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- xTaO 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.