MgAuGa and MgAu2Ga: first representatives of the Mg-Au-Ga system

MgAuGa (magnesium gold gallium), the first ternary representative of the Mg-Au-Ga system, crystallizes in the space group P62m and adopts the Fe2P structure type (Pearson symbol hP9). Various phases with the general composition AB2 have been reported in the surrounding binary systems, viz. Mg2Ga (hP18), MgGa2 (hP6; CaIn2 type), AuGa2 (cF12; CaF2 type), Au2Ga (oS24; Pd2As type) and Mg2Au (oP12; Co2Si type). In principle, MgAuGa can be obtained from each of them by partial replacement of the major element with the missing element. In fact, the structure of MgAuGa closely resembles hexagonal Mg2Ga through a direct group-subgroup relationship. MgAu2Ga (magnesium digold gallium) also crystallizes hexagonally in the space group P6(3)/mmc and is isotypic with Na3As. It adopts the structure of another binary compound, viz. Mg3Au (hP8), but shows an unexpected distribution of Mg, Au, and Ga among the atomic positions of the asymmetric unit. Both MgAuGa and MgAu2Ga can be described as formally anionic Au/Ga frameworks, with pseudo-hexagonal tunnels around Mg in MgAuGa or cages in MgAu2Ga.