The structures of the reduced and oxidized forms of barium silicon tantalum oxide, Ba 3Si 4Ta 6O 23 and Ba 3Si 4Ta 6O 26

The compound once reported as Ba _{0.5- x}TaO _{3- x} has now been shown to be Ba ₃Si ₄Ta ₆O ₂₃. The silicon is readily introduced from the silica reaction capsule if fluoride is present. The symmetry is hexagonal, space group P overline62m, a = 8.997 ± 0.003, c = 7.745 ± 0.005 Å. The structure contains layers with tantalum coordinated by a square pyramid of oxygens and silicon coordinated tetrahedrally by oxygens. These layers are mirrored to form double layers which are interleaved with barium ions. The oxidized phase, Ba ₃Si ₄Ta ₆O ₂₆, which may be formed by oxidation of the reduced phase, has the same space group, and cell dimensions a = 8.99 ± 0.01, c = 7.79 ± 0.01 Å. The additional oxygens complete octahedra about the tantalum atoms.