I. P. Raevskii

Peculiar electric and photoelectric behavior of lead-containing perovskite-type oxides

Electrophysical properties and photoconductivity of several binary (PbTiO3, PbZrO3, PbHfO3) and ternary (Pb2ScNbO6, Pb2InNbO6) lead-containing perovskite-type oxides were investigated. Band gap energies for all studied compounds were measured to be somewhat smaller than for its perovskite analogs not containing lead. This fact is ascribed to a specific configuration of the outer electronic shell of lead.Owing to high photosensitivity of the lead-containing perovskites, a considerable effect of electronic subsystem (in particular of the non-equilibrium carriers) on crystal properties both in ferroelectric and antiferroelectric crystal phases was observed.

Phase transitions and electric properties of Pb2InNbO6 and Pb2ScNbO6 crystals with differing degree of ion ordering in the sites of the crystalline lattice

Abstract The effects of the compositional ordering on the properties of Pb2ScNbO6 and Pb2InNbO6(PIN) crystals were studied. For PIN an order-disorder transition temperature of 1010°C is yielded.

Growth and study of PbFe1/2Ta1/2O3 single crystals

Single crystals of the composition PbFe1/2Ta1/2O3 are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe1/2Ta1/2O3 ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, Tm, increases by more than 70 K with an increase in the frequency from 102 to 106 Hz. It is assumed that the unusual properties of the PbFe1/2Ta1/2O3 crystals are caused by mesoscopically inhomogeneous compositional ordering and comparatively high conductivity providing favorable conditions for the appearance of the volume-charge and thermal electron polarization.

Structural instabilities, incommensurate modulations and P and Q phases in sodium niobate in the temperature range 300–500 K

The relation between the phase transition in the vicinity of 420 K discovered earlier and the formation of an incommensurate phase in the temperature range 410–460 K is established based on the X-ray, dielectric, and dilatometric studies of sodium niobate (NaNbO3) single crystals and ceramics. It is also established that this phase is characterized by temperature and temporal instabilities. Anomalies in some physical characteristics in the vicinity of 350 K are revealed. It is also shown that the thermodynamic history of the samples is important for the coexistence of the regions of the ferroelectric Q and antiferroelectric P phases in NaNbO3.

Acoustic emission and thermal expansion of Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals

The temperature dependence of the elongation per unit length for Pb(Mg1/3Nb2/3)O3 crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb1−xBaxSc0.5Nb0.5O3 solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.

Phase states and dielectric properties of sodium-potassium niobate solid solutions

The temperature dependence of dielectric constant ɛ for single crystals of Na1−xKxNbO3 (0.04≲x≲0.15) is studied for the first time. From the shape of the ɛ(T) anomalies corresponding to rotational phase transitions, the type of interaction between the order parameters and the polarization is determined. A phenomenological model is developed which adequately describes the experimentally observed sequence of high-temperature (T>300°C) phase transitions, the dielectric anomalies associated with these transitions, and the changes in the phase states under the action of external factors (pressure, electric field).

Crystal-chemical effect of the thermodynamic history on the composition-temperature phase diagrams of (Na,Li)NbO3 and (Na,K)NbO3 solid solutions

Composition (x)-temperature (T) phase diagrams obtained for the first time for single-crystalline (Na,Li)NbO3 (system I) and (Na,K)NbO3 (system II) niobates are compared with the known phase diagrams for ferroelectric niobate ceramics prepared by various techniques (standard firing and hot pressing). In system I, the thermodynamic history (process conditions) is shown to have a pronounced effect on the diagrams throughout the range of perovskite stability, while in system II, the effect is the most significant at low potassium concentration (x≲0.15). These data are associated with a small value of the tolerance factor t of the solid solutions, whose properties are sensitive to the thermodynamic history. Small (≲0.9–0.93) values of t indicate the “porosity” of the structure of solid solutions I and II in the above concentration ranges. Under these condi-tions, polarization-and rotation-related structure instabilities compete on equal terms and substitutional and interstitial solid solutions may coexist.

Growth and study of single crystals of (Na, K)NbO3 solid solutions

Abstract(Na, K)NbO3 crystals with a perovskite structure and a KNbO3 content up to 40 mol % were grown from flux with the use of the solvent NaBO2. The dielectric measurements of the crystals grown revealed phase transitions that had never been observed before in ceramic samples.

A new phase transition in sodium niobate

The temperature dependences of the dielectric constant ε of sodium niobate single crystals NaNbO3 and sodium niobate-based (Na,Li)NbO3 and (Na,K)NbO3 solid solutions with low content of the second component have been studied. A weak anomaly observed in ε(T) indicates a new phase transition in the vicinity of 150°C in NaNbO3. The introduction of lithium or potassium into NaNbO3 crystals increases the temperature of this transition. The X-ray powder diffraction data suggest that the new transition takes place between two rhombohedral phases.