Ceramic and Glass Materials

10 Zirconia 173

in a neutral atmosphere, while the latter two measurements were done in a reducing

atmosphere [6]. At increasing pressures, the lattice parameter changes to 0.4947,

0.4925, and 0.4916 nm for pressures of 28.9, 33.8, and 37.3 GPa, respectively [7].

2.2 Tetragonal Zirconia

The tetragonal zirconia structure (Fig. 3 [6], Table 2), with space group P4 2 /nmc
(primitive) [8, 9] and a cation coordination number of 8, is derived from the cubic
fluorite structure by the movement of oxygen anions along one of the cubic axes,
which results in a tetragonal distortion along that axis, as shown in Fig. 3 for a distor-
tion along the c-axis. The two zirconium ions in the primitive structure are located at
(0, 0, 0) and (½, ½, ½), and the four oxygen ions are located at (0, ½, z), (½, 0, −z),
(0, ½, ½ + z), and (½, 0, ½ −z), where z = 0.185. This results in a body-centered
tetragonal (bct) structure, which is sometimes described as a pseudofluorite
structure.
The transition from cubic to tetragonal is displacive in which four Zr4+ cations in
the 4a cubic positions split into two groups to occupy the 2b positions in the tetragonal
structure, and the O2− anions in the 8c cubic positions also split into two groups to
occupy the 4d positions in the tetragonal structure. The directions of a and b axes in
the tetragonal primitive lattice are 45° from those in the cubic cell. The c axis in both
structures remains the same. Following the literature convention of reporting the
parameterdz = 0.25 − z, where z represents the third coordinate of the oxygen position
4d (0, ½, z), the calculated c/a ratio and internal parameter dz of the tetragonal phase
as a function of volume are shown in Fig. 4 [2].

Fig. 3 Tetragonal zirconia unit cell in both the body-centered tetragonal and pseudofluorite descrip-
tions (adapted from Aldebert and Traverse [6])