Lattice variation of cubic Y₂O₃ in three dimensions: Temperature, pressure and crystal size

Yttrium oxide (Y₂O₃) is a ceramic material in use in various fields such as catalysis, thermal barrier coatings, dental ceramics, solid-state lasers, solid oxide fuel cells and etc. In nanoscale metal oxides, unlike metals, the dependence of the lattice parameter on the crystal size is in dispute. Y₂O₃ and 5 vol% Y₂O₃ in MgO powders were synthesized via the sol-gel technique. The latter was made in order to produce Y₂O₃ inclusions in MgO, allowing for the study of differences resulting from crystal size with inhibited thermal growth. However, the lattice parameters of the C-type Y₂O₃ were significantly lower than expected due to misfit high compression stress. High-temperature X-ray diffraction (HT-XRD) was used in order to investigate in situ crystal structure and microstructure variations of Y₂O₃ imbedded in MgO as formed at high temperatures free of misfit strain, comparing with in situ study of pure Y₂O₃ formation at the same temperature range. It was found that nanocrystalline C-type Y₂O₃ was formed at a temperature range of 873–1163 K. Lattice parameters and crystal size were evaluated by using Rietveld refinement and high resolution scanning electron microscopy (HR-SEM). The relative lattice parameter changes as a function of crystal size were obtained by analyzing the in situ HT-XRD data from 873 to 1573 K. Those results along with those obtained at room temperature showed a non-monotonic trend: lattice contraction versus decreasing crystal size at the range of 60–45 nm crystal size and lattice expansion versus decreasing crystal size for smaller crystals (approximately 0.11%). The non-monotonic lattice variation was modeled by assuming two effects acting simultaneously as a function of crystal size: lattice contraction due to hydrostatic pressure caused by the surface tension and Madelung model predicting lattice expansion due to weakening of bonding force in small ionic crystals. The calculated theoretical lattice parameters values which were obtained by using this model were compared to the experimental results yielding a better understanding of the lattice variations in nano-Y₂O₃.