L. L. Surat

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.

Crystal structure and optical properties of germanates Ln2Ca(GeO3)4 (Ln = Gd, Ho, Er, Yb, Y)

This paper reports on the results of structural and optical investigations of a new class of layered compounds Ln2Ga(GeO3)4 (Ln = Gd, Ho, Er, Yb, Y), which are promising for the use in photonics as converters of laser radiation.

Synthesis, crystal structure, and luminescence properties of CaY2Ge3O10:Ln3+, Ln = Eu, Tb

The crystal structure and luminescence properties of CaY2Ge3O10:Ln3+ (Ln = Eu, Tb) germanates synthesized via a conventional solid-state reaction and an ethylenediaminetetraacetic acid complexing process are studied. The CaY2 − xLnxGe3O10 (Ln = Eu, Tb; x = 0–1.0, 2.0; Δx = 0.1) solid solutions have a monoclinic structure (space group P21/c, Z = 4), in which dopant ions occupy three nonequivalent noncentrosymmetric sites with different Ca2+/Ln3+ ratios. The effect of the synthesis methods, dopant concentrations, and excitation wavelengths on the luminescence properties of the compounds obtained is determined.

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.

Synthesis and structural study of a new group of trigermanates, CaRE2Ge3O10 (RE = La–Yb)

A new series of germanates CaRE2Ge3O10 (RE = Y, La–Yb) has been prepared using an EDTA-assisted route. Rietveld refinement of room temperature powder X-ray diffraction patterns shows that these compounds crystallize in the monoclinic system (S.G. P21/c, Z = 4) and have two morphotropic transitions. The calcium and rare earth atoms are distributed among three nonequivalent sites and form layers along the [0 0 1] direction connected into the framework through [Ge3O10]. The morphotropic transitions are accompanied by changes in the site occupancy factors of metal cations and by variation in the first and second coordination spheres of Ca2+/RE3+.

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.

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.

Infrared luminescence of CaLa2 – xNdxGe3O10:Ho3+, Er3+

Solid solutions CaLa2–xNdxGe3O10:Ho3+, Er3+ have been synthesized, their optical characteristics have been studied, and the dependences of the Stokes luminescence in the 0.9–3.0 µm region on the concentration of neodymium ions under 808 nm laser diode excitation have been determined. Based on analysis of the obtained dependences, the optimal composition of the phosphor with the maximum efficiency of the energy conversion is established.

Systems Li2O(Na2O)-CdO-V2O5: Phase composition and phase diagrams

The subsolidus phase composition of the M2O-CdO-V2O5 systems with M = Li or Na is studied. Double orthovanadates MCdVO4 and MCd4(VO4)3 form solid solutions of composition Li1 − 2x/3Cdx/3CdVO4 (0 ≤ x ≤ 1, orthorhombic space group Cmcm, modulation at x = 0.6) and Na3 − 2xCd3 + x(VO4)3 (0 ≤ x ≤ 0.10 and 0.30 ≤ x ≤ 1, orthorhombic space group Cmcm and Pn21a or Pnma, respectively). In the range 0.10 < x < 0.30, the end-members of the solid solutions coexist. Isothermal sections of the systems are mapped.