Configuration and coordination number of cation polyhedra of tungstenbronze-type-like Ba6−3xSm8+2xTi18O54 solid solutions

We have undertaken investigations of the crystal structures and microwave dielectric properties of the Ba6−3xR8+2xTi18O54 solid solutions. The dielectric constant is affected by the volumes of cation polyhedra in which ions are located: octahedral B-site for Ti ion; rhombic A1-site mainly for R ions, deformed from cuboctahedron in perovskite structure; pentagonal A2-site for Ba ions and trigonal C-site for small ions. In the more precise crystal structure, the A1- and A2-sites separate into five and two independent positions, respectively. In this study, the configurations and coordination numbers of the cation polyhedra are presented. The dielectric constant is also discussed based on the volume of these polyhedra.