The title compound, the TI-richest in the K-TI system, has been synthesized in Ta containers via direct reaction of the elements at 400 degreesC followed by quenching to room temperature and subsequent annealing at 150 degreesC for 4 weeks. It crystallizes in the orthorhombic space group Cccm (No. 66) with a = 16.625(l) Angstrom, b = 23.594(2) Angstrom, c 15.369(2) Angstrom (22 degreesC), and Z = 8. Two different TI12 units consisting of augmented tetrahedral stars are condensed into layers of such tetrahedra, and further TI2 dumbbells and the potassium cations also interconnect the stars and layers into a three-dimensional network. The former anionic TI8 subunits clearly resemble those in the heteroatomic 3-D structure of cubic Cr3Si before their augmentation with bridging atoms. The compound is metallic (rho(270) = 22.6 muOmega(.)cm, alpha = 0.0023 K-1) and shows Pauli-like paramagnetic susceptibility (chi(296) = 1.1 X 10(-4) emu/mol). EHTB calculations illustrate the importance of TI p-orbital bonding, the positive TI-TI overlap populations up to E-F, and greater strengths of the TI-TI bonding between and about the surface of the augmented TI12 units. Cations between the thallium layers play specific and important roles in the structure.