THE 4-CONNECTED NET IN THE CECU2 STRUCTURE AND ITS TERNARY DERIVATIVES - ITS ELECTRONIC AND STRUCTURAL-PROPERTIES

The crystallochemistry of and the bonding in the orthorhombic four-con nected nets of BaIn2 (CeCu2 structure) and of CaPtSn (TiNiSi structure , a derivative of the CeCu2 structure) are analyzed with approximate m olecular orbital calculations. Following the Zintl concept, in BaIn2 t he In- ions are isoelectronic with group IV tin and should adopt a fou r-connected structure. In contrast to alpha-tin, which has a cubic dia mond structure, the indium ions in BaIn2 build up an orthorhombic thre e-dimensional four-connected net containing distorted tetrahedra and l adder polymers of four-membered rings. In the CeCu2 structure (space g roup Imma) two bond angles in these distorted tetrahedra are fixed at 90 degrees. The four-connected net in the CeCu2 structure is topologic ally related to the layers in black phosphorus (space group Cmca). In CaPtSn (TiNiSi structure) the orthorhombic four-connected net is forme d by (PtSn)(2-) ions in an ordered arrangement. Calculations on BaIn2 and CaPtSn show that the four-connected nets are increasingly stabiliz ed as the valence electron count is increased from 16 to 30 valence el ectrons per 4 formula units. For more than 30e, the nets are destabili zed due to filling of M-E antibonding states. Structural data obtained by precise single crystal investigations for the TiNiSi series CaPdIn (20e), CaPdSn (24e), CaPdSb (28e), and CaAgSb (32e), confirm the resu lts of the extended Huckel calculations. We find an interesting and un derstandable angular asymmetry of the tetrahedral sites in these terna ry compounds.