Distorted Square Nets of Tellurium in the Novel Quaternary Polytelluride K_0.33Ba_0.67AgTe₂

The synthesis, structure, and band structure analysis of the quaternary compound K_0.33Ba_0.67AgTe₂are reported. Crystals of K_0.33Ba_0.67AgTe₂were obtained in a K₂Te/BaTe/Te flux by the reaction of 1 mmol of K2Te, 0.5 mmol of BaTe, 0.5 mmol of Ag, and 4 mmol of Te in an evacuated Pyrex tube at 450 °C for 3 days followed by a slow cooling to 150 °C. The compound has a substructure in the tetragonal space group I4/mmm (no. 139) with a_sub = 4.624 (2) Å, c_sub = 23.326 (4) Å, V = 498.7 (3) ų, at 20 °C (Mo Ka radiation): Z = 4, D_calc = 6.23 g/cm³, 20_max = 50°, data collected: 592, independent data: 172, observed with I > 3σ (I): 108, variables: 13, final R = 0.054, R_w= 0.067. K_0.33Ba_0.67AgTe₂has a lamellar structure related to that of Na_1.9Cu₂Se₂•CCu₂O. The substructure contains a readily recognizable [Te₂]^4/3-square net. The closest Te-Te distance in the net is 3.269 (2) A, not a full covalent bond, but too short for a simple van der Waals contact. While it is predicted that the square [Te₂]^4/3-net has metallic properties, the experimental data show a semiconductor behavior which has its origins in a structural distortion. Electron diffraction measurements reveal the presence of two different but related superstructures; an incommensurate orthorhombic superstructure of the tetragonal cell with a_super = 2.84a_SUb, b_Super = b_sub, and c_super = c_Sub, and Acommensurate tetragonal superstructure with a_Super = 3a_SUb, b_super = 36_SUb, and c_super = c_sub. Both extended-Hiickel and Hückel calculations suggest that this distortion is a charge density wave. In the case of the incommensurate cell, the theoretically predicted supercell corresponds to the experimentally observed. We also used the μ₂-scaled Hückel method to predict the actual atomic positions within the supercell. The theoretically predicted superstructures have calculated diffraction patterns similar to the experimentally observed ones.