Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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3.6 Ceramic Crystal Structures • 51

Ti4+ Ba2+ O2–

Figure 3.9 A unit cell for the perovskite crystal

structure.

as F−ions, and therefore the crystal structure would be similar to CsCl (Figure 3.6),

except that only half the center cube positions are occupied by Ca^2 +ions. One unit

cell consists of eight cubes, as indicated in Figure 3.8. Other compounds that have

this crystal structure include ZrO 2 (cubic), UO 2 , PuO 2 , and ThO 2.

AmBnXp-Type Crystal Structures

It is also possible for ceramic compounds to have more than one type of cation;

for two types of cations (represented by A and B), their chemical formula may be

designated as AmBnXp. Barium titanate (BaTiO 3 ), having both Ba^2 +and Ti^4 +cations,

falls into this classification. This material has aperovskite crystal structureand rather

VMSE

Unit Cells–BaTiO 3 interesting electromechanical properties to be discussed later. At temperatures above

120 ◦C (248◦F), the crystal structure is cubic. A unit cell of this structure is shown in

Figure 3.9; Ba^2 +ions are situated at all eight corners of the cube and a single Ti^4 +is

at the cube center, with O^2 −ions located at the center of each of the six faces.

Table 3.5 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 struc-

tures are possible.

Table 3.5 Summary of Some Common Ceramic Crystal Structures

Coordination

Structure Numbers

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©c1976 by

John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.