Physical Properties of Crystalline and Amorphous Solids - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 70

CHAPTER 6
Physical Properties of
Crystalline and Amorphous
Solids
6.1 Crystalline and Amorphous Solids: A Comparison
The word ''solid'' is in everyday parlance applied to those forms of
matter that, on a macroscopic scale, tend to retain their shape and
volume, and possess some degree of rigidity. On a molecular scale,
however, a distinction must also be drawn between two very different
types of solids. The crystalline state, in which the atoms or molecules are
arranged in a regular and precise spatial array, is the familiar solid form,
characterised by its long-range order and thermodynamic stability. For
completeness, it must be emphasised that certain crystalline types,
obtained by cooling from the fused state, may not necessarily result in
the most stable form, i.e. the form of lowest energy. Some elements
(e.g. phosphorus and sulfur) and many chemical compounds can exist in
a variety of crystalline states (polymorphs), each of which is stable only
within a well-defined domain in phase space. Outside that domain, they
are metastable and liable to spontaneous solid/solid transitions, resulting
in a more stable form. Ice is a well-known example of complex poly-
morphic behaviour. Up to the present, 12 distinct polymorphs, some of
them metastable, have been prepared and characterised by X-ray and/
or neutron diffraction; 84 some of them are included in Figure 9 in
Chapter 4. There is no reason why additional polymorphs may yet be
discovered, because even ''ordinary'' ice (hexagonal ice) is not the form
of lowest energy; the statistical distribution of protons gives rise to the
well-known residual entropy, first described by Linus Pauling. 85
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