Stability of Monoclinic and Orthorhombic Zirconia: Studies by High-Pressure Phase Equilibria and Calorimetry

The stability fields of two high‐pressure polymorphs of ZrO2 (ortho I and ortho II) were determined by both calorimetry and phase equilibrium experiments. Enthalpies of transition were measured by transposed temperature drop calorimetry. The entropies of transition and slopes of phase boundaries were calculated using the measured enthalpies and free energies calculated from the results of phase equilibrium experiments. From the thermodynamic measurements, it is seen that the entropy increases and the volume decreases during the monoclinic–ortho I transition, whereas both the entropy and the volume decrease during the ortho I–ortho II transition. Accordingly, the gradient of phase boundaries, dP/dT, is negative in the former and positive in the latter. These trends are consistent with those of phase equilibrium experiments.