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Size effects in rare earth sesquioxides
EP34839
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: 164
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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.
References: 1. Shefer MW, Roy R. Rare earth polymorphism and phase equilibria in rare earth oxide water systems. J. of Am. Cer. Soc., 1959; 42: 563-560
2. Roth RS, Schneider SJ. J. Phase equilibria in systems involving the rare earth oxides. part 1. polymorphism of the oxides of the trivalent rare earth ions. J. Res. National Bureau of Standards A. Physics and Chemistry. 1960; 64A(4) :309-316.
3.Warshaw I, Roy RJ. Polymorphism of the rare earth sesquioxides. Phys. Let. 1961;65:2048-2051.
4. Brauer G. Precipitation of rare‐earth oxides from melt salts. Rare
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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