Ceramic and Glass Materials

10 Zirconia 175

Fig. 5 Crystal structure of monoclinic zirconia as seen from the

[13–1] direction

Table 3General crystallographic correlations in monoclinic zirconia

Space group P2 1 /c

Reflection conditions h 0 l:h + l = 2n

0 k0:k = 2n

h00:h = 2n

00 l:l = 2n

Coordination number 7

Z 4

Lattice parameters am≠bm≈ac

cm > ac

Unit cell volume Vm≈ 2 Vc

Ionic positions Zr4+: 4e

OI2−: 4e

OII2−: 4e

Zr–Zr bonds are 7, 4, and 1, respectively. The zirconium ions form layers parallel to

the (100) planes of the unit cell, with the O 1 ions on one side and the O 2 ions on the

other side. The distance between two layers of zirconium ions is larger when they are

separated by O 1 ions.

2.4 High-Pressure Phases

High-pressure experimental measurements on ZrO 2 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 [13], depending upon the crystallite size of the
material – lower crystallite size results in a higher transformation pressure [14],
although the phase transformation is not completed until 10 ± 1 GPa, as determined
by Desgreniers and Lagarec [15]. The calculated (ab initio) transition pressure,
according to Stapper et al. [2], is 5.7 GPa, so experimental measurements and ab initio