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 E_F, 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.