High-pressure experimental measurements on ZrO2 have revealed that the ambient monoclinic baddeleyite phase transforms under increasingly higher pressures to a series of orthorhombic phases. The first orthorhombic phase starts appearing at an applied pressure of about 3.5 GPa , depending upon the crystallite size of the material – lower crystallite size results in a higher transformation pressure , although the phase transformation is not completed until 10 ± 1 GPa, as determined by Desgreniers and Lagarec . The calculated (ab initio) transition pressure, according to Stapper et al. , is 5.7 GPa, so experimental measurements and ab initio
calculations are in general agreement. This first orthorhombic phase (Pbca , Z = 8, coordination number = 7) is observed to exist up to about 25 GPa when a second orthorhombic structure appears (Pnma, Z = 4, coordination number = 9), although the precise onset pressure has not been determined accurately. This phase is stable at ambient temperature up to at least 70 GPa. A projection of the crystal structure is illustrated in Fig. 6 . The change in volume with pressure is shown in Fig. 7 , where the initial volume, V, is taken as 70.32 A.
Aside from the two high-pressure phases of zirconia, a hexagonal high-temperature and high-pressure phase was found by Ohtaka et al.  by quenching pure ZrO2 powders from above 1,000°C and 20 GPa. This hexagonal structure (Z = 8) reverts to the baddeleyite structure when pressure is released below 1 GPa.
Fig. 6 A projection into (100) of the orthorhombic-I structure. The crosses indicate the atom positions in the tetragonal structure, and the arrows the presumed displacements of these atoms during the transition to orthorhombic  (reprinted with permission)
0.800- 0.750- 0.700
Fig. 7 Pressure dependence of the volume of the dense phases of zirconia  (reprinted with permission)