**3. Control of the TCε value by influencing the phonon spectrum**

As follows from the analysis of expressions (3), the trend of the plot of ε against temperature in the MW range can be controlled by influencing the phonon spectrum. One of the ways of influencing the phonon spectrum in some types of structures can be iso- and heterovalent substitutions in cation sublattices. As an example, we chose La2/3-x (Na, K)3x TiO3 materials, which crystallize in defect-perovskite structure in a wide x range (Fig 1).

**Figure 1.** Crystal structure of La2/3-x(Na, K)3xTiO3 perovskite

In this system, lanthanum ions in the oxidation state +3 are partially substituted by alkali (sodium or potassium) ions in the oxidation state +1. Substitution is performed so that the electroneutrality condition is satisfied. The value of x was varied from 1/24 to 1/6. In this case, lanthanum ions, alkali metal (sodium or potassium) ions and a structural vacancy (vacant crystal site) could be in one crystal sublattice at the same time. We hoped that in the case of such heterovalent substitution, the phonon spectrum, and hence the trend of the plot of ε against temperature in the MW range, had to change.

Microwave Dielectrics Based on Complex Oxide Systems 117

La1/2Na1/4K1/4TiO3(ε = 106) La7/12Na1/4 • 1/6TiO3(ε = 87)

cm-1 cm-1

**Table 1.** Parameters of (La2/3-x M3x • 1/3 – 2x)TiO3 dispersion oscillators\*

the MW range, including the temperature dependence of ε.

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

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

\*ε∞ = 5.1.

ωTO ω\_LO ∆E g ωTO ωLO ∆E g

116 163 74.0 0.75 133 178 55.0 0.64 198 258 22.0 0.44 201 224 19.5 0.31 237 334 3.0 0.21 230 265 4.1 0.30 336 375 0.2 0.33 270 343 1.8 0.26 381 489 0.4 0.16 345 460 0.2 0.10 554 747 1.2 0.10 563 694 1.0 0.11 785 816 0.1 0.12 789 860 0.3 0.30

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

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

La2/3-x (Na, K)3x TiO3 materials are not characterized by high temperature stability of dielectric parameters. The dielectric properties of these materials had been studied in a wide frequency range [8, 9].

It had been found that by decreasing the number of structural vacancies, using heterovalent substitution in sublattices and locating different ions in vacant crystal sites, one can influence greatly the dielectric loss level (Fig 2) [10].

**Figure 2.** Plots of dielectric loss (tg δ) in the La2/3-x(Na, K)3xTiO3 system at 1.2 × 1010 Hz against temperature: (I) La1/2Na1/2TiO3, (II) Nd1/2Na1/2TiO3, (III) La1/2Na1/4K1/4TiO3, (IV) La7/12Na1/4•1/6TiO3, where • is the structural vacancy

Investigations showed that heterovalent substitutions in cation sublattices affect greatly the value of TCε too. To explain this effect, IR reflection spectra of La2/3-x M3x TiO3 materials have been analyzed [11]. The analysis of IR reflection spectra made it possible to calculate the parameters of dispersion oscillators (Table 1). It is known that in the materials that crystallize in perovskite structure, a low-frequency lattice vibration exists which is responsible for the high ε value in the MW range [12].

The partial heterovalent substitution of ions in crystal sublattice gives rise to a lowfrequency vibration, which affects noticeably the value of permittivity. In this case, the temperature stability of dielectric parameters increases greatly. Thus, the proposed method of influencing the phonon spectrum can be employed in the development of novel MW dielectrics with high temperature stability of dielectric properties.


\*ε∞ = 5.1.

116 Dielectric Material

frequency range [8, 9].

• is the structural vacancy

In this system, lanthanum ions in the oxidation state +3 are partially substituted by alkali (sodium or potassium) ions in the oxidation state +1. Substitution is performed so that the electroneutrality condition is satisfied. The value of x was varied from 1/24 to 1/6. In this case, lanthanum ions, alkali metal (sodium or potassium) ions and a structural vacancy (vacant crystal site) could be in one crystal sublattice at the same time. We hoped that in the case of such heterovalent substitution, the phonon spectrum, and hence the trend of the plot

La2/3-x (Na, K)3x TiO3 materials are not characterized by high temperature stability of dielectric parameters. The dielectric properties of these materials had been studied in a wide

It had been found that by decreasing the number of structural vacancies, using heterovalent substitution in sublattices and locating different ions in vacant crystal sites, one can

**Figure 2.** Plots of dielectric loss (tg δ) in the La2/3-x(Na, K)3xTiO3 system at 1.2 × 1010 Hz against

temperature: (I) La1/2Na1/2TiO3, (II) Nd1/2Na1/2TiO3, (III) La1/2Na1/4K1/4TiO3, (IV) La7/12Na1/4•1/6TiO3, where

Investigations showed that heterovalent substitutions in cation sublattices affect greatly the value of TCε too. To explain this effect, IR reflection spectra of La2/3-x M3x TiO3 materials have been analyzed [11]. The analysis of IR reflection spectra made it possible to calculate the parameters of dispersion oscillators (Table 1). It is known that in the materials that crystallize in perovskite structure, a low-frequency lattice vibration exists which is

The partial heterovalent substitution of ions in crystal sublattice gives rise to a lowfrequency vibration, which affects noticeably the value of permittivity. In this case, the temperature stability of dielectric parameters increases greatly. Thus, the proposed method of influencing the phonon spectrum can be employed in the development of novel MW

of ε against temperature in the MW range, had to change.

influence greatly the dielectric loss level (Fig 2) [10].

responsible for the high ε value in the MW range [12].

dielectrics with high temperature stability of dielectric properties.

**Table 1.** Parameters of (La2/3-x M3x • 1/3 – 2x)TiO3 dispersion oscillators\*
