SYNTHESES AND CRYSTAL-STRUCTURES OF CACUGE, CAAUIN, AND CAAUSN - 3 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 crucib les in a high-frequency furnace. Their crystal structures were determi ned from single crystal X-ray data. The three compounds crystallize wi th the same subcell structure (KHg2), however, they form three clearly perceptible superstructures with different unit. cells, but all in sp ace groups Pnma: a = 2124.9(6) pm, b = 436.0(2) pm, c=749.4(5)pm, Z=12 , wR2=0.0789, 1303 F-2 values, 56 variables for CaCuGe town structure type), a = 738.2(1) pm, b = 459.4(1) pm, c = 839.4(2) pm, Z = 4, wR2 = 0.0651, 656 F-2 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 F-2 values , 92 variables for CaAuSn (new structure type). The three structures m ay be considered as superstructures of the KHg2 type with an ordered a rrangement of the transition metal and germanium (indium, tin) atoms o n the mercury position. Each calcium atom in the structures of CaCuGe, CaAuIn, and CaAuSn has an distinctly ordered near-neighbor environmen t 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-sub-group scheme.