Monoclinic Zirconia

EXAFS analysis by Winterer [10] and Li et al. [11] shows that monoclinic ZrO2 can be described by a sevenfold Zr-O shell with an average distance of 0.216 nm and a mean vibrational frequency of 410 cm-1. The structure consists of layers of triangular coordination polyhedra of three O1-Zr bonds and four distorted tetrahedral O2-Zr bonds for a total of seven oxygen ions surrounding the zirconium [10], as shown in Fig. 5. In this illustration, there are three unit cells that help with visualization. The Zr4+ sublattice is marked with dashed lines and the seven coordinating oxygen ions surrounding the darker-colored zirconium ion are marked with numbers. The plane that constitutes the O1 atoms is nearly parallel to the plane of O2 atoms. The general crystallographic correlations for this structure are listed in Table 3. The Zr-O distances range from 0.1885 to 0.2360 nm for the O1-Zr shell and 0.1914 to 0.2511 nm for the O2-Zr shell.

There are also three distinct Zr-Zr subshells at 0.346,1 0.396 (0.401),2 and 0.454 (0.455) nm with a total of 12 zirconium next nearest neighbors at an average distance of 0.372 nm. The coordination numbers for the first, second, and third subshells of the

Winterer and Li et al. report the same value.

First value by Winterer; value in parentheses by Li et al.

Fig. 5 Crystal structure of monoclinic zirconia as seen from the [131] direction

Table 3 General crystallographic correlations in monoclinic zirconia

Space group

P2/c

Reflection conditions

h0l: h + l = 2n 0k0: k = 2n h00: h = 2n 00l: l = 2n

Coordination number

7

Z

4

Lattice parameters

a Ф b ~ a

mmc

c > a

mc

Unit cell volume

V = 2V

mc

Ionic positions

Zr4+: 4e 0I2-:4e On2-:4e

Monoclinic ZirconiaZr-Zr bonds are 7, 4, and 1, respectively. The zirconium ions form layers parallel to the (100) planes of the unit cell, with the Oj ions on one side and the O2 ions on the other side. The distance between two layers of zirconium ions is larger when they are separated by Oj ions.