**8. M1+ xNb2O6 – Based MW dielectrics (M = Mg, Co, Zn) with columbite structure**

Among the MW dielectrics known to date, M2+ Nb2O6 niobates (M = Mg, Co, Zn) are of considerable interest. The crystal structure of A2+ Nb2O6 columbite is infinite zigzag chains of oxygen linked by shared edges (Fig 12) [45]. For this structure, redistribution of crystal sites, which are in the oxygen octahedral, among A2+ ion size and the cation ratio Nb5+ : A2+ in the unit cell will affect the crystallographic distortions of the columbite structure and hence the phase composition and electrophysical properties of synthesized materials.

The ZnNb2O6 and MgNb2O6 materials have a high Q (3000 and 9400 respectively) and permittivity (23 and 20 respectively) [46, 47, 48]. In contrast to magnesium- and zinccontaining niobates, the literature data on the dielectric properties of cobalt niobate (CoNb2O6) are very contradictory. For instance, Ref [46] reported low Q values for CoNb2O6 (Q × f = 40000). Earlier it was shown [49, 50, 51, 52, 53] that the making of single-phase M2+ Nb2O6 materials depends largely upon their synthesis conditions. For instance, the difficulty of making single-phase magnesium niobate with columbite structure is accounted for, in particular, by the simultaneous formation of two phases: MgNb2O6 columbite and Mg4Nb2O9 corundum [51]. It should be noted that in the Mg-Nb-O system, a number of compounds: MgNb2O6, Mg4Nb2O9, Mg5Nb4O15, Mg1/3Nb11 (1/3) are formed [52, 53]; however, only the phases MgNb2O6 (columbite structure) and Mg4Nb2O9 (corundum structure) are stable at room temperature [54]. Therefore, even after long heat treatment at highj temperature (T > 1100 0C) [50], there were intermediate phases in the end product (generally a corundum phase). In such cases, the phase composition and electrophysical properties can be greatly affected even by a small deviation from stoichiometry. In view of this, we have studied the effect of small deviations from stoichiometry in A2+ Nb2O6 materials (A = Co, Mg, Zn) with columbite structure on phase composition, microstructure and MW properties [55, 56, 57].

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

126 Dielectric Material

purposes.

**structure** 

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].

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,

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

**8. M1+ xNb2O6 – Based MW dielectrics (M = Mg, Co, Zn) with columbite** 

Among the MW dielectrics known to date, M2+ Nb2O6 niobates (M = Mg, Co, Zn) are of considerable interest. The crystal structure of A2+ Nb2O6 columbite is infinite zigzag chains of oxygen linked by shared edges (Fig 12) [45]. For this structure, redistribution of crystal sites, which are in the oxygen octahedral, among A2+ ion size and the cation ratio Nb5+ : A2+ in the unit cell will affect the crystallographic distortions of the columbite structure and hence the

The ZnNb2O6 and MgNb2O6 materials have a high Q (3000 and 9400 respectively) and permittivity (23 and 20 respectively) [46, 47, 48]. In contrast to magnesium- and zinccontaining niobates, the literature data on the dielectric properties of cobalt niobate (CoNb2O6) are very contradictory. For instance, Ref [46] reported low Q values for CoNb2O6 (Q × f = 40000). Earlier it was shown [49, 50, 51, 52, 53] that the making of single-phase M2+ Nb2O6 materials depends largely upon their synthesis conditions. For instance, the difficulty of making single-phase magnesium niobate with columbite structure is accounted for, in particular, by the simultaneous formation of two phases: MgNb2O6 columbite and Mg4Nb2O9 corundum [51]. It should be noted that in the Mg-Nb-O system, a number of compounds: MgNb2O6, Mg4Nb2O9, Mg5Nb4O15, Mg1/3Nb11 (1/3) are formed [52, 53]; however, only the phases MgNb2O6 (columbite structure) and Mg4Nb2O9 (corundum structure) are stable at room temperature [54]. Therefore, even after long heat treatment at highj temperature (T > 1100 0C) [50], there were intermediate phases in the end product (generally a corundum phase). In such cases, the phase composition and electrophysical properties can be greatly affected even by a small deviation from stoichiometry. In view of this, we have studied the effect of small deviations from stoichiometry in A2+ Nb2O6 materials (A = Co, Mg, Zn) with columbite structure on phase composition, microstructure and MW properties [55, 56, 57].

for the solution of which high ε values are required, are especially important.

phase composition and electrophysical properties of synthesized materials.

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

2A32+O4 + 6Nb2O5 6А2+Nb2O6 + O2;

$$\text{4A} \text{\AA}^{2+} \text{O}\_{4} \text{+} \text{\AA} \text{\text{\text\textbullet}} \text{\textbullet} \text{\textbullet}^{2} \text{\textbullet}^{2} \text{\textbullet}^{2} \text{\textbullet}^{2} \text{\textbullet}^{2} \text{\textbullet}^{2} \text{\textbullet}^{2}$$

At higher temperatures (> 1000 0C), the formation of columbite structure took place by interaction between the A42+Nb2O9 phase and unreacted Nb2O5:

$$\mathrm{Al^{2+}NbxO\bullet + 3NbxO\bullet} \to 4\mathrm{A^{2+}NbxO\bullet} \text{ (A^{2+}-Co^{2+})Mg^{2+} ).}$$

At the same time, the synthesis of Zn Nb2O6 with columbite structure takes place in the temperature range 500-800 0C without formation of intermediate products.

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 cobalt, magnesium or zinc, extra phases are formed.

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 main phase MgNb2O6 (columbite structure).

Microwave Dielectrics Based on Complex Oxide Systems 129

This is accounted for by the high Q values (Q × f ~ 230000) of the extra phase Mg2Nb4O9. However, further deviation from stoichiometry leads to a considerable decrease in

This instance is interesting in that each of the phases MgNb2O6 and Mg2Nb4O9 characterized by certain merits and demerits. Only multiphase materials based on them have both a

The first information about ZrTiO4 as a promising high-Q dielectric was presented in Ref [58]. Later, in the 1950s, investigations of solid solutions in the ZrO2-TiO2-SnO2 system were carried out [59]. It was shown that the composition Zr0.8Sn0.2TiO4 has the highest Q values [60, 61].

ZrTiO4 crystallizes in orthorhombic structure (space group Pbcn) [62, 63, 64] with the space lattice parameters: a = 4.806Å, b = 5.447Å, c = 5.032Å. The unit cell contains two formula units, theoretical density 5.15 g/cm3. It should be noted that in ZrTiO4 there is an order-disorder phase transition in the temperature range 1100-1200°C [65, , , , 69]. When the temperature is decreased, this transition is from an -PbO2 – type high-temperature phase of which disordered arrangement of Zr and Ti ions is typical, to a low-temperature phase with ordered arrangement of Zr and Ti ions [70, 71]. Addition of Sn to ZrTiO4 results in the stabilization of the disordered distribution of cations. The variation of the lattice parameters in the Zr1 xSnxTiO4 system with increasing x is shown in Fig15. As is seen from Fig 15, there are no noticeable changes in the behavior of the parameters a and b in the phase transition region (1100-1200°C) with increasing Sn content. At the same time, as Sn ions are added, a noticeable change in the dependence of the parameter c in the phase transition region is observed [69].

**Figure 15.** Variation of the lattice parameters of Zr1-xSnxTiO4 materials as a function of temperature: ( ) - ZrTiO4; ( ) Zr0,95Sn0,05TiO4; ( ) Zr0,9Sn0,1TiO4; ( ) Zr0,8Sn0,2TiO4; ( ) Zr0,7Sn0,3TiO4 [69].

permittivity, which is due to the fact that ε of the corundum phase was 11.

**9. ZrO2-TiO2-SnO2 – Based Mw dielectrics** 

relatively high permittivity (ε ~ 20) and a highQ(Q × f ~ 128000) in the MW range.

**Figure 13.** Micrographs of microsections of polycrystalline Co1-xNb2O6-x samples with x = 0.05 (a), x = 0.03 (b), x = 0 (c), x = - 0.03 (d), x = - 0.05 (e, f), sintered at 1500 0C for 1h (a - f), 6 h (e): A = Nb2O5, B = Co4Nb2O9

**Figure 14.** Plots of the product Q × ƒ of A1-xNb2O6 samples (where A = Mg (1), Zn (2), Co(3)) against concentration. The samples were sintered in air for 8 h at 1400 0C (1 and 3) and 1300 0C (2).

This is accounted for by the high Q values (Q × f ~ 230000) of the extra phase Mg2Nb4O9. However, further deviation from stoichiometry leads to a considerable decrease in permittivity, which is due to the fact that ε of the corundum phase was 11.

This instance is interesting in that each of the phases MgNb2O6 and Mg2Nb4O9 characterized by certain merits and demerits. Only multiphase materials based on them have both a relatively high permittivity (ε ~ 20) and a highQ(Q × f ~ 128000) in the MW range.
