Pressure‐Temperature Phase Diagram of Zirconia

The pressure‐temperature phase diagram of zirconia was determined by optical microscopy and X‐ray diffraction techniques using a diamond anvil pressure cell. At room temperature, monoclinic ZrO 2 transforms to a tetragonal phase ( t II) which is related to the high‐temperature tetragonal structure ( t I). The transformation pressure exhibits hysteresis and is cycle dependent. At room temperature, the initial transformation pressure for the monoclinic‐ t II transition on a virgin monoclinic crystal can be as high as 4.4 GPa; on subsequent cycling the transition pressure ultimately lowers to 3.29 ± 0.06 GPa. The pressure for the reverse transition is essentially constant at 2.75 ± 0.06 GPa. At pressures > 16.6 GPa, the t II form transforms to the orthorhombic cotunnite (PbCl 2 ) structure. With increasing temperature, the t II form transforms to the high‐temperature tetragonal phase. For increasing P and T , the monoclinic‐ t I‐ t II triple point is located at T = 596°± 18°C and P = 2.26 ± 0.28 GPa, whereas for decreasing P and T , the triple point is found at T = 535°± 25°C and P = 1.7 ± 0.28 GPa.