On the crystal structure of γ-AgMg4

• Published in: Zeitschrift für Kristallographie. Crystalline materials Vol. 220; no. 2; pp. 102 - 114
• Main Authors: Kudla, Christian, Prots, Yurii, Leineweber, Andreas, Kreiner, Guido
• Format: Journal Article
• Language: English
• Published: De Gruyter Oldenbourg 01.03.2005
• ISSN: 2194-4946; 2196-7105
• DOI: 10.1524/zkri.220.2.102.59134

The title compound with the trivial name -AgMg is obtained by melting the elements in weld-sealed tantalum ampoules and subsequent annealing. The alloys were characterized by chemical, metallographic, thermal and X-ray powder analyses. -AgMg is formed by a peritectoid reaction at 472(2) °C from ′-Ag Mg and -(Mg). The exact composition corresponds to the formulas AgMg or Ag Mg with 80.3(1) at-% Mg concentration. The title compound is a complex metallic alloy phase due to the large number of atoms and intrinsic structural disorder. -AgMg crystallizes hexagonal in the space group with = 12.4852(8) Å and = 14.4117(9) Å in a unique structure type without a perceptible homogeneity range. The crystal structure has been studied including disorder phenomena by means of Rietveld refinements of X-ray and neutron powder diffraction data and X-ray single crystal structure analyses. The crystal structure is a hitherto unknown 3 cluster phase, i.e., an intermetallic compound with building blocks of three vertex connected icosahedra as the fundamental structural units joined via a small number of connection types (here B- and L-type). Channels along [001] reveal distinct structural disorder described as a packing of approximately 1.9 icosahedra, 1.3 tricapped trigonal prisms and 0.4 Frank-Kasper Z15 polyhedra per unit cell. The findings support the usefulness of the 3 cluster concept as a construction kit concerning not only simple subunits but also more complex “patches” with inherent structural disorder.