O. N. Razumovskaya

Specific features of the thermal, magnetic, and dielectric properties of multiferroics BiFeO3 and Bi0.95La0.05FeO3

Thermophysical, magnetic, and dielectric properties of multiferroic BiFeO3 and Bi0.95La0.05FeO3 ceramic compounds were comprehensively studied. Anomalies of the permittivity near an antiferromagnetic phase transition related to the structural variations were detected. The temperature TN was determined from the temperature dependences of the thermal expansion coefficient, heat capacity, and differential susceptibility. It is shown that the transition point is shifted to higher temperatures as the rare-earth La ion substitutes for Bi. It is established that an insignificant substitution of lanthanum for bismuth enhances the magnetic properties of bismuth ferrite and the magnetodielectric effect.

Dielectric and Piezoelectric Properties of NaNbO3-Based Solid Solutions

Detailed data are presented on the structure and electrical properties of Na1 – xLixNbO3 (0 ≤x ≤ 0.145), Na1 – xKxNbO3 (0 ≤ x ≤ 1.0), and (Na1 – xPbx)(Nb1 – xTix)O3 (0 ≤ x ≤ 1.0) solid solutions prepared by solid-state reactions followed by hot pressing. The sequences of phase changes and the stability ranges of the resulting phases are identified. The results are used to map out the compositional phase diagrams of the NaNbO3-based binary systems.

Crystallographic shear in niobium oxides of different compositions

Defect structures related to crystallographic displacement are revealed in niobium oxides of different composition (Nb2O5, NaNbO3, and solid solutions based on NaNbO3) by X-ray powder diffraction, X-ray fluorescence spectroscopy, and Raman scattering. It is suggested that lattice shear in NaNbO3 and NaNbO3 based solid solutions lead to the block structure of crystals. The effect of the thermodynamic history of objects under study on the formation of a defect structure of this type is demonstrated.

Crystal Chemistry of Lead Titanate in Relation to Its Electrical Properties

The origin of the discrepancy between the measured average parameter of the perovskite unit cell of PbTiO3and the unit-cell parameter calculated for an ideal perovskite structure is analyzed with the use of extended x-ray absorption fine structure spectroscopy data, which demonstrate that PbTiO3contains Pb4+ions. The amount of Pb4+depends on the preparation conditions and may attain ≃6 at. %. Assumption is made that the presence of crystallographic shear planes allows a fraction of the Pb2+ions to occupy octahedral sites near the shear planes. The maximum content of Pb2+on the B site is ≃2.5 at. %. The formula proposed for the cation composition of imperfect lead titanate differs from the ideal formula Pb2+Ti4+O3and indicates that lead titanate can be thought of as an internal solid solution. It is shown that the composition of lead titanate influences its electrical properties: ε/ε0increases with Pb4+concentration and exceeds that in single crystals by an order of magnitude.

Invar effect in n-Nb2O5, αht-Nb2O5, and L-Nb2O5

An X-ray diffraction analysis of two commercial sets of niobium pentaoxide (Nb{sub 2}O{sub 5}) of Nbo-Pt grade has been performed. Each set reveals the coexistence of three modifications: n-Nb{sub 2}O{sub 5}, {alpha}{sub ht}-Nb{sub 2}O{sub 5}, and L-Nb{sub 2}O{sub 5}. Anomalous behavior in the structural characteristics, with the occurrence of plateaus upon heating (the invar effect), is established for each phase. It is suggested that the coincidence of the temperature ranges with constant unit-cell parameters in Nb{sub 2}O{sub 5} and complex Nb-containing oxides indicates the unified nature of the invar effect, which is related to the defect state of objects under study.An X-ray diffraction analysis of two commercial sets of niobium pentaoxide (Nb2O5) of Nbo-Pt grade has been performed. Each set reveals the coexistence of three modifications: n-Nb2O5, αht-Nb2O5, and L-Nb2O5. Anomalous behavior in the structural characteristics, with the occurrence of plateaus upon heating (the invar effect), is established for each phase. It is suggested that the coincidence of the temperature ranges with constant unit-cell parameters in Nb2O5 and complex Nb-containing oxides indicates the unified nature of the invar effect, which is related to the defect state of objects under study.

Invar effect in n-Nb{sub 2}O{sub 5}, {alpha}{sub ht}-Nb{sub 2}O{sub 5}, and L-Nb{sub 2}O{sub 5}

An X-ray diffraction analysis of two commercial sets of niobium pentaoxide (Nb{sub 2}O{sub 5}) of Nbo-Pt grade has been performed. Each set reveals the coexistence of three modifications: n-Nb{sub 2}O{sub 5}, {alpha}{sub ht}-Nb{sub 2}O{sub 5}, and L-Nb{sub 2}O{sub 5}. Anomalous behavior in the structural characteristics, with the occurrence of plateaus upon heating (the invar effect), is established for each phase. It is suggested that the coincidence of the temperature ranges with constant unit-cell parameters in Nb{sub 2}O{sub 5} and complex Nb-containing oxides indicates the unified nature of the invar effect, which is related to the defect state of objects under study.An X-ray diffraction analysis of two commercial sets of niobium pentaoxide (Nb2O5) of Nbo-Pt grade has been performed. Each set reveals the coexistence of three modifications: n-Nb2O5, αht-Nb2O5, and L-Nb2O5. Anomalous behavior in the structural characteristics, with the occurrence of plateaus upon heating (the invar effect), is established for each phase. It is suggested that the coincidence of the temperature ranges with constant unit-cell parameters in Nb2O5 and complex Nb-containing oxides indicates the unified nature of the invar effect, which is related to the defect state of objects under study.

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.

Effect of barium on the structure and dielectric properties of multicomponent ceramics based on ferroelectric relaxors

High-density ceramic samples of mPb(Mg1/3Nb2/3) · yPb(Zn1/3Nb2/3) · nPb(Ni1/3Nb2/3) · xPbTiO3 (m = 0.4541, y = 0.0982, n = 0.1477, x = 0.3) solid solutions, unmodified and barium-modified, with the composition lying in a morphotropic phase region, have been prepared by a conventional ceramic processing technique. We demonstrate that barium substitution for 5% of the lead on the A site allows one to obtain ceramics containing no pyrochlore phase. Modification with barium shifts the solid solution from the morphotropic to a tetragonal phase region. In addition, barium doping increases the average grain size of the ceramic from 2–3 to 3–4 μm and changes the type of fracture from intergranular to mixed (intergranular and transgranular). Modification with barium increases the relative dielectric permittivity ɛ/ɛ0 (at E = 0) of the material by more than a factor of 2 (from 4300 to 9100). We conclude that materials based on the ceramics studied can find practical application and examine ways of further improving their piezoelectric performance.

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.

Magnéli phases in Ti-containing oxides and their solid solutions

The ordered planes of crystallographic shear inherited from titanium dioxide TiO2 were revealed in complex Ti-containing oxides (Magnéli phases) by powder X-ray diffraction. It is assumed that periodic shear in the structure results in the modulation of titanium and oxygen displacements along the direction normal to the shear planes. A method of calculating the amplitude of the displacements of titanium atoms from the centers of oxygen octahedra is suggested. The method provides good agreement of the calculated and experimental data for single crystals. The degree of A-nonstoichiometry in alkaline earth, cadmium, and lead titanates is evaluated as 2–3%.