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

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

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 high temperature (> 520 0C).

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 Pmmm [39].

Microwave Dielectrics Based on Complex Oxide Systems 125

When x is increased, the space group changes, independent of the kind of Ln, from Pmmm to Pmmn. In neodymium-containing solid solutions, the change of space group doespractically lead to change in normalized unit cell volume V/Z; in lanthanum-containing solid solutions, monotonic variation of V/Z as a function of x is accompanied by a slight inflection in the sodium concentration range corresponding to the change from Pmmm to Pmmn. The observed differences in the trend of plots of V/Z (x) are probably due to the fact that the ionic radius of Na+ is smaller than that of the substituted La3+ ion, whereas the difference in the ionic radii of Na+ and Nd3+ is small. The closeness of the ionic radius values of Na+ and Nd3+ leads to the existence of a wider concentration range (0.24 ≤ x ≤ 0.54), which corresponds to the space group Pmmm , for neodymium-containing solid solutions as

In the intervals 0.54 ≤ x ≤ 0.66, (for neodymium) and 0.45 ≤ x ≤ 0.66 (for lanthanum), the crystal structure of La2/3-XNa3X • 4/3-2XNb2O6 solid solutions has the space group Pbcn, which is typical of NaNbO3 at room temperature [40]. In fact, when the sodium content of the system is increased, a decrease in the symmetry of solid solutions from Pmmm to Pmmn and to

La2/3 Nb2O6 and Nd2/3 Nb2O6 materials (x = 0) are characterized by a high permittivity value (130 and 160 respectively) and a relatively low dielectric loss (in both cases, tg δ is of the order of 2 – 5 × 10-3 at a frequency of 10 GHz). There is no permittivity dispersion. The plot of ε (T) for La2/3 Nb2O6 and Nd2/3 Nb2O materials exhibits deflections in the MW range. In the low-temperature range, the ε value varies only slightly with rising temperature. As the x values in La2/3-XNa3X • 4/3-2XNb2O6 materials (Ln = La, Nd) increases, TCε changes its sign from negative to positive. Plots of dielectric parameters against concentration in the MW range are shown in Fig 11. In the interval 0 ≤ x ≤ 0.24 (space group ) Pmmm, increasing the sodium concentration leads to a slight increase in permittivity independent of the kind of rare-earth element (La or Nd), which is accounted for by increase in cation

**Figure 11.** Plots of permittivity (a) and the temperature coefficient of permittivity (b) against the sodium content of the solid solutions La2/3-XNa3X • 4/3-2XNb2O6 (1) and Nd2/3-XNa3X • 4/3-2XNb2O6 (2)

compared with lanthanum-containing ones (0.24 ≤ x ≤ 0.45).

Pbcn is observed.

vacancy concentration.

**Figure 9.** Defect-perovskite structure Ln2/3 • 4/3Nb2O6 (space group Pmmm). Atomic positions: La (1a) 000; Nb (2t) ½ ½ z; 0 (1) ( 1ƒ) ½ ½ 0; 0 (2)

**Figure 10.** Perovskite structure of NaNbO3 at room temperature (space group Pbcm). Atomic positions: Na (1) (4c) x ¼ 0; Na (2) (4d) x y ¼; Nb (8c) x y z; O (1) (4c) x ¼ 0; O (2) (4d) x y ¼ ; O (3) (8c) x y z; 0 (4) (8c) x y z

When x is increased, the space group changes, independent of the kind of Ln, from Pmmm to Pmmn. In neodymium-containing solid solutions, the change of space group doespractically lead to change in normalized unit cell volume V/Z; in lanthanum-containing solid solutions, monotonic variation of V/Z as a function of x is accompanied by a slight inflection in the sodium concentration range corresponding to the change from Pmmm to Pmmn. The observed differences in the trend of plots of V/Z (x) are probably due to the fact that the ionic radius of Na+ is smaller than that of the substituted La3+ ion, whereas the difference in the ionic radii of Na+ and Nd3+ is small. The closeness of the ionic radius values of Na+ and Nd3+ leads to the existence of a wider concentration range (0.24 ≤ x ≤ 0.54), which corresponds to the space group Pmmm , for neodymium-containing solid solutions as compared with lanthanum-containing ones (0.24 ≤ x ≤ 0.45).

124 Dielectric Material

**Figure 9.** Defect-perovskite structure Ln2/3 • 4/3Nb2O6 (space group Pmmm). Atomic positions: La (1a)

**Figure 10.** Perovskite structure of NaNbO3 at room temperature (space group Pbcm). Atomic positions: Na (1) (4c) x ¼ 0; Na (2) (4d) x y ¼; Nb (8c) x y z; O (1) (4c) x ¼ 0; O (2) (4d) x y ¼ ; O (3) (8c) x y z; 0

000; Nb (2t) ½ ½ z; 0 (1) ( 1ƒ) ½ ½ 0; 0 (2)

(4) (8c) x y z

In the intervals 0.54 ≤ x ≤ 0.66, (for neodymium) and 0.45 ≤ x ≤ 0.66 (for lanthanum), the crystal structure of La2/3-XNa3X • 4/3-2XNb2O6 solid solutions has the space group Pbcn, which is typical of NaNbO3 at room temperature [40]. In fact, when the sodium content of the system is increased, a decrease in the symmetry of solid solutions from Pmmm to Pmmn and to Pbcn is observed.

La2/3 Nb2O6 and Nd2/3 Nb2O6 materials (x = 0) are characterized by a high permittivity value (130 and 160 respectively) and a relatively low dielectric loss (in both cases, tg δ is of the order of 2 – 5 × 10-3 at a frequency of 10 GHz). There is no permittivity dispersion. The plot of ε (T) for La2/3 Nb2O6 and Nd2/3 Nb2O materials exhibits deflections in the MW range. In the low-temperature range, the ε value varies only slightly with rising temperature. As the x values in La2/3-XNa3X • 4/3-2XNb2O6 materials (Ln = La, Nd) increases, TCε changes its sign from negative to positive. Plots of dielectric parameters against concentration in the MW range are shown in Fig 11. In the interval 0 ≤ x ≤ 0.24 (space group ) Pmmm, increasing the sodium concentration leads to a slight increase in permittivity independent of the kind of rare-earth element (La or Nd), which is accounted for by increase in cation vacancy concentration.

**Figure 11.** Plots of permittivity (a) and the temperature coefficient of permittivity (b) against the sodium content of the solid solutions La2/3-XNa3X • 4/3-2XNb2O6 (1) and Nd2/3-XNa3X • 4/3-2XNb2O6 (2)

Within the limits of the space group Pmmn, the permittivity value decreases greatly with increasing x, passing through a maximum, when the space group changes from Pmmn to Pbcn . Investigations showed that it is possible to create thermostable dielectrics based on the system in Ln2/3Na3X •4/3-2XNb2O6 (Ln = La, Nd), which have a high permittivity (ε ~ 300- 600) and a relatively low dielectric loss (tg δ ~ 2 – 7 × 10-3) in the MW range [41, , , 44].

Microwave Dielectrics Based on Complex Oxide Systems 127

**Figure 12.** Columbite structure of A2+ Nb2O6

We have shown that when the solid-state reaction method is used, the formation of cobaltand magnesium-containing niobates with columbite structure is a multistage process. In this case, two concurrent processes of formation of niobates with columbite structure (A2+

2A32+O4 + 6Nb2O5 6А2+Nb2O6 + O2;

4A32+O4 + 3Nb2O5 3А42+Nb2O9 +2O2.

At higher temperatures (> 1000 0C), the formation of columbite structure took place by

А42+Nb2O9 + 3Nb2O5 4А2+Nb2O6 (А2+ – Со2+, Mg2+).

At the same time, the synthesis of Zn Nb2O6 with columbite structure takes place in the

In the case of deviation from stoichiometry in the A1+x2+ Nb2O6 system (A2+ = Mg2+, Co2+, Zn2+), when x < 0, samples contained two phases: the main phase A2+ Nb2O6 with columbite structure and the Nb2O5 phase, whose concentration increased with x (Fig 13). At x > 0, a narrow concentration range, in which samples are single-phase ones, exists in all three systems. On further deviation from stoichiometry in the direction of increasing excess of

The results of investigating electrophysical properties in the MW range turned out unlooked-for. At x < 0, when there were traces of the minor phase Nb2O5, the samples had a low Q. At the same time, extremely high Q values (Q × f) were observed at x > 0 (Fig 14). For example, in Mg1+xNb2O6, Q × f reached a value of 128000 at x ≥ 0.03 – 0.05 in multiphase samples, in which the phase Mg4Nb2O9 (corundum structure) was present together with the

Nb2O6) and corundum structure (A42+Nb2O9) (A2+ = Co2+, Mg2+) take place:

temperature range 500-800 0C without formation of intermediate products.

interaction between the A42+Nb2O9 phase and unreacted Nb2O5:

cobalt, magnesium or zinc, extra phases are formed.

main phase MgNb2O6 (columbite structure).

The systems considered above have a relatively high thermostability of electrophysical properties (TCε ~ 10-5 – 10-6K-1), Q × f ≤ 12000 and relatively high permittivity values (ε ≥ 80 600) in the MW range. This makes it possible to develop on their basis elements for decimeter wave band communication systems, where the problems of microminiaturization, for the solution of which high ε values are required, are especially important.

In the centimeter wave band and especially in the millimeter wave band, however, materials with relatively low permittivity (10 -30) are required, which must possess very high Q valies (Q × f ≥ 80000 - 100000). Let us consider some systems, which have promise in gaining these purposes.
