The Structure of Materials
1.2 STRUCTURE OF CERAMICS AND GLASSES
1.2.3 Silicate Structures*
The silicates, made up of base units of silicon and oxygen, are an important class of ceramic compounds that can take on many structures, including some of those we have already described. They are complex structures that can contain several additional atoms such as Mg, Na, K. What makes the silicates so important is that they can be either crystalline or amorphous (glassy) and provide an excellent opportunity to compare these two disparate types of structure. Let us first examine the crystalline state, which will lead us into the amorphous state.
The structural unit for the simplest silicate, SiO2, also known assilica, is the tetra- hedron (see Figure 1.43). This is the result of applying Pauling’s principles (Section
STRUCTURE OF CERAMICS AND GLASSES 61
Ti4+ Ca2+ O2−
Figure 1.42 The perovskite crystal structure of CaTiO3. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann,Introduction to Ceramics. Copyright1976 by John Wiley & Sons, Inc.
This material is used by permission of John Wiley & Sons, Inc.
Figure 1.43 The(SiO4)4−tetrahedron. The silicon atom is the solid circle at the center of the tetrahedron; large open circles are oxygens.
Table 1.17 Structural Units Observed in Crystalline Silicates
O/Si Ratio
Silicon – Oxygen
Groups Structural Units Examples
2 SiO2 Three-dimensional network Quartz
2.5 Si4O10 Sheets Talc
2.75 Si4O11 Chains Amphiboles
3.0 SiO3 Chains, rings Pyroxenes, beryl
3.5 Si2O7 Tetrahedra sharing one oxygen ion Pyrosilicates 4.0 SiO4 Isolated orthosilicate tetrahedra Orthosilicates
1.2.1) to a compound between silicon and oxygen. The data in Table 1.9 indicate that the anion/cation ratio in SiO2 is RO/RSi=(1.32)/(0.39)=3.3, which, according to Figure 1.37, dictates the tetrahedron as the base structural unit. Note that the SiO4tetra- hedron has a formal charge of −4, which must be neutralized with cations, such as other Si atoms, in real compounds. Pauling’s second rule tells us that the bond strength in silicon is 1, and the third and fourth rules tell us that corners of the tetrahedra are generally shared. This is not always the case, and different macroscopic silicate struc- tures result depending on how the tetrahedra are combined. Corners, edges, or faces of tetrahedra can be shared. As the nature of combination of the tetrahedra changes, so must the O/Si ratio, and charge neutrality is maintained through the addition of cations.
These structures are summarized in Table 1.17 and will be described separately.
1.2.3.1 Crystalline Silicate Network. When all four corners of the SiO4 tetra- hedra are shared, a highly ordered array of networked tetrahedra results, as shown in Figure 1.44. This is the structure ofquartz, one of the crystalline forms of SiO2. Notice that even though the O/Si ratio is exactly 2.0, the structure is still composed of iso- lated (SiO4)4−tetrahedra. Each oxygen on a corner is shared with one other tetrahedron, however, so there are in reality only two full oxygen atoms per tetrahedron. There are actually several structures, orpolymorphs, of crystalline silica, depending on the tem- perature. Quartz, with a density of 2.655 g/cm3, is stable up to about 870◦C, at which point it transforms into tridymite, with a density of 2.27 g/cm3. At 1470◦C, tridymite transforms tocristobalite(density=2.30 g/cm3), which melts at around 1710◦C. There are “high” and “low” forms of each of these structures, which result from slight, albeit rapid, rotation of the silicon tetrahedra relative to one another.
1.2.3.2 Silicate Sheets. If three of the four corners of the (SiO4)4−tetrahedron are shared, repeat units of (Si2O5)2−or (Si4O10)4−result, with a corresponding O/Si of 2.5.
Table 1.17 tells us, and Figure 1.45 shows us, that sheet structures are the result of shar- ing three corners. In these structures, additional cations or network modifiers, such as Al3+, K+, and Na+, preserve charge neutrality. Through simple substitution of selected silicon atoms with aluminum atoms, and some hydroxide ions (OH−) for oxygen atoms, complex and amazing sheet structures can result. One such common example ismus- covite, K2Al4(Si6Al2)O20(OH)4, more commonly known as mica (Figure 1.46). The large potassium ions between layers create planes that are easily cleaved, leading to the well-known thin sheets of mica that can be made thinner and thinner in a seemingly endless fashion. It is, in fact, possible to obtain atomically smooth surfaces of mica.
STRUCTURE OF CERAMICS AND GLASSES 63
Figure 1.44 The structure of quartz, showing the three-dimensional network of SiO4 tetrahe- dra. Reprinted, by permission, from L. G. Berry, B. Mason, and R. V. Dietrich, Mineralogy:
Concepts, Descriptions, Determinations, p. 388, 2nd ed. Copyright1983, Freeman Publish- ing, Inc.
Silicon Oxygen
Figure 1.45 Top view of a silicate sheet structure resulting from sharing three corners of the SiO4 tetrahedra. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright1976 by John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.
b axis c
axis
OH OH
OH OH
O O
O O O
O b = 95½°
~9.94 Å
2 K 6 O
6 O 4 Al 3 Si + Al
3 Si + Al 2 (OH) + 4 O
2 (OH) + 4 O O
O
Figure 1.46 The structure of muscovite (mica), a sheet silicate. Reprinted, by permission, from L. G. Berry, B. Mason, and R. V. Dietrich,Mineralogy: concepts, descriptions, determinations, p. 431, 2nd ed. Copyright1983 by Freeman Publishing, Inc.
1.2.3.3 Silicate Chains and Rings. Sharing two out of the four corners of the SiO4 tetrahedra results in chains. The angle formed between adjacent tetrahedra can vary widely, resulting in unique structures such as rings (see Figure 1.47). In all cases, when only two corners are shared, the repeat unit is (SiO3)2−, and the O/Si is 3.0.
Slight variations in the O/Si ratio can also take place, and result in partially networked structures such as double chains, in which two silicate chains are connected periodically by a bridging oxygen.Asbestos is such a double chain, with O/Si=2.75.
1.2.3.4 Pyrosilicates. One corner of the SiO4 tetrahedron shared results in a (Si2O7)6− repeat unit and a class of compounds called the pyrosilicates. Again, counterions are necessary to maintain charge neutrality. The pyrosilicates are non- networked and have an O/Si of 3.5.
1.2.3.5 Orthosilicates. Finally, no tetrahedral corners shared gives an O/Si of 4.0, and it results in isolated (SiO4)4− tetrahedra. These class of materials are referred to as theorthosilicates.