Ceramic and Glass Materials

176 O.A. Graeve

calculations are in general agreement. This first orthorhombic phase (Pbca [16], 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 [17]. The change in volume with pressure is shown in Fig. 7 [15],

where the initial volume, Vo, is taken as 70.32 Å.

Aside from the two high-pressure phases of zirconia, a hexagonal high-temperature

and high-pressure phase was found by Ohtaka et al. [18] by quenching pure ZrO 2

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 [17]
(reprinted with permission)

Fig. 7Pressure dependence of the volume of the dense phases of zirconia [15] (reprinted with
permission)