M. G. Komova

Phase Relations in the Solidus Region of the SrO-Bi2O3-B2O3 System

Phase equilibria in the SrO-Bi2O3-B2O3 system have been investigated by X-ray powder diffraction analysis and DTA. Ternary compounds SrBiBO4 and Sr7Bi8B18O46 congruently melting at 820 ± 5°C and 760 ± 5°C have been found. Quasi-binary sections are determined and the isothermal section of the system in the region Bi2O3-Sr2Bi2O6-Sr3B2O6-B2O3 at 600°C has been constructed.

Structure of the La12GdEuB6Ge2O34 borogermanate as probed by NMR and IR spectroscopy

The structure of fine crystalline borogermanate La12GdEuB6Ge2O34 has been studied by NMR and IR spectroscopy. It has been demonstrated that this compound is isostructural to the homonuclear Ln14B6Ge2O34 compounds (Ln = Pr-Gd) and crystallizes in space group P31. The rare-earth elements have been distributed over the LnOn polyhedra in La12GdEuB6Ge2O34 by analogy with the known structures. Lanthanum can occupy positions with CN 7–10, and the symmetry of these LnOn coordination polyhedra is not higher than C2v. In the La12GdEuB6Ge2O34 structure, the LnOn coordination polyhedra are formed by oxygen atoms of oxo groups and anions, some of the oxygen atoms being shared by LnOn polyhedra. The BO3 and GeO4 groups in the structure are also bridging, i.e., are involved in bonding of LnOn polyhedra. One of the B-O bonds in La12EuGd(BO3)6(GeO4)2O8 is elongated as compared with the B-O bond lengths in homonuclear compounds Pr14(BO3)6(GeO4)2O8 and Nd14(BO3)6(GeO4)2O8. In the La12GdEuB6Ge2O34 structure, germanium is located in isolated GeO4 tetrahedra with distorted Td symmetry. The local symmetry of lanthanum in fine crystalline La12GdEuB6Ge2O34 have been assessed using 139La NMR (B0 = 7.04 T, room temperature). For comparison, binary lanthanum compounds with a simpler structure— LaBO3, La(BO2)3, and La2GeO5—have been used. The spectra of all compounds are rather broad (ν1/2 = 180–240 kHz). The 139La NMR spectra of the LaBO3, La(BO2)3, and La12GdEu(BO3)6(GeO4)2O8 borates show a signal at (1080 ± 40) ppm, which is absent in the spectrum of La2GeO5. The shape of the 139La NMR spectra of La12GdEu(BO3)6(GeO4)2O8 and LaBO3 is characterized by the second-order quadrupole splitting with a downfield shoulder. The similarity of these spectra points to close 139La NMR chemical shifts of La12GdEu(BO3)6(GeO4)2O8 and LaBO3. No quadrupole splitting was observed in the spectra of La(BO2)3 and La2GeO5.

MgAl0.4Fe1.6O4 powders prepared via gel combustion

Powdered ferrite of composition MgAl0.4Fe1.6O4 is prepared by the combustion of gels consisting of a mixture of magnesium, aluminum, and iron nitrates, citric acid, and NH4NO3 added into the reaction mixture. The process is demonstrated to involve a single stage without the formation of volatile carbon-containing compounds when the ratio of the number of NH4NO3 moles to the total number of metal nitrate moles n = 10.1.

Cationic networks in the structures of borates, tungstates, and borotungstates forming in the Ln2O3-B2O3-WO3 systems

The influence of the structures of binary compounds on the formation of ternary mixed-anion compounds—borotungstates—in ternary Ln2O3-B2O3-WO3 systems was studied. The structures of borotungstates either inherit the predominant type of cationic network from the structures of the binary compounds (anisotropic cationic networks in Ln (BO2)(WO4)) or represent a sum of equivalent anisotropic and isotropic cationic networks, forming combined cationic networks (in Ln3BWO9). In the lanthanide series, the ranges of existence of borotungstates coincide with the ranges of existence of the structure types most abundant in the binary compounds.

Phase relations in the CaO-Bi2O3-B2O3 system in the subsolidus region

Phase relations in the CaO-Bi2O3-B2O3 system have been investigated by X-ray powder diffraction and differential thermal analyses, and the isothermal section at 600°C has been constructed. The formation of ternary compounds at the component ratios 1CaO: 1Bi2O3: 1B2O3 (CaBi2B2O7) and 1CaO: 1Bi2O3: 2B2O3 (CaBi2B4O10) has been established X-ray diffraction characteristics of these phases are presented.

Synthesis of IR phosphors based on germanatoborate Gd14Ge2B6O34

Processes of the formation of germanatoborates Gd14B6Ge2O34 and Gd13.02Nd0.98B6Ge2O34 have been studied using different methods of synthesis (solid-state interaction, direct and inverse co-precipitation, self-propagating high-temperature synthesis (SHS)). It has been established that the synthesis of germanatoborates Gd14B6Ge2O34 and Gd13.02Nd0.98B6Ge2O34 using the inverse precipitation and SHS methods occurs with the formation of an intermediate apatite-like phase, which upon heating to above 1100°С is reconstructed into the Ln14B6Ge2O34 (Ln = Gd, Nd) structure. The germanatoborates synthesized crystallize in the trigonal system (space group P31). The lattice parameters of Gd13.02Nd0.98B6Ge2O34 are a = 9.746(4) Å and c = 25.795(13) Å. The thermal stability of the Gd14B6Ge2O34 and Gd13.02Nd0.98B6Ge2O34 germanatoborates has been studied. The obtained materials of composition Gd13.02Nd0.98B6Ge2O34 show luminescence properties and can be employed as infrared phosphors.

Combustion synthesis of germanium phosphates Gd 11– x – y Yb x Er y GeP 3 O 26 and their luminescence properties

The formation of germanium phosphates Gd11–x–yYbxEryGeP3O26 containing 1 at % Er and 5 to 20 at % Yb upon gel combustion and annealing of intermediates was studied. Structure formation in germanium phosphates was shown to start with the crystallization of phosphate Gd3PO7 followed by its reaction with Gd2GeO5 to yield Gd11GeP3O26. The compounds prepared are crystallized in triclinic crystal system (space group P1). The unit cell parameters and unit cell volumes of Gd11–x–yYbxEryGeP3O26 decrease with increasing dopant concentration in the phosphor: from V = 555 Å3 in Gd10.34Yb0.55Er0.11GeP3O26, where Yb: Er = 5: 1, to V = 549 Å3 in Gd9.24Yb1.65Er0.11GeP3O26, where Yb: Er = 15: 1. Upconversion luminescence spectra of germanium phosphates Gd11–x–yYbxEryGeP3O26 were recorded. The luminescence spectra featured two bands: a strong band in the red, corresponding to the transition 4F9/2→ 4I15/2 (660 nm), and a weaker band in the green, corresponding to two transitions: 2H11/2 → 4I15/2 (λ = 525 nm) and 4S3/2 → 4I15/2 (λ = 550 nm) of Er3+ ions. The energy yields (Ben, %) were determined for the prepared phosphors. The highest value Ben = 0.54% was observed in phosphor Gd10.765Yb0.125Er0.11GeP3O26 where Yb: Er = 15: 1.

Isotropic cationic networks in the structures of rare earth compounds

The isotropic cationic networks (CNs) completing the anisotropic CNs ai isotropic CNs binary opposition are identical in the structures of various rare earth compounds.

Trends in the cation distribution in the structures of the compounds forming in the Ln2O3-GeO2-P2O5 systems

The cationic networks in the structures of the initial oxides and all binary and ternary compounds forming in the Ln2O3-GeO2-P2O5 systems have been studied. In the phase diagrams of the Nd2O3-GeO2-P2O5 and Er2O3-GeO2-P2O5 systems, the regions of the structural influence of individual compounds with topologically identical cationic networks—anisotropic (A), combined (C), and isotropic (I)—are united into common areas. The A: C: I area ratio is 1: 1: 1 in the neodymium system and 1.7: 1: 3.4 in the erbium system.

Anisotropic distribution of cations in the structures of rare earth compounds

Anisotropic cationic networks, which constitute the left-hand side of the binary opposition with isotropic cationic networks, are topologically identical in the structures of quite different rare earth compounds.