Multiple Nonstoichiometric Phases with Discrete Composition Ranges in the CaAu5 -CaAu4 Bi-BiAu2 System. A Case Study of the Chemistry of Spinodal Decomposition
- Ames Lab., Ames, IA (United States); Iowa State Univ., Ames, IA (United States)
Synthetic explorations in the CaAu5-CaAu4Bi-BiAu2 system at 400 °C reveal five separate solid solution regions that show three distinct substitution patterns in the CaAu5 parent: (I) CaAu4(Au1-mBim) with 0 ≤ m ≤ 0.15(1), (II) 0.33(1) ≤ m ≤ 0.64(1), (III) 0.85(4) ≤ m ≤ 0.90(2); (IV) (Ca1-rAur)Au4(Bi1-sAus) with 0 ≤ r ≤ 0.39(1) and 0 ≤ s ≤ 0.12(2); (V) (Ca1-p-qAupBiq)Au4Bi with 0.09(2) ≤ p ≤ 0.13(1) and 0.31(2) ≤ q ≤ 0.72(4). Single crystal X-ray studies establish that all of these phase regions have common cubic symmetry F43m and that their structures (MgCu4Sn-type, an ordered derivative of MgCu2) all feature three-dimensional networks of Au4 tetrahedra, in which the truncated tetrahedra are centered and capped by Ca/Au, Au/Bi, or Ca/Au/Bi mixtures to give 16-atom Friauf polyhedra. TB-LMTO-ASA and -COHP calculations also reveal that direct interactions between Ca-Au and Ca-Bi pairs of atoms are relatively weak and that the Bi-Au interactions in the unstable ideal CaAu4Bi are antibonding in character at EF but that their bonding is optimized at ±1 e. Compositions between the five nonstoichiometric phases appear to undergo spinodal decompositions. The last phenomenon has been confirmed by HRTEM, STEM-HAADF, EPMA, and XRD studies of the nominal composition CaAu4.25Bi0.75. In conclusion, its DTA analyses suggest that the phases resulting from spinodal decomposition have nearly the same melting point (~807 °C), as expected, and that they are interconvertible through peritectic reactions at ~717 °C.
- Research Organization:
- Ames Lab., Ames, IA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- DOE Contract Number:
- DE-AC02-07CH11358
- OSTI ID:
- 985516
- Report Number(s):
- IS-J7557
- Journal Information:
- Journal of the American Chemical Society, Vol. 132, Issue 16; ISSN 0002-7863
- Publisher:
- American Chemical Society (ACS)
- Country of Publication:
- United States
- Language:
- English
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