Synthesis and Structure of the Metallic K<sub>6</sub>Tl<sub>17</sub>:  A Layered Tetrahedral Star Structure Related to That of Cr<sub>3</sub>Si

The title compound, the Tl-richest in the K−Tl 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 <i>Cccm</i> (No. 66) with <i>a</i> = 16.625(1) Å, <i>b</i> = 23.594(2) Å, <i>c</i> = 15.369(2) Å (22 °C), and <i>Z</i> = 8. Two different Tl<sub>12</sub> units consisting of augmented tetrahedral stars are condensed into layers of such tetrahedra, and further Tl<sub>2</sub> dumbbells and the potassium cations also interconnect the stars and layers into a three-dimensional network. The former anionic Tl<sub>8</sub> subunits clearly resemble those in the heteroatomic 3-D structure of cubic Cr<sub>3</sub>Si before their augmentation with bridging atoms. The compound is metallic (ρ<sub>270</sub> = 22.6 μΩ·cm, α = 0.0023 K<sup>-1</sup>) and shows Pauli-like paramagnetic susceptibility (χ<sub>296</sub> = 1.1 × 10<sup>-4</sup> emu/mol). EHTB calculations illustrate the importance of Tl p-orbital bonding, the positive Tl−Tl overlap populations up to <i>E</i><sub>F</sub>, and greater strengths of the Tl−Tl bonding between and about the surface of the augmented Tl<sub>12</sub> units. Cations between the thallium layers play specific and important roles in the structure.