Pressure-induced structural phase transitions in zirconia under high pressure

Angular-dispersive x-ray in situ powder-diffraction experiments have been performed on pure zirconia, ${\mathrm{ZrO}}_2$, at room temperature under high pressure up to 50 GPa. Under increasing pressure four phases were successively encountered: baddeleyite (monoclinic, P$2_1$/c) from normal pressure up to about 10 GPa, orthorhombic-I (Pbca) to 25 GPa, orthorhombic-II to 42 GPa, and orthorhombic-III above 42 GPa. The unit-cell parameters and the volume have been determined as a function of pressure. The bulk moduli of the two lower pressure phases have been calculated using Birch’s equation of state. The bulk modulus of baddeleyite, 95 GPa, is much lower than expected from bulk modulus-volume systematics, 195 GPa, while for the orthorhombic-I phase, the experimental and calculated values are almost identical. A generalized P-T diagram for ${\mathrm{ZrO}}_2$, including an orthorhombic-IV phase, is proposed and discussed. The phase transitions to orthorhombic-II and orthorhombic-III phases can be described by a simple rotation of the unit cell of the orthorhombic-I phase about either the b axis to form the orthorhombic-II phase or a axis to form the orthorhombic-III phase. All high-pressure cells (orthorhombic-I, -II, and -III) have eight formula units (Z=8). The orthorhombic-II phase was found not to have the cotunnite ${\mathrm{PbCl}}_2$-type structure which was proposed previously. There is no longer any example of a compound which transforms to such a cotunnite-type structure under high pressure. The behavior of zirconia and hafnia under high pressure is different although they have very close chemical properties at ambient pressure and identical structures in the two lower-pressure phases.