Callister - An introduction - 8th edition

460 • Chapter 12 / Structures and Properties of Ceramics
Table 12.4
Summary of Some Common Ceramic Crystal Structures
Coordination
Numbers
Structure
Structure Name Type Anion Packing Cation Anion Examples
Rock salt (sodium chloride) AX FCC 6 6 NaCl, MgO, FeO
Cesium chloride AX Simple cubic 8 8 CsCl
Zinc blende (sphalerite) AX FCC 4 4 ZnS, SiC
Fluorite AX 2 Simple cubic 8 4 CaF 2 ,UO 2 ,ThO 2
Perovskite ABX 3 FCC 12(A) 6 BaTiO 3 , SrZrO 3 , SrSnO 3
6(B)
Spinel AB 2 X 4 FCC 4(A) 4 MgAl 2 O 4 , FeAl 2 O 4
6(B)
Source: W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd edition. Copyright © 1976
by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.
Table 12.4 summarizes the rock salt, cesium chloride, zinc blende, fluorite, and
perovskite crystal structures in terms of cation–anion ratios and coordination numbers,
and gives examples for each. Of course, many other ceramic crystal structures
are possible.
VMSE
tetrahedral position
octahedral position
Crystal Structures from the Close Packing of Anions
It may be recalled (Section 3.12) that for metals, close-packed planes of atoms
stacked on one another generate both FCC and HCP crystal structures. Similarly,
a number of ceramic crystal structures may be considered in terms of
close-packed planes of ions, as well as unit cells. Ordinarily, the close-packed
planes are composed of the large anions. As these planes are stacked atop each
other, small interstitial sites are created between them in which the cations may
reside.
These interstitial positions exist in two different types, as illustrated in Figure
12.7. Four atoms (three in one plane, and a single one in the adjacent plane) surround
one type; this is termed a tetrahedral position, because straight lines drawn
from the centers of the surrounding spheres form a four-sided tetrahedron. The
other site type in Figure 12.7 involves six ion spheres, three in each of the two
planes. Because an octahedron is produced by joining these six sphere centers,
this site is called an octahedral position. Thus, the coordination numbers for
cations filling tetrahedral and octahedral positions are 4 and 6, respectively. Furthermore,
for each of these anion spheres, one octahedral and two tetrahedral positions
will exist.
Ceramic crystal structures of this type depend on two factors: (1) the stacking
of the close-packed anion layers (both FCC and HCP arrangements are possible,
which correspond to ABCABC . . . and ABABAB . . . sequences, respectively), and
(2) the manner in which the interstitial sites are filled with cations. For example,
consider the rock salt crystal structure discussed earlier. The unit cell has cubic
symmetry, and each cation (Na ion) has six Cl ion nearest neighbors, as may be
verified from Figure 12.2. That is, the Na ion at the center has as nearest neighbors
the six Cl ions that reside at the centers of each of the cube faces. The
crystal structure, having cubic symmetry, may be considered in terms of an FCC