Thermal stability and electrical conductivity of multiferroics BiFeO<Subscript>3</Subscript>/REEs

Ceramic samples of bismuth ferrite and solid solutions of Bi_{1 − x} A _x FeO₃ type (where A = Lu, Yb, Tm, Er, Ho, Dy, Tb, Gb, Eu, Sm, Nd, Pr, La; 0.05 ≤ x ≤ 0.20; Δx = 0.05) were prepared. Spectra of the real part of electrical conductivity were studied within the range 10⁻⁴–10⁻⁶ Hz. The dependence of the samples’ thermal stability and electrical conductivity on the size of the substituting ions was established.     

Crystal chemical simulation of low-permittivity ferroelectrics

It is shown that the uniform strain parameter of the unit cell of solid solutions of alkali metal niobates can be raised and their permittivity can drastically be decreased by isovalent superstoichiometric modification in the A subcell. The underlying mechanism is suggested. It is concluded that these findings are helpful in studying (simulating) piezoelectric ferroelectrics of practical value.     

Formation of a cluster structure in the PbZr<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>Ti<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>O<Subscript>3</Subscript> system

Microdeformations of the crystal lattices in the phombohedral and tetragonal regions of the PbZr_1–xTi_xO₃ phase diagram have been calculated and their dependences on the titanium concentration have been constructed. Based on an analysis of these dependences, it has been concluded that tetragonal-phase clusters form in the range 0.11< x < 0.12 and rhombohedral-phase clusters form in the range 0.725 < x < 0.750.     

Invar effect in PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics

High-density lead ferroniobate PbFe_1/2Nb_1/2O₃ (PFN) is prepared by conventional ceramic technology. Its structural properties are studied in a wide temperature range (293 ≤ T ≤ 973 K). The following chain of phase transitions is established in the vicinity of the transition to the polar phase: Rh (rhombohedral phase) (T < 363 K) → Psc (pseudo-cubic phase) (363 < T < 387 K) → C (cubic phase) (T > 387 K). The paraelectric range contains five ranges of constant unit-cell volume (invar effect): I (387 ≤ T ≤ 413 K), II (433 ≤ T ≤ 463 K), III (553 ≤ T ≤ 613 K), IV (743 ≤ T ≤ 773 K), and V (798 ≤ T ≤ 823 K). It is shown that the anomalous behavior of the PFN dielectric characteristics above the Curie temperature, which was revealed previously, is associated with the specific features of its real (defect) structure, which is caused by the crystal-chemical specificity of the main structure-forming agents: α-Fe₂O₃ and α_ht-Nb₂O₅.     

Optimizing conditions of fabrication and the properties of BaNb2O6-SrNb2O6 binary ceramics

Conditions for fabricating (Ba_0.5Sr_0.5)Nb₂O₆ ceramics are optimized. It is shown that materials synthesized at T = 1250°C and sintered at T = 1375–21400°C exhibit the best ceramic characteristics. The (Ba_0.5Sr_0.5)Nb₂O₆ solid solution is found to be a ferroelectric with a diffuse phase transition. The possibility of fabricating Ba_0.5Sr_0.5Nb₂O₆ thin films from ceramic targets prepared in the resulting optimum regimes is shown.     

Growth and study of single crystals of (Na,K)NbO<Subscript>3</Subscript> solid solutions

(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.     

Phase analysis of high-temperature superconductors by selective dissolution combined with ICP-AES

Summary An apparatus for the selective dissolution as a chemical method of phase analysis and its operating conditions are described. Using ICP-AES as determination method an example for the YBCO system is found in the phase YBa_2–0.04Cu_3+0.03O_x and a BSCCO system is found in three phases of 35.6% Bi₂(Sr,Ca)_4–0.08Cu_3–0.05O_x, 30.1% Bi₂(Sr,Ca)_3–0.09Cu_2–0.08O_x and 34.3% Bi₂(Sr,Ca)_2–0.07Cu_1–0.05O_x.     

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

Six new polycrystalline bismuth-containing layered perovskite-like oxides described by the general formula A _m − 1Bi₂B _m O_{3m + 3} (m = 3) were synthesized by high-temperature solid-state reactions. The unit-cell parameters of these compounds were determined as a = 5.438(9) Å, b = 5.462(7) Å, and c = 33.08(6) Å for Bi₄Ti₂Nb_0.5Fe_0.5O₁₂; a = 5.408(6) Å, b = 5.425(9) Å, and c = 32.85(6) Å for Ca_0.5Bi_3.5Ti_2.75W_0.25O₁₂; a = 5.457(0) Å, b = 5.482(2) Å, and c = 32.75(2) Å for NaCaBi₂Nb₃O₁₂; a = b = 3.826(2) Å and c = 32.82(6) Å for Ca_0.25Th_0.25Bi_3.5Ti₃O₁₂; a = b = 3.816(2) Å and c = 32.81(5) Å for Nd_0.5Gd_0.5Bi₃Ti₃O₁₂; and a = b = 3.826(4) Å and c = 32.95(8) Å for Th_0.5Bi_3.5Ti_2.5Ga_0.5O₁₂. The compositions of these compounds are based on the composition of the basic Bi-containing layered perovskite-like Bi₄Ti₃O₁₂ oxide in which Bi^III and Ti^IV atoms are partially or completely replaced with other atoms. The experimental and calculated interplanar spacings determined from X-ray diffraction patterns and the reliability factors of their indexing are reported. __________ Translated from Kristallografiya, Vol. 48, No. 3, 2003, pp. 450–457. Original Russian Text Copyright ? 2003 by Geguzina, Shuvayev, Vlasenko, Shuvayeva, Shilkina. This study was presented at the symposium “Order, Disorder, and Properties of Oxides” (ODPO), Sochi, Russia, 2002.     

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

Single crystals of the composition PbFe_1/2Ta_1/2O₃ are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe_1/2Ta_1/2O₃ ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, T _m, 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 PbFe_1/2Ta_1/2O₃ 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.