E. S. Buyanova

Oxide ion conductors of the BIMEVOX family: Synthesis, structure, and conductivity

The synthesis, stable existence regions, structure, and migration characteristics of complex oxides of the BIMEVOX family were studied experimentally and theoretically. The temperature, composition, and oxygen partial pressure dependences of the conductivity of the samples obtained were determined. The contribution of oxide ion transport decreased as the temperature and oxygen partial pressure lowered. Quantum-chemical calculations of β-Bi4V2O11 and γ-Bi4V2O11 were performed to analyze the relative stabilities of the phases and the influence of dopants.

Synthesis, structure, and conduction of solid solutions BIMEVOX (Me = Cu, Ti)

Conditions for synthesizing single-phase solid solutions Bi4V2 − xCux/2Tix/2O11 − x and Bi4 − x/2V2 − x/2Cux/2Tix/2O11 − x/2 are studied. The sequence of phase transformations is determined. Possible domains of stability of polymorphic modifications and the overall conductivity as a function of temperature and composition are studied. The structure of the γ-modification is refined using Rietveld’s full-profile analysis.

Oxygen-ionic conductors based on substituted bismuth molybdates with column-type structural fragments

The possibility of synthesizing oxygen-ionic conductors from substituted bismuth molybdates containing [Bi12O14]n8n+ columns, MoO4 tetrahedra, and isolated Bi ions in their structure was studied. The specifics of their structure and electric conductivity were investigated. The general formula of the solid solutions can be recorded as Bi13Mo5 − xMexO34 − δ, where Me is the fouror five-valent d metal (Ti, Zr, V, Nb). The electric conductivity of doped bismuth molybdates considerably increased compared with that of the matrix compound. The electric conductivity reached 5.5 × 10−3 S cm−1 at 700°C and 1.8 × 10−4 S cm−1 at 350°C for the zirconium-doped compound with x = 0.4. The porosity of the ceramics was less than 5%; the thermal expansion coefficient was of the order of 14 × 10−6 K−1. Based on the set of their characteristics, these compounds are recommended as materials for membranes of electrochemical devices.

Crystal structure and conduction of BICUTIVOX

Existence boundaries, structure, and transport parameters were studied for Bi4V2 − xCux/2Tix/2O11 − x solid solutions. Doping levels within x = 0.025–0.15 distort the C2/m crystal lattice (this lattice is characteristic of individual the Bi4V2O11 phase) and lowers its symmetry to triclinic. The solid solutions with 0.25 ≤ x ≤ 0.30 crystallize in tetragonal space group I4/mmm. High-temperature X-ray diffraction and dilatometry measurements for Bi4V2 − xCux/2Tix/2O11 − x (x ≤ 0.35) solid solutions verified the existence of three structural varieties within 298–1023 K. Electrical conductivity of BICUTIVOX was studied by impedance spectroscopy as a function of temperature, composition, and oxygen partial pressure. Equivalent circuits of cells were analyzed. Features of electrical conductivity versus temperature for the structural varieties are noted. Above 873 K, the solid solutions samples with x = 0.05 have the highest conductivity. At lower temperatures, higher conductivities are in the solid solutions that retain the γ phase in the low-temperature region. The dominant oxygen-ion conduction mechanism was discovered in the solid solutions.

Preparation, structure, and charge transport characteristics of BIFEVOX ultrafine powders

Existence boundaries, structure, and transport parameters of ultrafine powders were studied in Bi4V2 − xFexO11 − x (BIFEVOX) solid solutions. The details of synthesis of the solid solutions via liquid precursors are analyzed comparatively. In general, BIFEVOX formation via liquid precursors is similar to phase formation in solid-phase synthesis. With low iron levels (x = 0.05–0.1), solid solutions are formed in the monoclinic α phase (space group C2/m) The compositions with x = 0.125 and 0.15 are mixtures of α- and β phases. In the range 0.2 < x < 0.7, the Bi4V2 − xFexO11 − x solid solution has the structure of the γ phase of Bi4V2O11 (space group I4/mmm). The β phase in the system in question has a very narrow existence range in the vicinity of x = 0.175. The average particle sizes of the powders prepared by various methods are within 0.5–3 μm. In the powders prepared via liquid precursors, however, the distribution peak shifts toward smaller sizes, to 0.3–1 μm. Mechanical activation conserves the structure of the γ phase of BIFEVOX, and unit cell parameters change only insignificantly; however, the crystal lattice is slightly distorted. The electrical conductivity of BIFEVOX was studied as a function of temperature, preparation technology, and composition using impedance spectroscopy. Equivalent circuits of cells were analyzed. The conductivity of samples prepared by solution technology is always higher than for samples prepared by the solid-phase process. Features of electrical conductivity versus temperature for various phases are noted. All transitions on the conductivity curves match the features of linear thermal expansion curves. Compositions with doping levels x= 0.1–0.3 have the highest total conductivities.

Electronic structure and chemical bonding in oxygen conductors β-Bi4V2O11 and γ-Bi4V2O11

Ab initio calculations of the electron density are fulfilled for bismuth vanadate polymorphs: β-Bi4V2O11 and γ-Bi4V2O11. A semiconductor-type spectrum is obtained for both phases. The calculated bandgap value decreases in the direction from the β to γ phase. Chemical bond analysis shows an increase in the strength of covalent interactions in the series of Bi-V, Bi-O, and V-O bonds. The V-O bond strength in β-Bi4V2O11 is appreciably higher than in the γ phase. The relatively weak binding of oxygen in the tetragonal structure of γ-Bi4V2O11 confirms the possibility of easier oxygen transport in the γ phase than in the β phase. The stability of the bismuth vanadate polymorphs and the oxygen-disorder features in the γ-Bi4V2O11 structure are discussed using the results of calculation of the total energies of the phases. Bond overlap population analysis explains the existence of the homogeneous range and the feasibility of doping β-Bi4V2O11 and γ-Bi4V2O11. The possibility of using four-and five-charged d cations as stabilizers of the crystal structure of bismuth vanadates is discussed.

Synthesis, structure, and conductivity of BIMEVOX oxide ceramics

The methods using solid-phase synthesis or liquid precursors converted compounds of BIMEVOX family (ME is a bi- to pentavalent cation) into stable materials in a sufficiently wide concentration and temperature ranges. The crystal structure of various BIMEVOX polymorphic modifications was refined and their behavior was analyzed with respect to temperature and oxygen partial pressure. The phase transition temperature ranges were fixed. The particle size distribution was determined by means of laser diffraction and optical microscopy. The impedance measurements of BIMEVOX electroconductivity against temperature, composition, oxygen partial pressure detected the most promising compositions with respect to the absolute conductivity value and linear conductivity relationship to temperature and Po2. Different conductivity of specimens was noted with respect to method of synthesis.

Oxide ceramics BIMEVOX: Conductivity, structure, and chemical bond

The processes of synthesis, ranges of stable existence, structure, and migration characteristics of complex oxides of the BIMEVOX series are studied experimentally and theoretically.

Structural and Transport Characteristics of Substituted Bismuth Niobates

The results of studying solid solutions with the composition of Bi3Nb1 − yZryO7 ± δ, Bi2.95Y0.05Nb1 − yZryO7 ± δ (y = 0–0.5; Δy = 0.1), and Bi6.95Y0.05Nb2 − yZryO15.5 (y = 0.1–1; Δy = 0.1) are presented. XRD and electron microscopy with X-ray microanalysis are used to determine the homogeneity regions of solid solutions; crystallochemical parameters are calculated. It is shown that irrespective of the ratio of Bi: Nb, two cubic phases are formed at an increase in the dopant amount. One of these represents a solid solution based on Bi3NbO7 (δ-phase) and the second one is a solid solution based on δ-Bi2O3 (δ′-phase). Conductivity of sintered samples is studied using the impedance spectroscopy technique. Introduction of yttrium into the bismuth sublattice results in no increase in conductivity of solid solutions, while in the case of the ratio of Bi: Nb = 3: 1, overall conductivity of solid solutions is somewhat higher at similar dopant concentrations.

Synthesis, region of existence, structural characteristics, and conductivity of BI(CR, FE)VOX solid solutions

The results of our studies of solid solutions of the general composition Bi4V2 − xFex/2Crx/2O11 − δ are presented. The crystal-chemical parameters of different polymorphic modifications of BIMEVOX were determined. The particle size distribution was determined by laser diffraction and optical microscopy. The surface of sintered preforms was studied by scanning electron microscopy. The conductivity of sintered poly-crystalline samples as a function of temperature and composition was studied by impedance spectroscopy. The conductivity was depends on the procedure for the synthesis of solid solutions. The most promising compositions were revealed.