Crystal Structure of Ba_0.5-x TaO_3-x

Abstract

A STUDY of the barium–tantalum–oxygen system has produced the compounds Ba₃TaO_5.5 with an ordered perovskite structure¹, Ba₅Ta₄O₁₅ with a hexagonal layer structure², Ba_0.5TaO₃ with the tetragonal bronze structure³, and Ba_0.44TaO_2.94, a hexagonal phase for which a trial structure has just been determined. The preparation of a powder and a single crystal of the reduced phase, Ba_0.44(Ta_0.74 ^IVTa_0.26 ^V)O_2.57, as well as analogous phases has previously been described⁴. It was later found that this reduced phase was produced only when the tantalum pentoxide used in the preparation was obtained from a particular source. The method of preparation is to mix appropriate quantities of tantalum pentoxide, tantalum metal, and barium oxide and heat in vacuum. Attempts to prepare larger amounts of the phase for density measurements after the original source of tantalum pentoxide was exhausted, were unsuccessful unless a barium chloride flux was added. The preparations obtained with the use of the flux, however, were not homogeneous, and good density measurements were therefore not obtained.