Polymorphism describes a situation whereby substances can crystallize in more than one form. Such substances are said to be polymorphic. If the number of crystalline forms are two, they are dimorphic; if three, trimorphic; and so on. The term polymorphic applies to crystalline forms of both elements and compounds, but the term allotropy is also used to describe different forms of the same element, in the same state, whether they be crystalline forms or not. Polymorphism, then, includes all cases of allotropy caused by variation in crystalline form, but some examples of allotropy, e.g. ozone and oxygen, are caused by different arrangements of atoms in molecules and do not involve crystal structure at all.
The crystals formed by polymorphic substances have structures which can readily change to another type, particularly if the temperature is changed. In some cases the change takes place at a definite temperature known as the transition temperature.
When the change take place reversibly, i.e. when one form of the substance is stable above the transition temperature and other form is stable below it, the type of polymorphism is known as enantiotropy, from the Greek meaning opposite change. When the change is not reversible and can only take place in one direction the polymorphism is known as monotropy, from the Greek meaning one change. In monotropic substances there is no definite transition temperature, and one form of the substances is more stable than the other. The unstable form will change into the stable form at all temperatures, but the change into the stable form at all temperatures, but the change may be very slow. So slow, in fact, that the unstable form can sometimes be kept for a very considerable time in what is known as a metastable condition.
The distinction between enantiotropy and monotropy is made clearer by a consideration of vapour pressure-temperature curves for sulphur and phosphorus
Examples of Polymorphic Compounds
The following are some examples of common polymorphic substances.
- Mercury(II) Iodide turns yellow on heating, the transition point being 126o the yellow form does not immediately revert on the red form on cooling for it can be kept in a metastable state, which passes over into the red form slowly, on standing, or quickly if disturbed by touching. The yellow crystals of mercury (II) iodide are rhombic; the red ones are tetragonal.
- Zinc Oxide: Zinc oxide, which can form crystals with the zinc blende or wurtzite structures is white at room temperature but changes into a yellow form above 250o
- Ammonium Chloride: Ammonium chloride forms crystals with a caesium chloride structure below the transition point of 184.3oC, and crystals with the sodium chloride structure above the transition point.
- Ammonium Nitrate(v): This salt has five polymorphic forms with transition temperatures as shown,
The polymorphism of ammonium compounds is due to the rotation of the ions in the crystal which enables them to take up different positions in relative to each other.
5. Silicon (iv) oxide, silicon dioxide, or silica. This can exist in the crystalline forms,
All the three forms are stable and all occur naturally.
Other polymorphic compounds include Ammonium Bromide, Iodide and Sulphate, Caesium Chloride (which changes to a Sodium chloride structure), and Zinc Sulphide (with zinc blende and wurtzite structures).