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Size effects in rare earth sesquioxides
Poster Title: Size effects in rare earth sesquioxides
Submitted on 17 Feb 2021
Author(s): Giora Kimmel(1), Witold Łojkowski(2) and Roni Z. Shneck(1)
Affiliations: (1) Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel. (2)Institute of High-Pressure Physics, Polish Academy of Sciences, Poland.
This poster was presented at Israel Physical Society Annual Meeting 2021
Poster Views: 103
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Poster Information
Abstract: Below 2000 °C rare earth sesquioxides (RESOX) have three crystal structures: hexagonal, cubic and monoclinic, designated as A, C and B respectively [1-3]. Early studies, based on low temperature (LT) synthesis, suggested that RESOX phase stability versus temperature is a function of the metallic ion radii (MIR). La2O3 Ce2O3 and Nd2O3 with the highest MIR are A-type, while for Sm2O3, Eu2O3 and Gd2O3 with intermediate MIR the structure is C-type at LT and B-type at high temperature (HT) [1-3]. All other RESOX including Y2O3 and Sc2O3 were assumed to be cubic (C-type) at all temperatures below 2000 °C. The transformation from LT cubic to high temperature (HT) monoclinic structure in Sm2O3, Eu2O3 and Gd2O3 is unusual and therefore, Brauer [4] and Yokogawa et al. [5] suggested that the stable phase is monoclinic at all temperatures below 2000 °C. To resolve the controversy, we have demonstrated that slowing down grain growth of Sm2O3 and Gd2O3 [9] prevented transition from C to B-types in the expected temperatures (1100 and 1300 °C respectively). Hence, we suggest that the grain size plays an important role in determining the structure of nano-REOXs [6,7]. The monoclinic Sm2O3, Eu2O3 and Gd2O3 is the stable structure at all temperatures below 2000 °C when the grains are large. However, for small nano-crystals the stable structure is cubic since it has a lower surface energy than the monoclinic phase. In addition, Kimmel et al. [9] suggested that for all RESOX with MIR lower than Gd3+ (except Sc2O3) obtained by HT synthesis [10-17] or under high pressure [18-20], the monoclinic phase is the stable phase at LT. Thus, for all RESOX with MIR lower than Gd3+ except Sc2O3, the assumption of a continuous cubic structure at all temperatures is wrong. Summary: When the two parameters, grain size and temperature were separated as independent variables, a true phase stability diagram of the rare earth sesquioxides was obtained.
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5. Yokogawa Y, Yoshimora M, Somyia S. Lattice energy and polymorphism of rare earth oxsides. J. Mater Sci Lett. 1991;16:509–12.
6. Kimmel G, Zabicky J. Stability, instability, metastability and grain size in nanocrystalline ceramic oxide systems. Solid State Phenomena 2008;140:29-36.
7. Navrotsky A. Energetics of nanoparticle oxides: interplay between surface energy and polymorphism. Geochem Trans. 2003;4:34-37.
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