O. Yu. Kravchenko

Properties of Na0.875Li0.125NbO3 ceramics

The structural, dielectric, and thermal properties of the Na0.875Li0.125NbO3 solid solution doped with strontium and other elements have been studied in wide temperature and frequency ranges. The material has been shown to undergo a sequence of phase transitions accompanied by anomalies in its structural, dielectric, and thermal properties. The observed low-frequency dispersion of its dielectric permittivity is attributed to the effect of electrical conductivity.

Phase transformations and properties of Ag1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics

We have studied the phase transformations, microstructure, and dielectric, piezoelectric, and thermophysical properties of Ag1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics. Within its homogeneity range (y ≤ 0.10), silver niobate undergoes a complex sequence of phase transformations, accompanied by anomalies in its physical properties. The observed dispersion effects are interpreted in terms of electrical conductivity above the Curie temperature and in terms of the motion of domain walls and interfaces below the Curie temperature.

The Invar Effect and the Devil's Staircase in Alkali and Alkaline Earth Niobates

Some anomalies of structural characteristics are revealed in the metaniobates of alkali metals (NaNbO3, AgNbO3, and KNbO3) and pyroniobates of alkaline earth metals (Ca2Nb2O7 and Sr2Nb2O7): the constancy of one or several parameters and/or volumes of the unit cell (the Invar effect) in the temperature ranges corresponding to the regions of phase transitions of different nature. These anomalies are explained by the coexistence of alternating phases with limiting cell parameters. Other reasons for the Invar effect may be compromises between the competing processes of thermal expansion of the unit cell upon sample heating and its compression as a result the occurrence of crystallographic shear, enhanced in the regions of structural instability. A complex morphology of the region of incommensurability in NaNbO3, including portions with an incomplete or complete devil’s staircase is established. It is suggested that the incommensurate phase has a soliton structure.

Phase composition, microstructure, and properties of Na1 − yNbO3 − y/2 ceramics

We have studied the phase composition, microstructure, and dielectric and thermophysical properties of Na1 − yNbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics and identified a complicated sequence of phase transformations within the homogeneity range of sodium niobate (y ≤ 0.10), accompanied by anomalous variations in its physical properties. At low y values, discontinuous secondary recrystallization occurs. We conclude that dielectric effects above the Curie temperature are related to the electrical conductivity of the material and that those at low temperatures are governed by the motion of domain walls and interfaces. The temperature-dependent structural, dielectric, and thermophysical properties of the materials studied are shown to correlate.

Phase pattern of sodium niobate ceramics with different porosities in the temperature range of 25–700°C

The effect of closed porosity (2 and 10%) on the sequence phase transitions in lead-free ceramics (based on sodium niobate) in the temperature range of 25–700°C has been studied. The phase patterns in relatively dense and highly porous ceramics are found to differ.

Influence of the misfit of the crystal chemical parameters of Na and Li cations on the dielectric properties of NaNbO3-LiNbO3 solid solutions

It is established that the Curie-Weiss temperature of Na1 − xLixNbO3 solid solutions determined by extrapolation of linear portions of the temperature dependence of the reciprocal of the permittivity ɛ−1 from the cubic phase sharply increases with x, although the temperature of the ɛ(T) maximum decreases. It is shown in terms of a simple theoretical model that the experimentally observed change in the dielectric properties of Na1 − xLixNbO3 is well explained under the assumption of formation of a dipole system due to the displacement of Li cations from the center of the cubooctahedral cavity because of the significant steric misfit between the Na and Li cations.

Phase composition and dielectric properties of NaNbO3 ceramics

The phase composition and dielectric properties of NaNbO3 ceramics ranging in closed porosity from 2.0 to 13.6% have been studied in wide temperature and frequency ranges. The phase composition of the ceramics has been shown to depend on their porosity and temperature. The temperature and frequency dependences of the dielectric properties of the NaNbO3 ceramics correlate with their phase composition and porosity.

Dielectric properties of Na1 − x K x NbO3 and Na1 − x Li x NbO3 ceramics

The dielectric properties of solid solutions based on sodium potassium and sodium lithium niobates have been studied in wide ranges of temperatures (25–750°C), frequencies (25 to 106 Hz), and electric fields (up to 30 kV/cm). We have identified solid-solution regions differing in the temperature and frequency dependences of the dielectric permittivity. It is reasonable to take into account the present results in device applications where wide variations in dc bias field and frequency are needed.

Dielectric properties of Na 1 − x K x NbO 3 and Na 1 − x Li x NbO 3 ceramics

The dielectric properties of solid solutions based on sodium potassium and sodium lithium niobates have been studied in wide ranges of temperatures (25–750°C), frequencies (25 to 106 Hz), and electric fields (up to 30 kV/cm). We have identified solid-solution regions differing in the temperature and frequency dependences of the dielectric permittivity. It is reasonable to take into account the present results in device applications where wide variations in dc bias field and frequency are needed.

Dielectric properties of Na1 − xKxNbO3 and Na1 − xLixNbO3 ceramics

The dielectric properties of solid solutions based on sodium potassium and sodium lithium niobates have been studied in wide ranges of temperatures (25–750°C), frequencies (25 to 106 Hz), and electric fields (up to 30 kV/cm). We have identified solid-solution regions differing in the temperature and frequency dependences of the dielectric permittivity. It is reasonable to take into account the present results in device applications where wide variations in dc bias field and frequency are needed.