B. V. Slobodin

Subsolidus Phase Relations in the Systems M2+O–M2+O–V2O5(M+ = Li, Na, K, Rb, Cs; M2+ = Mg, Ca)

Subsolidus phase relations in the M2O(M2CO3)–MgO–V2O5 and M2O(M2CO3)–CaO–V2O5 (M = Li, Na, K, Rb, Cs) systems are studied. Twenty mixed vanadates are obtained, of which Rb2CaV2O7 , Cs2CaV2O7 , LiMg4(VO4)3 , RbCaVO4 , and CsCaVO4 are identified for the first time. Structural data are summarized for all of the mixed vanadates: the space group and lattice parameters are indicated for 14 compounds (for 6 compounds, such data are obtained for the first time), and I and d data are presented for 8 compounds. Partial series of Ca3(VO4)2-based solid solutions with the general formula Ca3 – xM2x(VO4)2 (M = Na, K, Rb, Cs) are identified in the range 0 < x ≤ 0.14. Six phase diagrams (M+ = Li, Rb, Cs; M2+ = Mg, Ca) are investigated and are compared with the phase diagrams of the other ternary systems in question. The key features of the ternary phase diagrams and, hence, the reactivity of the constituent oxides are shown to vary systematically in going from Li2O to Cs2O and from MgO to SrO, which is interpreted in terms of the variation in the ionic radius of the alkali and alkaline-earth metals.

Thermochemical and luminescent properties of the K2MgV2O7 and M2CaV2O7 (M = K, Rb, Cs) vanadates

We have studied the compounds K2MgV2O7 and M2CaV2O7 with M = K, Rb, and Cs. These vanadates melt incongruently in the range 635–717°C. Cooling their decomposition products to room temperature leads to the formation of nonequilibrium phase assemblages characteristic of the corresponding oxide systems. The compounds offer broadband photo- and radioluminescence with an essentially white (to the human eye) emission spectrum. A model is proposed for luminescence centers in the vanadates, which involves the formation of defects in vanadium-oxygen groups, and an energy level diagram of the emission centers is constructed in the form of configuration curves in the harmonic oscillator approximation. The luminescent properties of these compounds suggest that they can be used as basic components of cathodo- and roentgenoluminescent screens and white-light-emitting diodes with improved color performance.

Charge carriers in La0.67−xYxBa0.33MnO3

Magnetic, magnetotransport and optical properties are experimentally studied. The mechanism of conduction is found to be different below and above the Curie temperature. This difference is explained by a shift of the mobility edge due to magnetic disorder.

Phase relationships in ‘MO–LaMnO3–manganese oxides’ systems where M=Ca, Ba

Abstract The phase formation in MO–LaMnO 3 –MnO 2 –Mn 3 O 4 systems (M=Ca, Ba) has been studied from 1400 to 1500°C in air. It was shown that the LaMnO 3 -based solid solution appears within a wide region due to vacancy formation in the cationic sublattice and heterovalent substitution. The regions of existence of the solid solutions La 1− x M x MnO 3 , M=Ca (0≤ x ≤0.70), Ba (0≤ x ≤0.45), and the solid solutions where cations in calcium manganites are replaced by lanthanum, namely, La 1− x Ca 1+ x MnO 4 (0.65≤ x ≤1.00) and La 2−2 x Ca 1+2 x Mn 2 O 7 (0.50≤ x ≤1.00), have been established. The formation of the solid solution La 1− x Ca x MnO 3 (0.90≤ x ≤1.00) was confirmed. Schematic phase diagrams of MO–LaMnO 3 –manganese oxides systems were constructed using the phase composition of the concentration regions bordering the solid solutions.

Preparation and luminescent properties of rubidium and cesium vanadates

Four rubidium and cesium vanadates, with the compositions Rb5V3O10, Rb3V5O14, Cs5V3O10, and Cs2V4O11, have been synthesized and identified, and their thermal stability ranges have been determined. Their spectroscopic properties and luminescence kinetics have been studied: we have measured their X-ray luminescence spectra at room temperature and liquid-nitrogen temperature, photoluminescence spectra, and photoluminescence excitation spectra. The spectra are compared to those of RbVO3 and CsVO3—phosphors that have optimal properties among the alkali metal vanadates.

Synthesis and crystal structure of the pyrovanadate Na2ZnV2O7

The compound Na 2 ZnV 2 O 7 with an akermanite-type structure has been synthesized. It has a tetragonal unit cell, a =8.2711(4), c =5.1132(2) A, and crystallizes with P -42 1 m symmetry, Z =2. Its crystal structure has been refined from a combination of X-ray and neutron powder diffraction data. The structure contains layers of corner-sharing VO 4 and ZnO 4 tetrahedra, the former in pairs forming pyrovanadate V 2 O 7 units. The sodium atoms are positioned between the layers, with a distorted antiprismatic coordination of oxygen atoms.

Chemistry of interactions in the La2O3-MO(MCO3)-Mn2O3, M = Ca, Sr, Ba, Cd systems

Abstract The sequence of phase transitions in the system La 2 O 3 -MO(MCO 3 )Mn 2 O 3 , (M=Ca, Sr, Ba, Cd) and the span range of La 1−x M x MnO 3±δ solid solutions formed were studied. It is demonstrated that interaction begins at 650°C due to formation of divalent metal manganites. Just simultaneously as a result of oxidation-reduction processes and structural transformations the synthesis of small quantities of solid solutions takes place, which is kinetically favoured in the case of smaller divalent metal cations. The chemical transformation finishes at 1200°C. The values of x at all systems are near 0,35. The absence of CdMnO 3 composition in the system CdO-MnO is proved.

Thermochemical and Luminescent Properties of RbVO 3 , CsVO 3 , and Rb 0.5 Cs 0.5 VO 3

RbVO3, CsVO3, and Rb0.5Cs0.5VO3 have been synthesized by the Pechini process. The vanadates have an orthorhombic structure (sp. gr. Pbcm), melt congruently in the range 650–530°C, and undergo a reversible phase transition in the range 520–340°C. We have determined the onset temperatures and end points of the transformations at a temperature scan rate of 3°C/min and their enthalpies, and measured the photo-, roentgeno-, and cathodoluminescence and diffuse reflectance spectra of the vanadates. The luminescence spectra each are well fitted with three pseudo-Voigt functions. CsVO3 has the highest integrated emission intensity. The emission intensity of the Rb0.5Cs0.5VO3 solid solution is lower than that of the simple vanadates because of the optical absorption around its intrinsic luminescence band. This may be due to the presence of stable vacancy-type structural defects in Rb0.5Cs0.5VO3.

Thermal stability and cathodoluminescence of potassium strontium vanadates

We have studied the thermal stability of five potassium strontium vanadates: KSr(VO3)3, K2Sr(VO3)4, K4Sr(VO3)6, K6Sr(VO3)8, and KSrVO4. The double orthovanadate undergoes a reversible polymorphic transformation at 1117°C and is stable up to 1500°C. The double metavanadates melt peritectically in the range 490–517°C to give Sr2V2O7 crystals and peritectic melt. Pulsed cathodoluminescence studies have shown that the potassium: strontium ratio in the vanadates and their crystal structure have little effect on their optical emission properties. The performance parameters of new vanadate-based phosphors have been determined.

Thermoanalytical study of the polymorphism and melting behavior of Cu2V2O7

We have developed a procedure for the synthesis of phase-pure α- and β-Cu2V2O7. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu2V2O7 (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu2V2O7, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu2V2O7, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu2V2O7, CuVO3, and other compounds.