High-Pressure Synthesis and Order-Disorder Transition of Layered Oxytelluride Ba2ZnO2Ag2Te2.

A layered oxytelluride, Ba2ZnO2Ag2Te2, which consists of alternating stacks of ZnO2 and Ag2Te2 layers with Ba ions in between, has been synthesized via high-pressure reaction. At ambient temperature, it contains discrete [ZnO2]2- linear units similar to Ba2ZnO2Ag2Se2 (Cmca), meaning that the unusual linear coordination around zinc center is stable even when a greater tensile strain is applied by the sandwiched Ag2Te2 layers in addition to barium ions. Upon heating, this compound undergoes an order-disorder phase transition from orthorhombic (Cmca) to tetragonal (I4/mmm) system at 350 K, transforming the ZnO2 lattice in linear coordination into a lattice with disordered oxide ions, suggesting the presence of cis/trans coordination coexistence as well as correlated disorder.