V. D. Zhuravlev

Synthesis of functional materials in combustion reactions

The conditions for obtaining oxide compounds in combustion reactions of nitrates of metals with organic chelating–reducing agents such as amino acids, urea, and polyvinyl alcohol are reviewed. Changing the nature of internal fuels and the reducing agent-to-oxidizing agent ratio makes possible to modify the thermal regime of the process, fractal dimensionality, morphology, and dispersion of synthesized functional materials. This method can be used to synthesize simple and complex oxides, composites, and metal powders, as well as ceramics and coatings. The possibilities of synthesis in combustion reactions are illustrated by examples of αand γ-Al2O3, YSZ composites, uranium oxides, nickel powder, NiO and NiO: YSZ composite, TiO2, and manganites, cobaltites, and aluminates of rare earth elements.

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.

Lead lanthanum and strontium lanthanum germanovanadates with apatite and oxyapatite structures

New compounds Pb4La(GeO4)2(VO4)(I) and Sr5La5(GeO4)5(VO4)O(II) were prepared and identified. Compound I has the structure of apatite, a = 10.108(1) Å, c = 7.369(1) Å, V = 652.1(2) Å3. Compound II has the structure of oxyapatite, a = 9.9028(5) Å, c = 7.3162(4) Å, V = 621.34(6) Å3.

Ba3 (VO4)2-K2 Ba(MoO4)2 and Pb3 (VO4)2-K2 Pb(MoO4)2 systems

Phase equilibria in the Ba3(VO4)2-K2Ba(MoO4)2 and Pb3(VO4)2-K2Pb(MoO4)2 systems have been investigated. In the first system, a continuous series of substitutional solid solutions with the palmierite structure is formed, and in the second one, the polymorphic transition in lead orthovanadate at 100°C restricts the extent of the palmierite-type solid solution to 10–100 mol % K2Pb(MoO4)2.

Mutual cation substitutions in palmierite orthovanadates: M3(VO4)2–M2Me2/3(VO4)2 systems where M = Sr, Ba, or Pb and Me = La or Bi

We performed the synthesis and studied the extents and compositions of solid solutions (ss) formed by heterovalent substitutions in M3(VO4)2–M2Me2/3(VO4)2 systems where M = Sr, Ba, or Pb and Me = La or Bi. A continuous solid solution (css) series was shown to be formed only when the initial compounds crystallize in palmierite structure. Bi3+ and La3+ were shown to occupy different positions (3а and 6с, respectively) in the crystal lattice of Sr3(VO4)2-base ss.

Variation of interatomic distances in oxides

The average interatomic distances D in oxygen polyhedra MOn of isostructural oxides were proposed to be estimated using the equation D = Kɛ(RM + RO) or D = Ax2 + Bx + C, where x = ɛ(RM + RO), ɛ is the ionicity of the M-O bond, RM is the ionic radius of the cation M with account for the coordination in the polyhedron, and RO is the ionic radius of oxygen. Calculations were made for MO oxides having the rock-salt structure; Ln2O3 oxides, where Ln = Ce-Yb; and the MO2 oxides having the rutile and fluorite structures.

Crystal structures of La1 − xSr2 + x(GeO4)(V1 − xMoxO4) (x= 0−0.4) solid solutions

The phase compositions of theLaVO4-SrMoO4(1) and Sr2GeO4-SrMoO4 (2) binary systems, which bound the Sr2GeO4-LaVO4-SrMoO4 (3) ternary system, and the LaSr2(VO4)(GeO4)-Sr2GeO4+SrMoO4 section (4) of system 3 are studied at subsolidus temperatures. Systems 1 and 2 consist of a mixture of the initial compounds, and the La1 − xSr2 + x(GeO4)(V1 − xMoxO4) (where 0 ≤ x ≤ 0.4) region of substitutional solid solutions with a palmierite structure is formed in system 3. The unit cell parameters of the solid solutions are determined. The distribution of the lanthanum and strontium cations over two positions of the cationic sublattice is described.

Calculation of Miscibility for Compounds with Zircon Structure

An algorithm is proposed for calculating isomorphic miscibility of LnEO4 compounds with zircon structure, where Ln = Sc, Y, or Ce-Lu and E = P, V, or As, in terms of Urusov’s theory. Miscibility in CeVO4-ScVO4 and NdVO4-ScVO4 systems is revised.

New Vanadium Germanium Garnets

New vanadatogermanates Ca3–xMe2 + xV2GeO12, Ca3Mе2VGe2O12, and Ca3Me2Ge3O12, where Me = Mg, Mn, Co, Ni, Fe, Zn, and x = 0–1, have been synthesized through coupled substitutions of cations in the initial garnets Ca2.5Mn2V3O12 or Ca3Fe2Ge3O12. The unit cell parameters of the synthesized samples have been determined. Probability analysis of the cation distribution in the octahedral positions of the crystal lattice as a function of crystallographic radii has been performed taking into account the spin state of the cations.

Synthesis, crystal structure, and Raman spectra of mixed oxides K2Pb(MoO4)2–x(CrO4)x and K2 – xPb1 + x(MoO4)(CrO4)1 – x(VO4)x, where x = 0–1

Specimens of K2Pb(MoO4)2–x(CrO4)x and K2–xPb1 + x(MoO4)(CrO4)1–x(VO4)x (x = 0–1) solid solutions, which crystallize in palmierite structural type with space group R3m, were prepared by a citrate method. Position 3a that has octahedral configuration of oxygen atoms in K2Pb(MoO4)2–x (CrO4)x is occupied mainly by lead atoms, the share of potassium in this position is not larger 0.056–0.060(2). The introduction of vanadium cations in K2Pb(MoO4)2–x(CrO4)x crystal lattice leads to the complete displacement of potassium cations from MO6 coordination polyhedron and decrease of Pb–O6 distances from 2.679(5) to 2.665(4) Å. Interactomic distances in (Pb/K)O6, (K/P)O10, and (Mo/Cr/V)O4 coordination polyhedra were calculated.