Based on the analysis of literature data from experimentally constructed phase diagrams of Y2O3 − Ln2O3 systems (Ln = Tb − Lu), as well as temperatures of polymorphic transformations of rare earth oxides (REE), tentative phase diagrams of Y2O3 − Ln2O3 systems (Ln = Tb − Lu) were constructed in wide intervals of temperatures and concentrations. Prediction of the binary phase diagrams structure of yttria − yttrium subgroup lanthanides systems was carried out on the basis of three principles: 1. Since double systems are formed by lanthanide oxides of one (yttrium) subgroup, it is very likely that in such systems continuous solid solutions will be formed between the components. 2. Intermediate binary phases are not formed in these systems. 3. The formation of continuous solid solutions occurs with a decrease in the temperatures of phase transformations in the solid state to a minimum shifted towards a lower transformation temperature of the system component. The forecast of the Y2O3 – Ln2O3 systems phase diagrams structure, where Ln = Tb – Lu, indicates the complete solubility of the components in the liquid and solid states. Binary compounds in the considered systems are not predicted. Phase transformations in the solid solutions on the basis of polymorphic modifications X, H, A, B and C of lanthanide oxides cascade at high temperatures by the peritectoid mechanism. Below 1850 °C regions of solid solutions with cubic C-structure of REE oxides are formed in the whole range of concentrations in the systems. Key words: REE oxides, yttria, polymorphs of REE oxides, phase diagram.
PHASE TRANSFORMATIONS OF PURE Mg2SiO4 INTO A SPINEL STRUCTURE UNDER HIGH PRESSURES AND TEMPERATURES
