Approximant phases and an icosahedral quasicrystal in the Ca-Au-Ga system: The influence of size of gallium versus indium

Two crystalline approximants (ACs) and their corresponding icosahedral quasicrystal (i-QC) are obtained in the Ca-Au-Ga system through conventional solid-state exploratory syntheses. Single crystal structural analyses reveal that the 1/1 AC, Ca(3)Au(x)Ga(19-x) (x = similar to 9.3-12.1) [Im3, a = 14.6941(6)-14.7594(6) angstrom], has the empty cubes in the prototypic YCd(6) (= Y(3)Cd(18)) now fully occupied by Ga, resulting in a 3:19 stoichiometry. In parallel, the distorted cubes in the 2/1 AC, Ca(13)Au(57.1)Ga(23.4) [Pa3, a = 23.9377(8) angstrom] are fully or fractionally occupied by Ga. The valence electron count per atom (e/a) for the 2/1 AC (1.64) is smaller than that over the 1/1 AC composition range (1.76-2.02), and the e/a of the Ca(15.2)Au(50.3)Ga(34.5) i-QC, 1.84, is somewhat distant from typical values for Tsai-type i-QCs (similar to 2.0). Comparisons of the gallium results with the corresponding In phases suggest that the structural differences result mainly from size rather than electronic factors. The 1/1 and 2/1 appear to be thermodynamically stable on slow cooling, as usual, whereas the i-QC isolated by quenching decomposes on heating at similar to 660 degrees C, mainly into 2/1 AC and Ca(3)(Au,Ga)(11). Calculations of the electronic structure of 1/1 AC suggest that the Fermi sphere-Brillouin zone interactions remain important for the Ca-Au-Ga i-QC.