The 1 MHz dielectric properties of Ca5Nb2TiO12 and Ca5Ta2TiO12 compounds were reported recently. Both compounds have a common characteristic dependence of the temperature coefficient of dielectric constant (TCK) and dielectric constant (εr) on sintering temperature. For both compounds a TCK < 5 ppm/deg can be achieved by selecting the right processing conditions. It was suggested that an order-disorder transition among the B-site ions in these perovskite-based compounds plays a major role in the determination of TCK. Since X-ray powder diffraction does not appear to be a good technique to detect the structural changes, transmission electron microscopy (TEM) was applied in this work. It was found that different types of ordering between (111) planes, namely 1:1, 1:2 and 1:3, as well as distortions by tilting of octahedra, are involved in the formation of a microstructural state at an ambient temperature. The compounds in the as-sintered conditions both have a microdomain structure but with a different type of ordering, 1:3 for Ca5Nb2TiO12 and 1:2 for Ca5Ta2TiO12. Both compounds have a tilting phase transition from the disordered Pm-3m to distorted Pnma (with the a-b+a- tilt of octahedra) structures at temperatures that coincide with the observed kinks at TA1 and TA2 in the TCK(TS) and K(TS) curves.
All Science Journal Classification (ASJC) codes
- Ceramics and Composites
- Materials Chemistry