V. F. Popova

Phase diagram of the LaFeO3-LaSrFeO4 system

The phase relationships in the LaFeO3-LaSrFeO4 system at temperatures in the range 1000–2100°C in air are investigated, and the phase diagram of the system is constructed. One perovskite-like compound, namely, La2SrFe2O7, which belongs to the Ruddlesden-Popper phases, is revealed in the system.

Mechanism of formation of the complex oxide Gd2SrFe2O7

The mechanism of formation of the perovskite-like layered structure of the oxide Gd2SrFe2O7 was studied. The limiting stages are those of formation of phases with perovskite (GdFeO3, SrFeO3−x) and K2NiF4 (GdSrFeO4) structures. The Mössbauer study has shown that iron atoms exist in a heterovalent state (Fe3+ and Fe4+) only in the structure of SrFeO3−x).

Phase and chemical transformations in the SiO2-Fe2O3(Fe3O4) system at various oxygen partial pressures

The SiO2-Fe2O3(F3O4) system has been studied in air, oxygen, and an inert atmosphere. The dissociation temperatures for iron oxides, the onset and full melting temperatures for coexisting phases, and the melt demixing temperatures have been determined as a function of the oxygen partial pressure. A scenario of the phase and chemical transformations in the title systems has been developed.

Y2O3-Ga2O3 phase diagram

Phase relations in the Y2O3-Ga2O3 system were studied by the anneal-and-quench technique in air within 1000–2300°C, and a phase diagram was plotted. Three compounds were found to form: Y3GaO6, Y4Ga2O9, and Y3Ga5O12; the temperature and concentration bounds of stability were determined for these compounds. Indexing results for Y3GaO6 are given.

Mechanism and kinetics of formation of La2SrFe2O7 and Nb2SrFe2O7

Phase formation processes in the systems Ln2O3-SrO-Fe2O3 (Ln = La, Nd) in air in the temperature range 1200–1500°C were studied. The synthesis of the complex ferrites La2SrFe2O7 and Nb2SrFe2O7 involves the formation of the intermediate compounds LnFeO3 and LnSrFeO4 and occurs by the same mechanism as the synthesis of the corresponding aluminates, but much faster.

The influence of the particularities of synthesis on the physicochemical properties of nanosized powders and ceramic samples of REE orthophosphates

Sol–gel synthesis of La1–xYxPO4 · nH2O orthophosphates has been conducted via a method of reverse precipitation, and the obtained powders have been studied with X-ray phase analysis and simulteneous thermal analysis, including differential scanning calorimetry and thermogravimetry (DSC/TG). For the ceramic samples obtained from the powder, the microhardness has been determined depending on the temperature of the heat treatment. The thermal behavior of the samples has been studied via the method of dilatometry.

Synthesis of nanopowders and physicochemical properties of ceramic matrices of the LaPO 4 –YPO 4 –(H 2 O) and LaPO 4 –HoPO 4 –(H 2 O) systems

Sol-gel method was used to synthesize nanosize powders in the LaPO4–YPO4–(H2O) and LaPO4–HoPO4–(H2O) systems. Dense ceramic samples with high microhardness (up to 25 GPa) were formed from these powders by sintering at temperatures of up to 1600°C. The isomorphic capacity of the monoclinic LaPO4 matrix for the second component (yttrium or holmium) simulating radioactive nuclides of the actinide-rare-earth fraction was found to be high. The composites are stable in aqueous solutions, which is indicated by the low concentration of lanthanum and yttrium ions during leaching test (~10–7 g L–1). The results obtained in the study can be used to develop new high-efficiency ceramic matrices for solidification of the actinide-rare-earth fraction of liquid wastes formed in processing of the spent nuclear fuel.

Chemical and thermal stability of phosphate ceramic matrices

Nanosized powders of orthophosphates in the LaPO4–YPO4–H2O system have been synthesized by the sol–gel method using the reverse precipitation. The obtained powders served as a base to produce compact ceramic matrices. The matrices’ microhardness has been determined and the dependence of microhardness on the sintering temperature and duration has been established. The dilatometry method was used to study the thermal behavior of ceramic matrix samples and to estimate their thermal expansion coefficient. The stability of La1–xYxPO4 ceramic matrices to leaching in distilled water at room temperature has been determined.

Nanopowders of orthophosphate LaPO4-YPO4-H2O system and ceramics based on them

Nanopowders of orthophosphate LaPO4-YPO4-H2O system have been synthesized via the solgel method, and the limits of the mutual solubility of components have been specified through the calculation of the parameters of the unit cell. The dense ceramics were prepared based on the La1 − xYxPO4 · nH2O nan-opowders at 1000 and 1200°C. The porosity, microhardness, and bending strength of the ceramics were determined, and the dependence of the microhardness on the calcination time was established. The thermal behavior of the samples was studied by dilatometric method and the thermal coefficient of linear expansion of the ceramics was estimated.

Phase equilibria in the Ho2O3-SrAl2O4 system

The phase formation is investigated and the phase diagram of the Ho2O3-SrAl2O4 system is constructed. A ternary compound, namely, Ho2SrAl2O7, is revealed. It is established that this compound undergoes incongruent melting.