First-Principles Study of Phase Stability of bcc XZn (X = Cu, Ag, and Au) Alloys

Journal Article · 04 November 2018 · Physical Review Materials

DOI: https://doi.org/10.1103/PhysRevMaterials.2.113601 · OSTI ID:1650605

Albers, Robert C. ^[1]; Alsalmi, Omar ^[2]; Sanati, Mahdi ^[2]; Lookman, Turab  ^[1]; Saxena, Avadh Behari  ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Texas Tech Univ., Lubbock, TX (United States)

First-principles density-functional theory is used to study the phase stability/instability and anomalies in the formation of the high-temperature bcc phases of X Zn (X = Cu, Ag, and Au) alloys. Although from perhaps a naive point of view, their properties are expected to monotonically depend on the noble-metal (X) column position in the periodic table, this is not the case. For example, the middle-column AgZn alloy has a lower bcc order-disorder (critical) temperature than the CuZn and AuZn alloys above and below in the column. It is shown that this and other nonmonotonic behaviors can be explained in terms of a competition between atomic-size effects and X-atom d-orbital spatial extent. For example, charge-density studies and pair-potential modeling of X Zn alloys show that the effective Ag-Zn bond is significantly weaker than either the Cu-Zn or Au-Zn bond at their respective equilibrium lattice constants. Furthermore, we find that an increased atomic-core size effect initially weakens the X-Zn bonding as one goes from CuZn to AgZn, but then the larger d-orbital spatial extent for higher principal quantum numbers becomes a more dominant effect and increases the bonding from AgZn to AuZn. In conclusion, this study is focused on the highly symmetric cubic high-temperature phases, where relative bond-strength magnitudes should be far more important than any bond-directionality effects; the lattice parameters, bulk moduli, elastic constants, Debye temperatures, heats of formation, and order-disorder temperatures for the bcc phases of the three X Zn alloys are calculated and compared with experiment.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: 89233218CNA000001; AC52-06NA25396
• OSTI ID: 1650605
• Report Number(s): LA-UR-18-24361
• Journal Information: Physical Review Materials, Journal Name: Physical Review Materials Journal Issue: 11 Vol. 2; ISSN 2475-9953
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English