Structural reasons for the high dielectric anisotropy of tungsten bronze-like dielectrics

Single crystal x-ray analyses of Ba6-3xR8+2xTi18O54 were performed for the La, Pr, and Sm analogues. Calculations based on refined atomic fractional c oordinates showed that 53% of [100], 100% of [010], and 67% of [001] direct ion tilt angles increase with a reduction in the ionic radius of ions occup ying pentagonal or tetragonal channels. This shows a significant anisotropy in the flexibility of the lattice. High flexibility in the [010] direction is also reflected in the anisotropy of the thermal expansion and in variat ions of the cell parameters across particular solid solubility regions. Var iations in the dielectric properties are correlated to modifications of the octahedral network indicating that the high dielectric anisotropy is relat ed to the high anisotropy in the network flexibility.