The synthesis, structure, and band structure analysis of the quaternary compound K0.33Ba0.67AgTe2are reported. Crystals of K0.33Ba0.67AgTe2were 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 asub = 4.624 (2) Å, csub = 23.326 (4) Å, V = 498.7 (3) Å3, at 20 °C (Mo Ka radiation): Z = 4, Dcalc = 6.23 g/cm3, 20max = 50°, data collected: 592, independent data: 172, observed with I > 3σ (I): 108, variables: 13, final R = 0.054, Rw= 0.067. K0.33Ba0.67AgTe2has a lamellar structure related to that of Na1.9Cu2Se2•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 asuper = 2.84aSUb, bSuper = bsub, and csuper = cSub, and Acommensurate tetragonal superstructure with aSuper = 3aSUb, bsuper = 36SUb, and csuper = csub. 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry