**4. MW dielectrics based on Ba6-xLn8+2x/3Ti18O54 (Ln = La-Gd)**

The Ba6-xLn8+2x/3Ti18O54 materials (Ln = La - Gd) (BLTss) have promise in the development on their basis of thermostable high-Q MW dielectrics with high permittivity (ε ≈ 80 - 100) [, 14]. They crystallize in KW bronze structure (Fig 3), which includes elements of perovskite structure [15, , 17]. In this structure, the octahedra are linked, as in perovskite, by their apices into parallel rectilinear chains. Unlike perovskite structure, however, the oxygen octahedra are linked so that they form pentangular, quadrangular and triangular channels, in which A ions can be, having in this case the coordination numbers 15, 12 and 9 respectively. This structure allows one to perform iso- and heterovalent substitutions in cation sublattices in a wide range, to control the number of vacant crystal sites in the A sublattice, to influence the partial redistribution of A ions among the pentangular, quadrangular and triangular channels and hence to control the electrophysical properties in the MW range, including the temperature dependence of ε.

**Figure 3.** Unit cell of Ba6-x Ln8+2x/3Ti18O54 [17]

When investigating Ba6-xLn8+2x/3Ti18O54 materials (Ln = La - Gd), which crystallize in KW bronze structure, a special attention was given to the study of formation reaction and the anomalous behavior of the temperature characteristics of dielectric parameters. It should be noted that the knowledge of the reactions proceeding during the synthesis of compounds can allow dielectric loss to be reduced. The formation of Ba6-xLn8+2x/3Ti18O54 materials (Ln = Nd, Sm) when using the solid-state reaction method was studied on compositions with x = 0.75, 1.5, 2.0. BaCO3, Sm2O3 and Tio2 were used as starting reagents. It was shown that it is a multistage process, which is accompanied by the formation of intermediate phases, e.g. Ln2Ti2O8, BaTi4O9, BaTiO3 [18]. It has been found that independent of x value, the phase Ba3.9Ln9.4Ti18O54 is formed at first, which belongs to Ba6-xLn8+2x/3Ti18O54 solid solutions and corresponds to the maximum x value. The formation of the other phases, which belong to the region of Ba6-xLn8+2x/3Ti18O54 – type solid solutions, takes place as a result of interaction between intermediate Ba3.9Ln9.4Ti18O54 phases and barium metatitanate (BaTiO3). The phase Ba3.9Ln9.4Ti18O54 crystallizes, as Ba6-xLn8+2x/3Ti18O54 materials, in KW bronze structure, which makes the identification of the phase Ba3.9Ln9.4Ti18O54 only by the data of X-ray phase analysis impossible. Therefore, EDS and TEM analyses were used additionally [18]. The latter analysis showed that even when all BaTiO3 had reacted, the homogeneity of materials was not reached yet (Fig 4(a)). If the ceramic sintering time was relatively short, a phase, e.g. Ba3.9Ln9.4Ti18O54, was present (Fig 4(a)), which had low and high x values within the limits of formation of Ba6-xLn8+2x/3Ti18O54 solid solutions. The structural-defect concentration decreased, and the homogeneity of Ba6-xLn8+2x/3Ti18O54 materials increased only in the case of long ceramic sintering time (t ≥ 3h) (Fig 4(b)).

Microwave Dielectrics Based on Complex Oxide Systems 119

**T, °C**

**T, °C**

**Gd**

**Gd**

**Sm**

Moreover, there was no information about the existence of temperature dependence anomalies of dielectric parameters in other barium-lanthanide analogs, including La-, Nd-, Gd-containing Ba6-xLn8+2x/3Ti18O54. Therefore, we tried to find out the cause of the temperature dependence anomalies of permittivity and dielectric loss since this makes it possible to establish the nature of the thermostability of electrophysical properties in these

It has been found that temperature dependence anomalies of dielectric parameters in Laand Nd-containing BLTss are observed at low temperatures in a wide frequency range, including the submillimeter-wave region (Fig 5) [23], the position of these anomalies on the temperature scale depending not on measurement frequency, but on chemical composition.

**-200 -100 0 100 200 300**

**-200 -100 0 100 200 300**

**Figure 5.** Plots of the dielectric parameters of Ba6-x Ln8+2x/3Ti18O54 solid solutions (Ln = La, Nd, Sm, Gd) at

In Sm- and Gd-containing systems, anomalies of dielectric parameters appear at temperatures above room temperatures (Fig 5). The plots of dielectric parameters against temperature have a similar trend when ferroelectric or antiferroelectric ordering occurs. In the case of BLTss, however, no hysteresis loops were observed, and the temperature dependence of ε did not obey the Curie-Weiss law, which indicated the absence of spontaneously polarized state in these materials. It may be supposed that the appearance of dielectric anomalies is due to the presence of unknown phase transitions. Therefore, Ba6 xLn8+2x/3Ti18O54 systems (x = 1.0), in which dielectric anomalies of ε and tg δ were observed at 100-120 0C, have been studied by low-temperature differential scanning calorimetry (LT-DSC) and high-temperature X-ray structural analysis of samples. We did not find any phase

**Sm Nd**

**La**

systems.

**60 70 80**

**tg** 

**0.000 0.002 0.004**

10 GHz against temperature

**0.02**

**Nd**

**La**

**0.04**

**100**

**120**

**Figure 4.** Results of a TEM analysis of Ba6-x Ln8+2x/3Ti18O54 ceramic; sintering time: 1 h (a), over 3 h (b)

The electrophysical characteristics of Ba6-xLn8+2x/3Ti18O54 depend largely upon ions in the A sublattice (rA) [14, 19, 20]. When the rare-earth ion in the A sublattice is changed from La to Gd, the permittivity (ε) value and loss-angle tangent in BLTss decrease. At the same time, the temperature coefficient of permittivity, TCε, increases and changes its sign in the series of rare-earth ions, which are in the A sublattice, in going from Nd to Sm [20, 21]. When investigating Ba6-xLn8+2x/3Ti18O54 materials (where x = 1.5), we had found for the first time an anomaly on the plot of permittivity against temperature [21]. Later, anomalies on the plots of dielectric parameters (ε, tg δ) against temperature were detected in the other Ba6 xLn8+2x/3Ti18O54 materials too [22]. The nature of these anomalies remained uncertain. Moreover, there was no information about the existence of temperature dependence anomalies of dielectric parameters in other barium-lanthanide analogs, including La-, Nd-, Gd-containing Ba6-xLn8+2x/3Ti18O54. Therefore, we tried to find out the cause of the temperature dependence anomalies of permittivity and dielectric loss since this makes it possible to establish the nature of the thermostability of electrophysical properties in these systems.

118 Dielectric Material

long ceramic sintering time (t ≥ 3h) (Fig 4(b)).

anomalous behavior of the temperature characteristics of dielectric parameters. It should be noted that the knowledge of the reactions proceeding during the synthesis of compounds can allow dielectric loss to be reduced. The formation of Ba6-xLn8+2x/3Ti18O54 materials (Ln = Nd, Sm) when using the solid-state reaction method was studied on compositions with x = 0.75, 1.5, 2.0. BaCO3, Sm2O3 and Tio2 were used as starting reagents. It was shown that it is a multistage process, which is accompanied by the formation of intermediate phases, e.g. Ln2Ti2O8, BaTi4O9, BaTiO3 [18]. It has been found that independent of x value, the phase Ba3.9Ln9.4Ti18O54 is formed at first, which belongs to Ba6-xLn8+2x/3Ti18O54 solid solutions and corresponds to the maximum x value. The formation of the other phases, which belong to the region of Ba6-xLn8+2x/3Ti18O54 – type solid solutions, takes place as a result of interaction between intermediate Ba3.9Ln9.4Ti18O54 phases and barium metatitanate (BaTiO3). The phase Ba3.9Ln9.4Ti18O54 crystallizes, as Ba6-xLn8+2x/3Ti18O54 materials, in KW bronze structure, which makes the identification of the phase Ba3.9Ln9.4Ti18O54 only by the data of X-ray phase analysis impossible. Therefore, EDS and TEM analyses were used additionally [18]. The latter analysis showed that even when all BaTiO3 had reacted, the homogeneity of materials was not reached yet (Fig 4(a)). If the ceramic sintering time was relatively short, a phase, e.g. Ba3.9Ln9.4Ti18O54, was present (Fig 4(a)), which had low and high x values within the limits of formation of Ba6-xLn8+2x/3Ti18O54 solid solutions. The structural-defect concentration decreased, and the homogeneity of Ba6-xLn8+2x/3Ti18O54 materials increased only in the case of

**Figure 4.** Results of a TEM analysis of Ba6-x Ln8+2x/3Ti18O54 ceramic; sintering time: 1 h (a), over 3 h (b)

The electrophysical characteristics of Ba6-xLn8+2x/3Ti18O54 depend largely upon ions in the A sublattice (rA) [14, 19, 20]. When the rare-earth ion in the A sublattice is changed from La to Gd, the permittivity (ε) value and loss-angle tangent in BLTss decrease. At the same time, the temperature coefficient of permittivity, TCε, increases and changes its sign in the series of rare-earth ions, which are in the A sublattice, in going from Nd to Sm [20, 21]. When investigating Ba6-xLn8+2x/3Ti18O54 materials (where x = 1.5), we had found for the first time an anomaly on the plot of permittivity against temperature [21]. Later, anomalies on the plots of dielectric parameters (ε, tg δ) against temperature were detected in the other Ba6 xLn8+2x/3Ti18O54 materials too [22]. The nature of these anomalies remained uncertain. It has been found that temperature dependence anomalies of dielectric parameters in Laand Nd-containing BLTss are observed at low temperatures in a wide frequency range, including the submillimeter-wave region (Fig 5) [23], the position of these anomalies on the temperature scale depending not on measurement frequency, but on chemical composition.

**Figure 5.** Plots of the dielectric parameters of Ba6-x Ln8+2x/3Ti18O54 solid solutions (Ln = La, Nd, Sm, Gd) at 10 GHz against temperature

In Sm- and Gd-containing systems, anomalies of dielectric parameters appear at temperatures above room temperatures (Fig 5). The plots of dielectric parameters against temperature have a similar trend when ferroelectric or antiferroelectric ordering occurs. In the case of BLTss, however, no hysteresis loops were observed, and the temperature dependence of ε did not obey the Curie-Weiss law, which indicated the absence of spontaneously polarized state in these materials. It may be supposed that the appearance of dielectric anomalies is due to the presence of unknown phase transitions. Therefore, Ba6 xLn8+2x/3Ti18O54 systems (x = 1.0), in which dielectric anomalies of ε and tg δ were observed at 100-120 0C, have been studied by low-temperature differential scanning calorimetry (LT-DSC) and high-temperature X-ray structural analysis of samples. We did not find any phase transitions [22], which was confirmed by the authors of [24], who carried out synchrotron Xray diffraction studies of Ba4.5Sm9Ti18O54 samples in the temperature range 10-295 K. These data indicate the absence of structural transitions in the temperature ranges where anomalies of dielectric parameters were observed. It can be concluded that the temperature dependence anomalies of dielectric parameters are not coupled with the peculiarities of sample preparation and the presence of structural transitions.

Microwave Dielectrics Based on Complex Oxide Systems 121

Q (10 GHz)

**Figure 6.** Perovskite structure of La2/3-xLi 3xTiO3

ТC, ppm/°C (20–100°C)

0 80 –520 1300 0.30 95 –240 100 0.50 90 –140 150 0.55 85 –50 200 0.58 80 –5 200 0.60 75 +60 200 0.1 52 +580 100

**Table 2.** Dielectric parameters of the Sm1/2Li1/2TiO3 – (1-x) Sm1/2Na1/2TiO3 system at 10 GHz

whereas there are few of them with TCε > 0 (LiNbO3, LiTaO3, LiAlO3 single crystals).

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),

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

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

х

On the basis of an analysis it was assumed that the nature of the anomaly of dielectric parameters is coupled with harmonic and anharmonic BLTss lattice vibration, which is different in the character of influence on the temperature behavior of dielectric parameters [23].

Investigations showed that the plots of ε and tg δ against temperature depend largely upon harmonic and anharmonic lattice vibration modes. Therefore, using different hetero- and isovalent substitutions in cation sublattices, one can influence the lattice phonon spectrum and hence obtain materials with high temperature stability of dielectric parameters, which are used in modern decimeter and centimeter wave band communication systems [25, , , 28].
