Energetics of cubic and monoclinic yttrium oxide polymorphs: Phase transitions, surface enthalpies, and stability at the nanoscale

The energetics of Y₂O₃ 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/m²) than that of the cubic (1.25 ± 0.17 J/m²). However, the anhydrous surface of the monoclinic form is less stable (surface enthalpy = 2.78 ± 0.49 J/m²) than that of the cubic (1.66 ± 0.14 J/m²). The phase transformation enthalpy (monoclinic to cubic) for coarse Y₂O3́ 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 Y₂O₃ 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.