Page 36 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

interactions between adjacent species. For example, the extremely high melting
points of salts are directly associated with the strength of the ionic bonds between
adjacent ions. For molecular species, it is the degree of intermolecular interactions
such as van der Waal and hydrogen bonding forces that controls the lattice energy.
1
Ionic and covalent crystals have similar lattice energies (ca. 700–900 kJ mol ),
1
followed by metallic crystals (ca. 400–500 kJ mol ). By contrast, molecular
crystals such as solid carbon dioxide are much more readily broken apart
1
(ca. 5–20 kJ mol ) – a consequence of the weak van der Waal interactions
consisting between the discrete molecules that comprise the lattice.
The ions, molecules, or atoms pack in an arrangement that minimizes the total
free energy of the crystal lattice. For ionic crystals, there is an overall balance of
charge among all ions throughout the lattice. Non-ionic crystals exhibit a greater
variety of packing interactions between constituent molecules. One of the most
influential forces found in these lattices is hydrogen bonding. The molecules will
pack in such a manner to balance the number of hydrogen bond donor and acceptor
groups. Often, a residual polar solvent, capable of participating in hydrogen bond-
ing, will play an important role in the observed packing arrangement. Depending on
the polarity of the encapsulated solvent, a variety of arrangements of molecules will
be observed in the crystal lattice, with hydrophobic and hydrophilic groups being
preferentially aligned with respect to each other and the solvent.
Depending on how strongly a solvent is contained within the crystal lattice,

sometimes the encapsulated solvent is lost, an occurrence referred to as efflores-
cence. By contrast, if the solid contains ions with a high charge density (high
charge/size ratio) and is soluble in water, the crystals will readily adsorb water
from the atmosphere and may even be transformed to a solution. An example of such
a deliquescent crystal is calcium chloride, which is employed as a dehydrating agent
for removal of moisture from a flow of inert gases.
The overall shape or form of a crystal is known as the morphology. Often, there is
more than one crystalline form of the same substance. Each form is known as
a polymorph, differing in both the arrangement of constituents as well as unit
cell dimensions. Although polymorphs differ in both the shape and size of the
unit cell, most compounds may exhibit this behavior under appropriate experimental
conditions. Common reasons for a varying crystal structure are similar ionic ratios for
anions and cations in ionic crystals, or variations in temperature or pressure during
crystal growth. These latter effects alter the amount of disorder within the crystal
lattice, allowing for the migration of atoms/ions/molecules into lattice positions that
are thermodynamically disfavored at lower temperatures and/or pressures. [5]
Most often, the energy for the interconversion between polymorphs is small,
resulting in phase changes that occur after only moderate changes in temperature
or pressure. In general, exposing a crystal to an applied pressure forces neighboring
atoms closer together, causing a decrease in the volume of the unit cell, and an
increase in the coordination number of individual atoms. For instance, silicon is
transformed from a four-coordinate polymorph at ambient pressure to a variety of
[6]
higher-coordinate phases at elevated pressures.