Highly refractory oxide systems containing oxides of rare-earth elements

Abstract

The paper describes the interactions of rare earth element (REE) oxides with magnesium, calcium, strontium, and barium oxides in a wide range of concentrations and temperatures from 1000 to 2450°C. The most typical binary phase diagrams found for these oxide systems are illustrated in the paper. The existence of six types of compounds, i.e. MeLn 4O 7, MeLn 2O 4, Me 5Ln 8O 17, Me 3Ln 4O 9, Me 2Ln 2O 5, has been established. It has been shown that the compounds formed resemble other classes of compounds with REE oxide participation; this is demonstrated by spasmodic property changes in a given series of compounds of a single type (process of formation, X-ray and petrographic characteristics) and by changes of the composition of the compounds. These facts indicate an appreciable role of the 4f electrons of the REE atoms resulting in changes in the nearest environment of the REE atoms; in turn this leads to property and compositional changes of the compounds formed and, hence, to variations of the type of phase diagrams in the La-Lu series. Phase transformations in REE oxides (1800°C) have been investigated and the regularities observed in the effects of oxides of elements of the IIA subgroup on the transformations are discussed. The nature and kinetics of decomposition of magnesium oxide-based solid solutions have been studied in the Sc 2O 3MgO system. It is suggested that the regularities observed here may be valid for other oxide systems having a eutectic type of liquids where the decomposition of the solid solution occurs into the starting components or into solid solutions on their base, without formation of intermediate compounds. Theoretical calculations of the phase-field boundaries of some types of phase diagrams of the systems studied have been carried out, assuming ideal or regular solution models.