Abstract
Rare-earth (RE) oxides are important in myriad fields, including metallurgy, catalysis, and ceramics. However, the phase diagram of RE oxides in the nanoscale might differ from the phase diagrams for bulk, thus attracting attention nowadays. We suggest that grain size in the nanoscale also determines the obtained crystallographic phase along with temperature and pressure. For this purpose, nanoparticles of Sm2O3 and Eu2O3 were mixed in an inert MgO matrix via the sol-gel method. This preparation method allowed better isolation of the oxide particles, thus hindering the grain growth process associated with increasing the temperature. The mixed oxides were compared to pure oxides, which were heat-treated using two methods: gradual heating versus direct heating to the phase transition temperature. The cubic phase in pure oxides was preserved to a higher extent in the gradual heating treatment compared to the direct heating treatment. Additionally, in MgO, even a higher extent of the cubic phase was preserved at higher temperatures compared to the pure oxide, which transformed into the monoclinic phase at the same temperature in accordance with the phase diagram for bulk. This indicates that the cubic phase is the equilibrium phase for nanosized particles and is determined also by size.
Highlights
Rare-earth (RE) oxides have gained popularity over the years owing to their versatile applications, such as the nuclear field (Sm2 O3 is used as an absorber in control rods), solid oxide fuel cells (Sm2 O3 -doped ceria electrolyte was found to have a high ionic conductivity), phosphor materials (Eu2 O3 is used as red or blue phosphor), catalysis, laser, optical, and more [1,2,3].The crystallographic structure influences the physical and chemical properties of RE oxides and determines their functionality
This study investigates the crystallographic phases in some rare-earth oxides from the middle range of ionic radius in the group (Sm2 O3 and Eu2 O3 ), both as pure oxides and as embedded nanoparticles in an inert magnesium oxide (MgO) matrix
In order to better heighten the grain size effect, the crystallographic phases were investigated in systems where the desired oxide was embedded in a MgO matrix in which the grain growth process tends to be deliberately hindered [20,21,22]
Summary
Rare-earth (RE) oxides have gained popularity over the years owing to their versatile applications, such as the nuclear field (Sm2 O3 is used as an absorber in control rods), solid oxide fuel cells (Sm2 O3 -doped ceria electrolyte was found to have a high ionic conductivity), phosphor materials (Eu2 O3 is used as red or blue phosphor), catalysis, laser, optical, and more [1,2,3]. Regarding the polymorphism of rare-earth oxides, five different crystallographic phases are known: at temperatures lower than about. This study investigates the crystallographic phases in some rare-earth oxides from the middle range of ionic radius in the group (Sm2 O3 and Eu2 O3 ), both as pure oxides and as embedded nanoparticles in an inert MgO matrix. In order to better heighten the grain size effect (as grains tend to grow with increasing temperature), the crystallographic phases were investigated in systems where the desired oxide was embedded in a MgO matrix in which the grain growth process tends to be deliberately hindered [20,21,22]. The crystal structure and the microstructure were characterized by X-ray diffraction (XRD) and high-resolution scanning electron microscopy (HR-SEM)
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