CRYSTAL AND ELECTRONIC-STRUCTURES OF SCAUGE, CEAUGE, AND LUAUGE - A TRANSITION FROM 2-DIMENSIONAL TO 3-DIMENSIONAL [AUGE] POLYANIONS

New germanides ScAuGe, TmAuGe, and LuAuGe were prepared by melting mix tures of the elements in an are furnace and subsequent annealing at 10 70 K. The structures of ScAuGe and LuAuGe were refined from X-ray sing le-crystal diffractometer data: P6(3)mc (No. 186), Z=2, a=430.82(5) pm , c=684.58(10) pm, V=0.1100(1) nm(3), wR2=0.0688, 275 F-o(2) values, t en variables for ScAuGe; P6(3)me (No. 186), Z=2, a=437.75(4) pm, c=711 .38(6) pm, V=0.1181(1) nm(3), wR2=0.0340, 355 F-o(2) values, 11 variab les, and a batch scale factor of 0.47(3) for LuAuGe. The lattice const ants for TmAuGe are a=439.08(4) pm, c=716.59(7) pm, and V=0.1196(1) nm (3). The crystal structures of these germanides are derived from the C aIn2-type structure by an ordered arrangement of Au and Ge atoms at th e indium position. The crystal chemistry of ScAuGe and LuAuGe is compa red with that of the recently reported cerium compound. Although the A u-Ge intralayer distances at 259.6 pm in CeAuGe, 260.5 pm in LuAuGe, a nd 257.6 pm in ScAuGe are similar, the Au-Ge interlayer distances at 3 64.2 pm in CeAuGe, 292.7 pm in LuAuGe, and 275.2 pm in ScAuGe differ s ignificantly. Thus, the [AuGe] polyanions are changed from two-dimensi onal layers in CeAuGe to a three-dimensional network of distorted tetr ahedra in ScAuGe. Chemical bonding within the structures was investiga ted by TB-LMTO-ASA band structure calculations. The energy bands, the densities of states and the valence charge densities are discussed. Bo nding is characterized by intralayer bonds between Au and Ge within th e puckered AuGe hexagons. In addition, in the case of ScAuGe strong bo nds are directed from the Au atoms of one layer to the Ge atoms of the neighbouring layer. Weaker interlayer bonding is observed in LuAuGe a nd no interlayer interaction in CeAuGe, as already indicated by the pr onounced increase of the interlayer distances.