**6. MW dielectrics based on antiferroelectrics-paraelectrics**

The permittivity value of the above-mentioned thermostable MW dielectrics is not over 80 – 100. To achieve higher permittivity values, use must be made of other polarization mechanisms connected with spontaneously polarized state. One of the possible ways of developing thermostable MW resonant elements is the creation of two-layer systems. Each of the layers must have a high Q (Q = 1/ tg δ), a high permittivity and TCε of different sign. A large number of high-Q dielectrics which have TCε < 0 in the MW range is known (mainly paraelectrics), whereas there are few of them with TCε > 0 (LiNbO3, LiTaO3, LiAlO3 single crystals).

Besides, they are characterized by relatively low ε values (ε < 50) in the MW range, and the use of them in this range makes it possible to obtain two-layer resonant dielectric elements with an effective permittivity (εeff) of not over 50. Therefore, we examined the possibility to use as high-Q materials with TCε > 0 antiferroelectrics based on tellurium-containing Pb2BTeO6-type perovskites (B = bivalent metal ions) [36, 37]. It has been found that the plot of ε against temperature for lead-cobalt tellurate (Pb2CoTeO6) in the MW range passes through a maximum (phase transition from the antiferroelectric to the paraelectric state) at 380 K (Fig 7). At room temperature, the materials of this group have high permittivity values (ε > 110). In the temperature range 220-350 K, the dependence ε (T) is close to linear one and has TCε ~ 700 × 10-6 K-1. Two-layer resonant elements: Pb2CoTeO6 – TiO2, Pb2CoTeO6 – CaTiO3, Pb2CoTeO6 – SrTiO3 have been prepared on the basis of paraelectrics (TiO2, CaTiO3, SrTiO3) and the antiferroelectric Pb2CoTeO6. Known two-layer resonant dielectric LiNBO3 – TiO2 elements have been investigated for comparison. It can be seen from Table 3 and Fig 8 that Pb2CoTeO6 - SrTiO3 based two-layer resonant elements have in the MW range a high effective permittivity (εeff ≈ 135), a high Q (Q10GHz ~ 900) and a high temperature stability of dielectric parameters (TCε tends to zero) [37]. Of course, two-layer systems have a number of demerits since they require mechanical bonding of different layers.

Microwave Dielectrics Based on Complex Oxide Systems 123

Materials Qef. Qef.

Pb2CoTe6–TiO2 1000 110 Pb2CoTe6–CaTiO3 900 125 Pb2CoTe6–SrTiO3 900 135 LiNbO3–TiO2 3000 49

**7. MW dielectrics based on spontaneously polarized phases** 

crystal, in which phase transition is observed at high temperature (> 1200 0C).

Thermostable MW dielectrics with high permittivity (ε ~ 430) and Q (Q1GHz ~ 700) have been obtained by the authors of [38] on the basis of the Ag (Nb, Ta) O3 system, which is characterized by the spontaneously polarized state. However, in the materials in which the spontaneously polarized state is present, increase in permittivity is always accompanied by an increase in dielectric loss, which impairs the technical characteristics of MW elements

The materials in which there is a phase transition from the spontaneously polarized to the unpolarized state at high temperatures are characterized by positive TCε. In the phase transition region, the tg δ values in the MW range are, as a rule, large, which is due to the presence of mobile domain walls (ferroelectrics). A salient feature of antiferroelectrics is the immobility of domain walls. This results in the fact that antiferroelectric (e.g. Pb2CoTeO6) is characterized by a relatively low tg δ value at room temperatures in the MW range [37]. In some cases, ferroelectrics, too, have low tg δ values, e.g. single-domain LiNbO3 single

It was investing to find out whether it is possible to create thermostable MW dielectrics on the basis of solid solutions firmed by ferroelectrics and/or antiferroelectrics, which are characterized by high phase transition temperature, and materials having a defect crystal structure (with vacancies). To this end, we investigated Ln2/3 – xNa3xNb2O6 materials (Ln = La, Nd), which were formed by interaction between the La2/3 • 4/3Nb2O6 phase with defectperovskite structure (Fig 9) and the NaNbO3 phase with perovskite structure (Fig 10), in which transition from the spontaneously polarized to the unpolarized state is observed at

An analysis of X-ray data for polycrystalline La2/3-XNa3X • 4/3-2XNb2O6 samples (Ln = La, Nd) showed that depending on x, solid solutions having three different space groups are formed. The space group changes in the order Pmmm Pmmn Pbcn with increasing x. In the interval 0 ≤ x ≤ 0.24, the solid solutions have, independent of Ln, a defect-perovskite structure (La2/3 • 4/3Nb2O6), where is a vacancy in the cation sublattice with the space group

**Table 3.** Properties of two-layer resonant elements at 10GHz

made on their basis.

high temperature (> 520 0C).

Pmmm [39].

**Figure 7.** Plot of permittivity against temperature for Pb2CoTe6 at 10GHz

**Figure 8.** Dependence of the effective permittivity (εff) of two layer resonant elements on the temperature coefficient of frequency (τε) at 10 GHz


**Table 3.** Properties of two-layer resonant elements at 10GHz

122 Dielectric Material

Pb2BTeO6-type perovskites (B = bivalent metal ions) [36, 37]. It has been found that the plot of ε against temperature for lead-cobalt tellurate (Pb2CoTeO6) in the MW range passes through a maximum (phase transition from the antiferroelectric to the paraelectric state) at 380 K (Fig 7). At room temperature, the materials of this group have high permittivity values (ε > 110). In the temperature range 220-350 K, the dependence ε (T) is close to linear one and has TCε ~ 700 × 10-6 K-1. Two-layer resonant elements: Pb2CoTeO6 – TiO2, Pb2CoTeO6 – CaTiO3, Pb2CoTeO6 – SrTiO3 have been prepared on the basis of paraelectrics (TiO2, CaTiO3, SrTiO3) and the antiferroelectric Pb2CoTeO6. Known two-layer resonant dielectric LiNBO3 – TiO2 elements have been investigated for comparison. It can be seen from Table 3 and Fig 8 that Pb2CoTeO6 - SrTiO3 based two-layer resonant elements have in the MW range a high effective permittivity (εeff ≈ 135), a high Q (Q10GHz ~ 900) and a high temperature stability of dielectric parameters (TCε tends to zero) [37]. Of course, two-layer systems have a number of

demerits since they require mechanical bonding of different layers.

**Figure 7.** Plot of permittivity against temperature for Pb2CoTe6 at 10GHz

temperature coefficient of frequency (τε) at 10 GHz

**Figure 8.** Dependence of the effective permittivity (εff) of two layer resonant elements on the

Thermostable MW dielectrics with high permittivity (ε ~ 430) and Q (Q1GHz ~ 700) have been obtained by the authors of [38] on the basis of the Ag (Nb, Ta) O3 system, which is characterized by the spontaneously polarized state. However, in the materials in which the spontaneously polarized state is present, increase in permittivity is always accompanied by an increase in dielectric loss, which impairs the technical characteristics of MW elements made on their basis.
