In the solid state, many substances exist in several forms that have different physical and chemical properties. These forms are called polymorphs.
The word polymorphs is a blanket term for Allotropy, which are specific to pure elements, and isomers, which apply in the case of chemical compounds.
How does this happen?Polymorphs occur when the smallest particles in a crystal arrange themselves in different patterns.
For example, the ions of an ionic crystal, or the molecules of a molecular crystal take up different positions relative to one another. In the case of elements, the atoms in a crystal can occupy different positions relative to one another, or they may join together to form molecules of different sizes.
With elements and ionic compounds, different bonding relationships are often also present in the polymorphs.
How many are there?Lots. In most elements, we know of several allotropes. In sulphur, for example, more than 10. With some compounds – silicon dioxide or water, as examples – more than 10 isomers are likewise known. Other compounds have not been studied so extensively, and only one or two isomers have been found.
Polymorphs are often only stable at very high temperatures or pressures.
Can they be transformed?In principle, yes, but ... the rate of transformation is often extremely low. This is also the reason why, in Nature, we find several polymorphs of a substance, and not just the thermodynamically most stable one. Other transformations are possible only under conditions of extreme pressure and temperature, some not at all.
If it is not possible to produce a polymorph by transformation from another one, we need to take other routes: desublimation from the gaseous state, crystallisation from the melt or from a solution, and synthesis from other materials are just some examples.