It is generally convenient to consider the structures of metals
in terms of the observed lattice type at 298K and atmospheric
pressure,‡ but these data do not tell the whole story.
When subjected to changes in temperature and/or pressure,
the structure of a metal may change; each form of the
metal is a particular polymorph. For example, scandium
undergoes a reversible transition from an hcp lattice (a-Sc)
to a bcc lattice (b-Sc) at 1610 K. Some metals undergo
more than one change: at atmospheric pressure, Mn undergoes
transitions from the a- to b-form at 983 K, from the
b- to g-form at 1352 K, and from g- to s-Mn at 1416 K.
Although a-Mn adopts a complex lattice (see above), the
b-polymorph has a somewhat simpler structure containing
two 12-coordinate Mn environments, the g-form possesses
a distorted ccp structure, and the s-polymorph adopts a
bcc lattice. Phases that form at high temperatures may be
quenched to lower temperatures (i.e. rapidly cooled with
retention of structure), allowing the structure to be
determined at ambient temperatures. Thermochemical data
show that there is usually very little difference in energy
between different polymorphs of an element.