Page 37 - Materials Chemistry, Second Edition

24 2 Solid-State Chemistry

If the solid is an element, polymorphs are known as allotropes. One of the
best-known examples for elemental polymorphism (allotropy) is observed for
Group 16 elements (chalcogens). For instance, let’s consider the allotropism exhibited
by sulfur. The most stable form under ambient conditions is cyclooctasulfur, a yellow
powderconsistingof arrangements of eight-membered rings (Figure2.4a). At elevated
temperatures, the S 8 rings begin to open and cross-link with one another, resulting in a
highly viscous reddish solution (Figure 2.4b). If this solution is quickly quenched by

Figure 2.4. Illustration of allotropic transformations exhibited by elemental sulfur. Shown are
(a) cyclooctasulfur (S 8 ) at room temperature/pressure, (b) breaking apart of discrete S 1 rings at elevated
temperature to form a viscous liquid, (c) formation of S 1 (catenasulfur or “plastic sulfur”) via quenching in
cold water, and (d) re-conversion of catenasulfur back to the thermodynamic stable S 8 allotrope.