Energetics of Cubic and Monoclinic Yttrium Oxide Polymorphs:  Phase Transitions, Surface Enthalpies, and Stability at the Nanoscale

Abstract

The energetics of Y 2 O 3 cubic (C-type) and monoclinic (B-type) polymorphs as a function of surface area have been studied by high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The hydrated surface of the monoclinic phase has a lower surface enthalpy (0.64 ± 0.49 J/m 2 ) than that of the cubic (1.25 ± 0.17 J/m 2 ). However, the anhydrous surface of the monoclinic form is less stable (surface enthalpy = 2.78 ± 0.49 J/m 2 ) than that of the cubic (1.66 ± 0.14 J/m 2 ). The phase transformation enthalpy (monoclinic to cubic) for coarse Y 2 O 3 is −21.8 ± 3.2 kJ/mol. An energetic crossover is suggested between the polymorphs under ambient conditions, in which the surfaces are hydrated, at a size of 7 ± 6 nm; however, anhydrous monoclinic yttria of any particle size is thermodynamically metastable at atmosphere pressure. Surface hydration controls the energetic landscape of Y 2 O 3 polymorphs. The phase transformation process of monoclinic nanocrystals to the cubic phase was also studied using measurements of surface area, X-ray diffraction, and differential scanning calorimetry.