**5. MW dielectrics with "mobile sublattice"**

The authors of [29] reported the development of novel MW dielectrics with high temperature stability of dielectric parameters based on solid solutions, where one of the phases (lithiumcontaining La1/2Li1/2TiO3) had a positive temperature coefficient of permittivity (TCε > 0). However, there was no explanation of the nature of this fact in literature.

The phase La1/2Li1/2TiO3, which crystallizes in perovskite structure, belongs to Ln2.3-xM3xTio3 solid solutions (Ln = rare-earth elements, M = alkali metal ion) (Fig 6). In this system, the M ions partially substitute for rare-earth ions. In this case, the electroneutrality condition is satisfied [30, 31]. If M = Na, K, materials crystallize in perovskite structure and are characterized by low dielectric loss (tg δ ≤ 10-3) and high permittivity in the MW range (about 100) and TCε < 0 [32].

At the same time, when M = Li, a system of solid solutions is formed, in which rare-earth ions, lithium ions, structural vacancies, which are characterized by TCε > 0, are in one sublattice at the same time. Lithium ions can move along structural channels, ensuring a high lithium-ion conductivity [33, 34], positive temperature coefficient of permittivity (TCε > 0) and causing considerable dielectric loss in the MW range. The latter is inadmissible in the creation of high-Q dielectrics. Investigations showed, however, that the dielectric loss in the MW range can be greatly reduced by decreasing lithium ion conductivity. The latter is achieved by substituting rare-earth ions with smaller radius for lanthanum ions. This leads to a decrease in the size of structural channels, in which lithium ions are; this reduces dielectric loss in the MW range through a decrease in lithium ion mobility in the structure. In this case, TCε > 0 is retained. As a result, solid solutions have been obtained, in which rare-earth ions are simultaneously substituted by alkali metal ions with large (Na, K) and small (Li) radius and which have a high temperature stability of dielectric parameters in the MW range (Table 2) [35].

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

120 Dielectric Material

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

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

The authors of [29] reported the development of novel MW dielectrics with high temperature stability of dielectric parameters based on solid solutions, where one of the phases (lithiumcontaining La1/2Li1/2TiO3) had a positive temperature coefficient of permittivity (TCε > 0).

The phase La1/2Li1/2TiO3, which crystallizes in perovskite structure, belongs to Ln2.3-xM3xTio3 solid solutions (Ln = rare-earth elements, M = alkali metal ion) (Fig 6). In this system, the M ions partially substitute for rare-earth ions. In this case, the electroneutrality condition is satisfied [30, 31]. If M = Na, K, materials crystallize in perovskite structure and are characterized by low dielectric loss (tg δ ≤ 10-3) and high permittivity in the MW range

At the same time, when M = Li, a system of solid solutions is formed, in which rare-earth ions, lithium ions, structural vacancies, which are characterized by TCε > 0, are in one sublattice at the same time. Lithium ions can move along structural channels, ensuring a high lithium-ion conductivity [33, 34], positive temperature coefficient of permittivity (TCε > 0) and causing considerable dielectric loss in the MW range. The latter is inadmissible in the creation of high-Q dielectrics. Investigations showed, however, that the dielectric loss in the MW range can be greatly reduced by decreasing lithium ion conductivity. The latter is achieved by substituting rare-earth ions with smaller radius for lanthanum ions. This leads to a decrease in the size of structural channels, in which lithium ions are; this reduces dielectric loss in the MW range through a decrease in lithium ion mobility in the structure. In this case, TCε > 0 is retained. As a result, solid solutions have been obtained, in which rare-earth ions are simultaneously substituted by alkali metal ions with large (Na, K) and small (Li) radius and which have a high temperature stability of dielectric parameters in the

in modern decimeter and centimeter wave band communication systems [25, , , 28].

However, there was no explanation of the nature of this fact in literature.

sample preparation and the presence of structural transitions.

**5. MW dielectrics with "mobile sublattice"** 

(about 100) and TCε < 0 [32].

MW range (Table 2) [35].


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