Size effects in rare earth sesquioxides


  Giora Kimmel  ,   Witold Lojkowski  ,   Roni Z. Shneck  ,  Institute of High-Pressure Physics, Polish Academy of Sciences, Poland   ,     
Department of Materials Engineering
Ben Gurion University of the Negev

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.

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