Multiple Nonstoichiometric Phases with Discrete Composition Ranges in the CaAu5-CaAu4Bi-BiAu2 System. A Case Study of the Chemistry of Spinodal Decomposition

TitleMultiple Nonstoichiometric Phases with Discrete Composition Ranges in the CaAu5-CaAu4Bi-BiAu2 System. A Case Study of the Chemistry of Spinodal Decomposition
Publication TypeJournal Article
Year of Publication2010
AuthorsLin QS, Corbett JD
Journal TitleJournal of the American Chemical Society
Volume132
Pages5662-5671
Date Published04/28
ISBN Number0002-7863
Accession NumberISI:000276991700047
Keywordsalloys, cell, icosahedral quasi-crystal
Abstract

Synthetic explorations in the CaAu5-CaAu4Bi-BiAu2 system at 400 degrees 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)Au-4(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 F (4) over bar 3m and that their structures (MgCu4Sn-type, an ordered derivative of MgCu2) all feature three-dimensional networks of Au-4 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 E-F 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. Its DTA analyses suggest that the phases resulting from spinodal decomposition have nearly the same melting point (similar to 807 degrees C), as expected, and that they are interconvertible through peritectic reactions at similar to 717 degrees C.

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