p17p35ujimn9ejc9cap81i1j7e4.pdf

F PROBLEMS
and appreciated. For example, the phases that created the breakthrough in the
latter half of the 1980s, La 2x Ba x CuO 4y (Bednorz & Müller [18]) and La 2x
Sr x CuO 4y (Cava et al. [19]), had been identified more than five years before their
unique electronic properties were discovered (Shaplygin et al. [20] and Raveau et
al. [21]). In this particular case, Bednorz and Müller’s insight was rewarded with
the Nobel Prize.
Just as some ionic compounds have properties more often associated with
metals, some metals have properties associated with ionic or covalent compounds.
For example, many intermetallic compounds, such as NiAl, have melting temperatures
greater than 1500°C and little of the ductility normally associated with
metals. In fact, detailed studies of the bonding in this compound suggest that there
is a small peak in the electron density along the line connecting the Ni and Al
atoms, a clear signature of directional covalent bonding. The low density, good
oxidation resistance, high melting temperature, high thermal conductivity, and
appropriate stiffness of this intermetallic compound make it an excellent material
for advanced aerospace structures and propulsion systems [22]. This interesting
combination of properties has motivated research aimed at the development and
understanding of several Ni- and Ti-based intermetallic alloy systems.
We conclude with an example of a polymer, Li-doped polyacetylene,
(Li y CH) x , that has the optical and electrical properties of a metal combined with
the chemical composition and molecular structure of a plastic. This material can
potentially be used as an extremely pliable, low density, electronic conductor
[23–25].
The complexities demonstrated by the C allotropes, the high T c superconductors,
intermetallic compounds, and conducting polymers motivate materials
researchers to develop better descriptions of the relationships between crystal
structures, bonding, and the physical properties of materials. Throughout the
book, we will explore both formal, quantitative, physical models, as well as more
flexible, qualitative, chemical models.
F Problems
(1) In Table 1.2, we see that Ge is a semiconductor and that Pb is a metal. The
element Sn, omitted from the list, lies between them on the periodic table (see
Fig. 1.1). In one of its allotropic forms, Sn is a semiconductor and in another, it
is a metal. Thus, it bridges the gap between the two different electrical properties.
The structure of ‘gray tin’ is diamond cubic (see Fig. 1.10) and the structure
of ‘white tin’ is body centered tetragonal (bct), a structure that is closely related
to the more common bcc structure. Which one do you think is a semiconductor
and which do you think is a metal and why?
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