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E THE LIMITATIONS OF SIMPLE MODELS
Note that C 60 and graphite were omitted from Table 1.2 because their
properties are not easily justified in terms of the simple models. In these cases,
the simultaneous presence of both strong primary (covalent) bonds and weak
secondary (van der Waals) bonds is responsible for their properties. The
difference between the properties of C 60 and diamond illustrates the difference
between covalent molecular bonds and extended covalent bonds. The same phenomenon
distinguishes the properties of CO 2 and SiO 2 .
There are, of course, many other examples of solids that can take more
than one structure. Many metallic materials (Fe, for example) can take either
the fcc or bcc structure. TiO 2 is an example of an ionically bonded compound
that can take different crystal structures (rutile, anatase, brookite, and TiO 2 -B).
Silicon carbide exhibits a special type of polymorphism that is known as
polytypism. Polytypes are structures distinguished by different stacking
sequences along one direction. In the more than 70 known polytypes of SiC,
both Si and C are always in tetrahedral coordination, as we would expect for a
three dimensional covalently bonded compound of group IV elements. The
difference between the polytypes is in the long range order (the stacking
sequence of its close packed layers). This phenomenon will be discussed in
greater detail in Chapter 4.
ii. Systems with mixed properties and mixed bonding
In Section D, generalizations relating bonding type to properties were reviewed.
However, one must remember that these bonding models represent limiting cases
and, therefore, do not provide accurate descriptions of many materials. In most
cases, bonding is best described as a mixture of these limiting cases and, because
of this, the generalizations that relate bonding type to properties are often misleading.
Consider, for example, YBa 2 Cu 3 O 7 and Y 3 Fe 5 O 12 . Both of these materials
are yttrium-transition metal oxides and meet the established criterion for ionic
bonding. However, the most important properties of these materials, superconductivity
in the first and magnetism in the second, are often associated with
metals. YBa 2 Cu 3 O 7 is a high critical temperature (high T c ) superconductor and
the yttrium iron garnet (known as YIG) is ferrimagnetic and used for ‘soft’ magnetic
applications. According to accepted bonding and property generalizations,
ionically bonded compounds should not have metallic electrical properties, but
many do. This example illustrates a failure of the simple generalizations and
demonstrates the need for improved models.
In fact, based on the progress of superconductivity research over the past
eight decades (see Fig. 1.15), one must conclude that the ‘conventional wisdom’
regarding the electrical properties of metallic and ionic compounds impeded the
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