Energetics of X-ray-amorphous zirconia and the role of surface energy in its formation

X-ray-amorphous zirconia was synthesized by a low temperature process. The difference in the energetics of monoclinic and amorphous zirconia was calculated based on drop solution calorimetry of amorphous and monoclinic zirconia in lead borate (2PbO·B2O3) at 1073 K. X-ray-amorphous zirconia is about 58 kJ/mol less stable in enthalpy than the monoclinic phase. Amorphous zirconia heated to 573 K shows crystalline nanoparticles (cubic or tetragonal) of 3.8 nm average diameter. Formation of amorphous zirconia at low temperatures can be explained by its surface energy stabilization relative to monoclinic zirconia, with a difference Δσ∼1.19±0.08 J/m2.