A(15)Tl(27) (A=Rb,Cs): A structural type containing both isolated clusters and condensed layers based on the Tl-11 fragment. Syntheses, structure, properties, and band structure

The isomorphous title compounds (and the ordered substitutional Rb14CsTl27) are obtained directly from reactions of the elements in sealed Ta below similar to 330 degrees C. Refinements of single-crystal data for the three established a structure with alternate layers of isolated pentacapped trigonal prismatic Tl-11(7-) (D-3h) ions and condensed (2.)(infinity)[Tl-16(8-)] networks that are separated by cations. The condensed layer consists of Tl-11 units that share prismatic edges and ate interbridged through waist-capping atoms (Tl6/2Tl3Tl2). (Rb15Tl27: P (6) over bar 2m, Z = 1, a = 10.3248(6) Angstrom, c = 17.558(2) Angstrom.) The rubidium phase is a poor metal (p(293) similar to 34 mu Omega.cm) and is Pauli-paramagnetic. Extended Huckel band calculations indicate partially filled bands and a non-zero DOS at E(F), consistent with the observed metallic behavior, although appropriate cation tuning or modest anion doping should provide a Zintl phase. The band structure and COOP curves are also used to rationalize the distortion of the Tl-11 unit on condensation and the critical role of the interfragment bonds between waist-capping atoms in stabilizing the layer.