Multiple Nonstoichiometric Phases with Discrete Composition Ranges in the CaAu5−CaAu4Bi−BiAu2System. A Case Study of the Chemistry of Spinodal Decomposition

Synthetic explorations in the CaAu(5)-CaAu(4)Bi-BiAu(2) system at 400 degrees C reveal five separate solid solution regions that show three distinct substitution patterns in the CaAu(5) parent: (I) CaAu(4)(Au(1-m)Bi(m)) with 0 < or = m < or = 0.15(1), (II) 0.33(1) < or = m < or = 0.64(1), (III) 0.85(4) < or = m < or = 0.90(2); (IV) (Ca(1-r)Au(r))Au(4)(Bi(1-s)Au(s)) with 0 < or = r < or = 0.39(1) and 0 < or = s < or = 0.12(2); (V) (Ca(1-p-q)Au(p)Bi(q))Au(4)Bi with 0.09(2) < or = p < or = 0.13(1) and 0.31(2) < or = q < or = 0.72(4). Single crystal X-ray studies establish that all of these phase regions have common cubic symmetry F43m and that their structures (MgCu(4)Sn-type, an ordered derivative of MgCu(2)) 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 CaAu(4)Bi 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 CaAu(4.25)Bi(0.75). Its DTA analyses suggest that the phases resulting from spinodal decomposition have nearly the same melting point (approximately 807 degrees C), as expected, and that they are interconvertible through peritectic reactions at approximately 717 degrees C.