Abstract. The crystal structures of two new topologically close-packed intermetallic compounds, MgIr (Cmca, 25 atoms) and Mg 2-x Ir 3+x (x=0.067, C2/m, 11 atoms) were fully characterized by high resolution synchrotron powder diffraction and global optimization of a structural model in direct space. The simulated annealing algorithm (in parallel tempering mode) and computer program Fox were used.

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The crystal structures of the ternary compounds Tb 0.67 PdAl 3 (hP14-4.67, P6 3 /mmc, a = 4.394(4), c = 9.372(6) Å) and Gd 1.33 Pt 3 Al 8 (hR51-14.00, R   m, a = 4.3090(14), c = 38.54(2) Å) were refined from X-ray single-crystal diffraction data. They contain monoatomic layers stacked along [001] with ordered or disordered distribution of rare-earth-metal atoms and Al-atom triangles. Together with the structures of Y 2 Co 3 Ga 9 , Sc 0.67 Fe 2 Si 5 , Er 4 Pt 9 Al 24 , and ErNi 3 Al 9 , they form a family of intergrowth structures built up of three kinds of monoatomic layer. Analyzing the relative positions of the monoatomic layers, some basic stacking rules, which may help to predict new structures, are formulated. Aluminide / Rare-earth metal / Noble metal / Single-crystal X-ray diffraction / Intergrowth layered structure

The crystal structure of the substitutional solid solution HfGa 1-x Sn x (x = 0-0.34(1)) was refined from X-ray single-crystal and powder diffraction data (space group Pbcm, Pearson symbol oP24, a = 9.3370(19), b = 8.6920(17), c = 5.6650(11) Å for Hf 3 Ga 1.97(2) Sn 1.03(2)). The structure of the ideal limiting formula Hf 3 Ga 2 Sn, ignoring partial Ga/Sn disorder, represents the first ternary ordering derivative of the structure type ThIn. The structure of Hf 3 Ga 2 Sn is closely related to that of Hf 5 GaSn 3 (limiting composition of the interstitial solid solution Hf 5 Ga x Sn 3 (x = 0-1), structure type Hf 5 CuSn 3 , space group P6 3 /mcm) and can be derived from it applying group-subgroup relations between the space groups.

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