 64 single vacancy SQS derived structures. The ECIs were used in a ternary cluster variation method (CVM) [67,68] calculation in the tetrahedron approximation to determine the Cu-Ni phase diagram, and the vacancy concentration as a function of composition and temperature. In the CVM calculations the ratio of Cu to Ni atoms is held constant, but the concentration of the vacancy species is freely varied.…”

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

“…Theoretical investigations on the electronic structure of alloys mostly concerned topics like ordering, phase stability [13,14] and systematic dependence of various properties on the alloy composition [15][16][17][18]. Studies of 2 local environment effects focused on charge transfer and energetics [2, [19][20][21][22].…”

Cluster Expansions for Thermodynamics and Kinetics of Multicomponent Alloys

“…(IV) Enumeration in combination with completeness and mandatory clusters. [55] Imposition of the completeness criterion drastically reduces the number of acceptable CEs. If furthermore it is imposed that certain clusters, such as the empty cluster, the single point and nearest neighbor pair clusters are always required in a valid CE the number of CEs can be reduced further.…”

“…(I) ''aufbau'' where a given CE is expanded with a single new cluster, one at a time. [55] All not yet included clusters are considered. Among the set of not yet included clusters the one that gives the best improvement in the fitness criterion is selected.…”