Question 15.3: Cubic Zirconia Cubic zirconia (ZrO2) is a less expensive alt......
Cubic Zirconia
Cubic zirconia (ZrO_{2}) is a less expensive alternative to diamond for jewelry. It is also used to make oxygen gas sensors used in, for example, automotive engines, and to measure dissolved oxygen in molten steels because of its high ionic conductivity. Zirconia is an additive in electronic ceramics, and it is a refractory material. Zirconia exists as several different polymorphs depending on temperature and pressure. Cubic zirconia is unstable at room temperature unless it has been doped with an addition such as Y_{2}O_{3}, known as yttria. (Note that the letters ia at the end of the terms “yttria” and “zirconia” mean “oxygen-containing,” i.e., “yttrium containing oxygen” and “zirconium containing oxygen.”) The unit cell of cubic zirconia can be described as a face-centered cubic lattice of zirconium with all tetrahedral sites occupied by oxygen.
What is the crystal structure of cubic zirconia? Explain how the stoichiometry of ZrO_{2} is consistent with this crystal structure.
This is the fluorite crystal structure. (See Table 15-4.) In a single unit cell of cubic zirconia, there are four Zr ions and eight O ions. The ratio of Zr^{4+} ions to O^{2–} ions is 4:8 or 1:2, which is the same as the stoichiometry of ZrO_{2}. Notice that this is a crystal structure for which we cannot think of the anions as being arranged in a close-packed fashion. In fact, the anions are the larger ions, but in this description of the structure, they occupy the smaller tetrahedral sites. The antifluorite structure (Na_{2}O) (see
Table 15-4) can be envisioned as a close-packing of anions in an FCC fashion with the cations occupying all of the tetrahedral sites.
| Table 15-4 Table of some ceramic crystal structures, lattice and basis descriptions, and exemplary ceramics adopting these forms. This list is by no means exhaustive. | ||||
| Structure | Name/Lattice and Basis | Some Examples | ||
| 1 |  |
Sodium chloride (NaCl) FCC lattice with a basis of Cl (0, 0, 0) and Na (1/2, 0, 0) |
BaO, CaO, MgO, SrO, TiC, TiO | |
| 2 |  |
Diamond cubic FCC lattice with a basis of C (0, 0, 0) and C (1/4, 1/4, 1/4) |
Diamond | |
| 3 |  |
Cubic zincblende (ZnS) also known as sphalerite FCC lattice with a basis of S (0, 0, 0) and Zn (1/4, 1/4, 1/4) |
β-SiC | |
| 4 |  |
Fluorite (CaF_{2}) FCC lattice with a basis of Ca (0, 0, 0), F (1/4, 1/4, 1/4), and F (1/4, 1/4, 3/4) |
BaF_{2} , CaF_{2} , CeO_{2}, ThO_{2}, UO_{2}, ZrO_{2} | |
| 5 |  |
Hexagonal zincblende (ZnS) also known as wurtzite Hexagonal lattice with a basis of S (0, 0, 0), S (2/3, 1/3, 1/2), Zn (0, 0, 3/8), Zn (2/3, 1/3, 7/8) |
α-SiC, BeO, ZnO, | |
| 6 |  |
Perovskite (CaTiO_{3}) Simple cubic lattice with a basis of Ca (0, 0, 0), Ti (1/2, 1/2, 1/2), and O (0, 1/2, 1/2), O (1/2, 0, 1/2), and O (1/2, 1/2, 0) | BaTiO_{3}, CaTiO_{3}, PbZrO_{3}, SrTiO_{3} | |
| 7 |  |
Antifluorite (Na2O) FCC lattice with a basis of O (0, 0, 0), Na (1/4, 1/4, 1/4), and Na (1/4, 1/4, 3/4) |
K_{2}O, Li_{2}O, Na_{2}O | |