Doping of LiNbO<Subscript>3</Subscript> and (Na,Li)NbO<Subscript>3</Subscript>-based solid solutions

Doping of LiNbO₃ and (Na,Li)NbO₃ with ions of different valence above stoichiometry has been performed. The effect of modifiers on the grain structure and electrical properties of these compounds has been investigated. It is shown that the conductivity of ceramics can be controlled.     

Properties of nonstoichiometric sodium niobate

Stoichiometric and nonstoichiometric sodium niobate of the composition Na_1−x NbO_3−x/2 (0≤x≤0.2) is obtained by the two-stage solid-phase reaction technique. X-ray diffraction study reveals the homogeneity region for compositions below x=0.1. At higher x, the samples become two-phase. The phase transition is found to occur at x=0.04. It is accompanied by a change in the multiplicity of the monoclinic subcell of the sodium niobate rhombic unit cell, as well as by the anomalous behavior of its structural and electrophysical characteristics. From the study of the dielectric and piezoelectric properties of the material, it is concluded that the stability of the antiferroelectric phase is enhanced, whereas the ferroelectric phase becomes less stable, as the nonstoichiometry in terms of Na₂O increases.     

Investigation of biodegradable composites of poly(3-hydroxybutyrate) ultrathin fibers modified by a complex of iron(III) with tetraphenylporphyrin

Ultrathin fibers based on poly(3-hydroxybutyrate) containing a small concentration of iron(III) complex with tetraphenylporphyrin are obtained by electrospinning. In the absence of the complex, the fibers are characterized by the presence of cylindrical regions with a diameter of 1–3 and spindlelike regions with an average diameter of 7–10 μm. Introduction of the iron(III) complex with tetraphenylporphyrin into the polymer solution leads to the disappearance of spindlelike thickenings on the fibers. Addition of the complex to the polymer matrix results in a sharp rise in crystallinity and slows down the molecular mobility in the amorphous regions of ultrathin fibers. Annealing of the fibers at 140°C involves a sharp increase in the crystallinity of the polymer, while after addition of the complex this characteristic decreases abruptly. The final fibrous materials possess bactericidal properties and should find direct application in the creation of antibacterial and antineoplastic therapeutic systems.     

Synthesis and structure of new phases of the Am?1Bi2BmO3m+3 (m = 2) type

Three polycrystalline bismuth-containing layered perovskite-like oxides are synthesized by high-temperature solid-state reactions. One of these compounds was described previously, namely, Bi₃Ti_1.5W_0.5O₉, for which the unit cell parameters a = 5.372(5) Å, b = 5.404(4) Å, and c = 24.95(2) Å are determined in this study. The other two compounds, namely, Na_0.75Bi_2.25Nb_1.5W_0.5O₉ with the unit cell parameters a = 5.463(1) Å, b = 5.490(7) Å, and c = 24.78(0) Å and Ca_0.5Bi_2.5Ti_0.5Nb_1.5O₉ with the unit cell parameters a = b = 3.843(2) Å and c = 24.97(6) Å, are synthesized for the first time. The compositions of these compounds are based on the composition of the well-known compound Bi₃TiNbO₉ with a high Curie temperature (T _C = 1223 K), in which bismuth, niobium, and titanium atoms are partially or completely replaced by other atoms. The experimental and calculated interplanar distances determined from the X-ray diffraction patterns of the studied compounds are presented. __________ Translated from Kristallografiya, Vol. 50, No. 1, 2005, pp. 59–64. Original Russian Text Copyright ? 2005 by Geguzina, Shuvaev, Shuvaeva, Shilkina, Vlasenko.     

Phase transitions and electrical properties of solid solutions of sodium, lithium, and strontium niobates

An additional study of solid solutions in the (Na, Li, Sr_0.5)NbO₃ ternary system was performed. More detailed information concerning its phase diagram and physical parameters in a wide range of component concentrations is obtained. The compositions obtained are of interest for application in highly sensitive and high-frequency transducers.     

On the prospects for technical applications of BiFeO<Subscript>3</Subscript> compounds substituted with rare-earth elements

Solid solutions of Bi_1-x Re _x FeO₃ (Re=La, Nd; x=0-0.2) compounds were synthesized, and their room-temperature magnetoelectric and magnetodielectric properties were studied. Amplification of magnetoelectric and magnetodielectric effects with increasing the rare-earth doping level was detected in the concentration range x under study. The results obtained confirm the prospects for applications of bismuth ferrite-based compounds as magnetoelectric converters and magnetic field sensors.     

Special features of the grain structure of polycrystalline NaNbO<Subscript>3</Subscript> with 3-0 connection

Grain structure of porous sodium niobate ceramics (with 3-0 connection and variable porosity changing from 1.0 to 13.6%) fabricated under different conditions is investigated and the microstructure parameters are calculated for different degrees of porosity. It is demonstrated that with increasing degree of porosity, the character of the sodium niobate grain structure changes due to the formation and ordering of the pore -multilayered grain coating clusters, sharp increase in the total pore surface area, and its significant excess over the external sample surface area. These structural changes can influence significantly the integral electrophysical characteristics of the porous sodium niobate ceramics and determine a high degree of its piezoanisotropy.     

Effect of external influences on the structural and dynamic parameters of polyhydroxybutyrate-hydroxyvalerate-based biocomposites

The EPR probe, DSC, and WARDX methods are applied to study the molecular dynamics and structure of 3-hydroxybutyrate-3-hydroxyvalerate copolymer (PHBHV, 5 mol %) and its mixtures with segmented polyester urethane (SPEU) upon ozone oxidation and hydrothermal treatment. It is shown that time-limited (3 and 5 h) treatment of the mixed compositions with water at 40°C has no noticeable effect on the dynamics of rotation of the probe (EPR) and the crystallinity of PHBHV (DSC). However, hydrothermal treatment at 70°C causes an increase in the molecular mobility of the probe in the system, accompanied, according to XRD and DSC analyses, by increases in the degree of crystallinity and density of PHBHV crystallites. The temperature of fusion of mixed compositions also increases. During ozonation, the mobility of the probe slows gradually in PHBHV all the time, whereas in mixed compositions, a slowdown is observed only up to 3 h of oxidation time, after which, up to 5 h of ozonation, the mobility increases, along with decreases in the fraction of the amorphous phase and in the density of the crystallites.     

Evolution of domain processes during the transition from classical ferroelectric to relaxor ferroelectric

Dependences of the relative permittivity on the external bias electric field of ceramics of the yPZN-mPMN-nPNN-xPT (x = 0.25?C0.40) system have been studied. The evolution of domain processes during the concentration transition from classical ferroelectric to relaxor ferroelectric has been shown. Tentative conclusions on the mechanisms of electric field-induced transitions from the relaxor state to the ??normal?? ferroelectric state have been drawn.     

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

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, α_ht-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. Original Russian Text ? L.A. Reznichenko, V.V. Akhnazarova, L.A. Shilkina, O.N. Razumovskaya, S.I. Dudkina, 2009, published in Kristallografiya, 2009, Vol. 54, No. 3, pp. 517–526.