Impurities

While the regularity and order of crystals have been stressed thus far, it is important to note that this order can be disturbed. Generally the cause is the inclusion of an impurity. Sometimes this is the result of a crystal forming around a foreign particle. This can usually be detected by microscopic examination. But other times it is actually an invasion by an atom with approximately the same size and shape as the host crystal, and the pattern is not disrupted, This is called a mixed crystal. The classic example of this in alum which is composed of potassium sulfate and aluminum sulfate in a one to one proportion. A similar compound is chrome alum, in which you find potassium sulfate combined with chromium sulfate.

Many crystals in nature demonstrate this mixed crystal condition in the replacement of aluminum by chromium or sometimes iron. Rubies are a good example of this, being composed of aluminum oxide with chromium replacing some of the aluminum, and also sapphires, which replace the aluminum with titanium and iron.

In some cases a slightly different atomic substance can enter a crystal but only in small quantities. This is called a substitutional impurity. A most relevant example of this is substitution of phosphorus or boron atoms in silicon crystals. These “impure” compounds are used to make transistors for electronic instruments.

Sometimes a different kind of impurity enters a crystal. These foreign atoms may be very small compared to the host substance and fit in between the orderly arranged host atoms. If the host substance has a generous size pattern the invading atoms could be as large as the host atoms themselves. The additional atoms are called interstitial impurities. A well known example of this is carbon and iron, which makes steel.

A third kind of defect could be called a vacancy or a hole. This results from very rapid crystal growth during which some of the atomic sites are simply not filled. The milky or veiled appearance of homegrown crystals, however, is caused by very large openings called voids. It generally occurs when the evaporation of solvent proceeds too rapidly and incomplete crystallization happens. The white coloration is caused by the presence of a liquid solution that is trapped in the open spaces of the crystal. Vacancies on the other hand are far too small to be visible. Vacancies are not an issue in most commercial resonators. It is only in some military applications where radiation hardness is a consideration where this kind of defect may be important.

Updated: 11/15/2010