The title compound with the trivial name γ-AgMg4 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. γ-AgMg4 is formed by a peritectoid reaction at 472(2) °C from ɛ′-Ag17Mg54 and δ-(Mg). The exact composition corresponds to the formulas AgMg4.08 or Ag9Mg36.7 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. γ-AgMg4 crystallizes hexagonal in the space group P63/m with a = 12.4852(8) Å and c = 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 I3 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  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 I3 cluster concept as a construction kit concerning not only simple subunits but also more complex “patches” with inherent structural disorder.
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