Phase Formation in Heterovalent Equimolar Quinary Oxide Systems of ZrO2-HfO2-CeO2-Nb2O5-RE2O3 Type (RE = Y, Yb, Nd, Gd)

Tailoring electrical and mechanical properties in the fluorite oxides family is of great interest for technological applications. Other than doping and substitution, entropy-driven stabilization is an emerging technique for new solid solutions formation and enhancing or exploring new functionalities. However, there is a high number of possible combinations for higher-order diagram investigations, and the current state of the art shows limited possibilities in predicting phase formation and related properties. In this paper, we expand the compositional space of fluorite oxides in ZrO2-HfO2-CeO2-Nb2O5-RE2O3 systems. X-ray diffractometry and scanning electron microscopy measurements showed the formation of cubic fluorite-type structures when processing compositions at 1600 °C.

Phase equilibria in the CeO2–La2O3–Ln2O3 system at 1250 and 1500 °С

Materials based on cerium oxide, stabilized by oxides of rare earth elements, are promising for use in medicine, energy and mechanical engineering due to the uniqueness of their properties. State diagrams of CeO2–La2O3–Ln2O3 systems are the physicochemical basis for the creation of solid electrolytes for fuel cells, oxygen gas sensors, catalyst carriers, protective coatings on alloys, etc. Phase equilibria and structural transformations in CeO2–La2O3–Gd2O3 systems at temperatures 1250 and 1500 °С and in the binary system La2O3–Gd2O3 at temperatures 1100, 1500 and 1600 ° С in the whole range of concentrations were investigated using X-ray phase and microstructural analyzes. It was found that solid solutions based on cubic (F) modification with CeO2 fluorite type, monoclinic (B) and cubic (C) modifications of Gd2O3 and hexagonal (A) modification of La2O3 are formed in the ternary system CeO2–La2O3–Gd2O3. The boundaries of the phase fields and the periods of the crystal lattices of the formed phases are determined. It is established that in the CeO2–La2O3 –Gd2O3 system at 1250 and 1500 °С the phases of cubic symmetry are in equilibrium: on the basis of F–CeО2 with the spatial group Fm3m and C-phase on the basis of Gd2O3 with the spatial group Ia3. As the temperature decreases, there is a narrowing of all areas of homogeneity.

Phase Equilibrium in ZrO2-CeO2-Eu2O3 System at a Temperature of 1500 °C

The phase equilibria and structural transformations in the ternary ZrO2-CeO2-Eu2O3 system at 1500 °C were studied by X-ray diffraction and scanning electron microscopy in the overall concentration range. The system was found to constitute fields of solid solutions based on the tetragonal (Т) modification of ZrО2, cubic (С) and monoclinic (B) modifications of Eu2O3, cubic with a fluorite-type structure (F) modifications of СеО2 (ZrО2), and ordered intermediate phase with a pyrochlore-type structure of Eu2Zr2O7 (Py). The refined lattice parameters of the unit cells corresponding to the solid solutions and microstructures of the definite field of compositions for the systems were determined. The peculiarity of the isothermal section of the phase diagram in the ZrO2-СеO2-Eu2O3 system at 1500 °С is the formation of phase equilibria on the basis of the fluorite solid solutions of ZrO2(CeO2) along with other components. There are at least three homogeneous fields of cubic phases. The isothermal section of the ZrO2-CeO2-Eu2O3 system at 1500 °С was constituted of four three-phase regions (C-Eu2O3 + F-CeO2+Py, C-Eu2O3 + F-ZrO2+Py, Py + F-ZrO2 + T-ZrO2, Py + F-CeO2 + T-ZrO2).