Cu-Au, Ag-Au, Cu-Ag, and Ni-Au intermetallics: First-principles study of temperature-composition phase diagrams and structures

V. Ozoliņš, C. Wolverton, Alex Zunger

Research output: Contribution to journalArticle

246 Scopus citations

Abstract

The classic metallurgical systems—noble-metal alloys—that have formed the benchmark for various alloy theories are revisited. First-principles fully relaxed general-potential linearized augmented plane-wave (LAPW) total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties, and bond lengths in Cu-Au, Ag-Au, Cu-Ag, and Ni-Au alloys. (i) Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at (Formula presented) K. (ii) Of all possible structures, (Formula presented) and CuAu (Formula presented) are found to be the most stable low-temperature phases of (Formula presented) with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and ≈670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. (iii) LAPW formation enthalpies demonstrate that (Formula presented), the commonly assumed stable phase of (Formula presented), is not the ground state for Au-rich alloys, but rather that ordered (100) superlattices are stabilized. (iv) We extract the nonconfigurational (e.g., vibrational) entropies of formation and obtain large values for the size-mismatched systems: 0.48 (Formula presented)/atom in (Formula presented) K), 0.37 (Formula presented)/atom in (Formula presented) K), and 0.16 (Formula presented)/atom in (Formula presented) K). (v) Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent extended x-ray-absorption fine-structure measurements.

Original languageEnglish (US)
Pages (from-to)6427-6443
Number of pages17
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume57
Issue number11
DOIs
StatePublished - Jan 1 1998

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics

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