TETRAHEDRAL STARS AS FLEXIBLE BASIS CLUSTERS IN SP-BONDED INTERMETALLIC FRAMEWORKS AND THE COMPOUND BALI7AL6 WITH THE NAZN13 STRUCTURE

The cluster units obtained by the capping of all faces of a tetrahedro n (tetrahedral star, TS) and a trigonal bipyramid (double tetrahedral star, DTS) are used as building units to describe and rationalize fram ework structures found in the intermetallic structure types NaBa, NaZn 13, Th6Mn23, Ba2Li4.21Al4.79, BaHg11, BaCd11, Cr23C6, beta-Mn, Ba3Li3G a4.1, BaLi4, CaZn3, EuMg5.2, ErZn5, Sr3Mg13, and Sr9Li17.5Al25.5. The electronic requirements for optimum structural stability of these fram eworks in the case of sp-bonding have been investigated with the simpl e tight-binding Huckel model. As a result the networks exhibit a prono unced maximum of stability in the range of 2.1-2.6 electrons per atom with the particular optimum values slightly depending on the kind of b asis cluster and its connectivity. Considering the sp-bonded represent atives of the above mentioned structure types, the frameworks describe d by the basis clusters TS and DTS are usually formed by the divalent metals Be, Mg, Zn, Cd, and Hg or a combination of a mono-and a trivale nt metal, e.g., (Li,AI), (Cu,AI), or(Ag,AI). More electropositive atom s, Like the heavier alkaline earth metals Ca, Sr, and Ba, are embedded in such frameworks. When applying a formal electron transfer from the se atoms to the slightly more electronegative framework forming atoms, the obtained values for the framework valence electron concentration are very close to the calculated optimum ones. Therefore it is argued that this family of intermetallic compounds can still be interpreted a s electron compounds following the Zintl-Klemm concept. The new repres entative BaLi7Al6, which has been synthesized by fusion of the element s and characterized by single-crystal X-ray diffraction methods, suppo rts this idea. BaLi7Al6 is isotypic to NaZn13 (Fm (3) over bar c, a = 12.9377 (9) Angstrom, Z = 8) with the Zn sites mixed occupied by Li an d Al atoms.