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−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 Cccm (No. 66) with a = 16.625(1) Å, b = 23.594(2) Å, c = 15.369(2) Å (22 °C), and Z = 8. Two different Tl12 units consisting of augmented tetrahedral stars are condensed into layers of such tetrahedra, and further Tl2 dumbbells and the potassium cations also interconnect the stars and layers into a three-dimensional network. The former anionic Tl8 subunits clearly resemble those in the heteroatomic 3-D structure of cubic Cr3Si before their augmentation with bridging atoms. The compound is metallic (ρ270 = 22.6 μΩ·cm, α = 0.0023 K-1) and shows Pauli-like paramagnetic susceptibility (χ296 = 1.1 × 10-4 emu/mol). EHTB calculations illustrate the importance of Tl p-orbital bonding, the positive Tl−Tl overlap populations up to EF, and greater strengths of the Tl−Tl bonding between and about the surface of the augmented Tl12 units. Cations between the thallium layers play specific and important roles in the structure.