Yttria-stabilized hafnia: Thermochemistry of formation and hydration of nanoparticles

The surface enthalpy of yttria-stabilized hafnia (YSH) (Y _xHf _1-xO _2-x/2) with different compositions was directly measured by a combination of high-temperature oxide-melt solution calorimetry and water adsorption calorimetry. The surface enthalpies for hydrated surfaces are 0.27 ± 0.06 J/m ² for x = 0.1, 0.77 ± 0.09 J/m ² for x = 0.17, and 1.30 ± 0.09 J/m ² for x = 0.24; and those for anhydrous surfaces are 0.51 ± 0.06, 1.08 ± 0.13, and 1.76 ± 0.09 J/m ² respectively. The enthalpies of both hydrated and anhydrous surfaces increase approximately linearly (R ² > 0.93) with increasing yttrium concentration. The surface enthalpies of Y0.1Hf0.9O1.95 were used to approximate those for pure anhydrous cubic hafnia. Combining the data relating to surface energies for monoclinic hafnia from our previous work and estimated data for tetragonal hafnia, a tentative stability map of HfO ₂ polymorphs as a function of surface area (SA) was constructed.