**11. Multiphase MW dielectrics**

High Q value is usually observed in single-phase systems. In the case of complex cation sublattices, it is necessary that the ions should be ordered by a definite type [105, 106]. In multiphase systems, which are chemically inhomogeneous, considerable dielectric loss (relatively low Q) is generally observed. When investigating barium polytitanates, however, we showed that multiphase systems having a high Q and thermostability of electrophysical properties can be formed. When zinc oxide is added to barium polytitanates, an extra BaZn2Ti4O11 phase is formed [132] which does not interact chemically with the main phase. A multiphase system is formed, in which the main and extra phases have the dependence ε(T) of different sign, which ensures the realization of the volume temperature compensation effect and hence a high thermostability of electrophysical properties (TCε = ±2 × 10-6K-1) in the MW range. The multiphase dielectrics obtained have a high Q (Q10GHz ~ 6500- 7000).

One more example of multiphase MW dielectrics is TiO2 materials, viz the compounds MgTiO3 and Mg2TiO4, which have a high Q (Q10GHz ~ 5000-10000) and permittivity (14 and 16 respectively) [133]. A demerit of these materials is the temperature instability of electrophysical parameters (TCε = (40-50) × 10-6K-1). To increase the Q value, cobalt ions were partially substituted for magnesium ions, and to increase the temperature stability of electrophysical properties, small amounts of the paraelectric phase CaTiO3, which has a high negative value of TCε, were added. Investigations showed that in this case multiphase systems of chemically noninteracting phases are formed (Fig 21) [134, , 136].

This made it possible to obtain MW dielectrics with a permittivity of 18-20, high Q values (Q × ƒ ≥ 5000-10000) and thermostable electrical properties. It may be supposed that high Q values are due to the fact that the size of chemical inhomogeneity is much smaller than the electromagnetic wavelength in dielectric and does not cause, therefore, noticeable electromagnetic scattering.

**Figure 21.** Micrograph of the microsection of 0.93 [0.98Mg2TiO4 – 0.02 Co2 TiO4] -0.07CaTiO3 ceramic
