Neutron powder diffraction studies on Ba3Ca1+xNb2−xO9−3x/2 complex perovskite-type oxides

High-resolution neutron powder diffraction data were collected on the Ba3Ca1+xNb2−xO9−3x/2 (x=0.10, 0.18, 0.25 and 0.35) at room temperature. All the data were precisely refined by the Rietveld method based on several structure models. Considering the variations of bond lengths and polyhedral volumes of [BO6] octahedra, the most suitable structure model for this system was found to be rhombohedral one based on the space group R3̄m (a≈5.95 Å, α≈60.0°), which was slightly distorted from the cubic model based on the space group Fm3̄m (a≈8.41 Å).