Synthesis and Structure of the Metallic K6Tl17: A Layered Tetrahedral Star Structure Related to That of Cr3Si

The title compound, the Tl-richest in the K-TI system, has been synthesized in Ta containers via direct reaction of the elements at 400 °C followed by quenching to room temperature and subsequent annealing at 150 °C for 4 weeks. It crystallizes in the orthorhombic space group Cccm (No. 66) with a = 16.625(1) A, b = 23.594(2) A, c = 15.369(2) A (22 °C), and Z= 8. Two different Tl12 units consisting of augmented tetrahedral stars are condensed into layers of such tetrahedra, and further T^ dumbbells and the potassium cations also interconnect the stars and layers into a three-dimensional network. The former anionic Tie subunits clearly resemble those in the heteroatomic 3-D structure of cubic CrsSi before their augmentation with bridging atoms. The compound is metallic (p270= 22.6 (mu)Q-cm, alpha = 0.0023 K^-1) and shows Pauli-like paramagnetic susceptibility (x296 = 1-1 x 10^-4 emu/mol). EHTB calculations illustrate the importance of Tl p-orbital bonding, the positive TI-TI overlap populations up to Ep, and greater strengths of the TI-TI bonding between and about the surface of the augmented Tl12 units. Cations between the thallium layers play specific and important roles in the structure.