Syntheses and Crystal Structures of CaCuGe,CaAuIn, and CaAuSn – Three Different Superstructures of the KHg2 Type

The intermetallic compounds CaCuGe, CaAuIn, and CaAuSn can be prepared from the elements in sealed tantalum tubes or in glassy carbon crucibles in a high-frequency furnace. Their crystal structures were determined from single crystal X-ray data. The three compounds crystallize with the same subcell structure (KHg₂), however, they form three clearly perceptible superstructures with different unit cells, but all in space groups Pnma: a = 2124.9(6) pm, b = 436.0(2) pm, c = 749.4(5) pm, Z = 12, wR2 = 0.0789, 1303 F² values, 56 variables for CaCuGe (own structure type), a = 738.2(1) pm, b = 459.4(1) pm, c = 839.4(2) pm, Z = 4, wR2 = 0.0651, 656 F² values, 20 variables for CaAuIn (TiNiSi type), a = 3690.3(3) pm, b = 470.5(1) pm, c = 813.6(2) pm, Z = 20, wR2 = 0.1294, 1730 F² values, 92 variables for CaAuSn (new structure type). The three structures may be considered as superstructures of the KHg₂ type with an ordered arrangement of the transition metal and germanium (indium, tin) atoms on the mercury position. Each calcium atom in the structures of CaCuGe, CaAuIn, and CaAuSn has an distinctly ordered near-neighbor environment of six transition metal (T) and six p element (X) atoms in the form of two counter-tilted T₃X₃ hexagons. All known superstructures of the KHg₂ type are described in terms of a group-subgroup scheme.