Crystals used for solid state electronics have impurities deliberately introduced into their structure that are used to control their electrical properties. Impurity atoms may substitute for the normal atoms in a crystal's structure or may fit in the spaces within the structure. Other defects include vacancies, where atoms are simply missing from the structure, and dislocations, in which a half plane of atoms is missing. The important thing about crystal defects is to be able to control their number and distribution. Uncontrolled defects can result in unreliable electronic properties or weaknesses in structural metals.
Many forces can affect the structure of a crystal.
One of the most important forces that can influence the structure of a growing crystal is gravity. Growing crystals in microgravity can reduce gravity effects to produce crystals with better defined properties. The information gained by microgravity experiments can lead to improved crystal processing on Earth. The connection between the force of gravity and the formation of defects varies from very simple and straightforward to complicated and nonintuitive. For example, mercury iodide crystals can form from the vapor phase.