Solid-State Photochemistry—A Method of Generating Unusual Valence States

High-energy radiation can give rise to pairs of complementary defects in nonmetallic solids by the transfer of electrons between various types of atoms. These “color centers” which are generally paramagnetic, can usually be described as unusual valence states of an element. They are destroyed by heating and in most cases regenerated by renewed irradiation. In a heteropolar solid the formation of color centers usually leads to cancellation of point charges due to foreign ions of other valence or to vacancies. This is shown by the examples of kunzite, brazilianite, smoky quartz, and citrine; the most important methods for the structural elucidation of color centers are also described. Application of the principle of charge balance opens up possibilities for the production of unusual valence states, e.g. Al²⁺, F^2?, Fe⁴⁺, and O^?. Moreover, the type of the color center often permits far-reaching conclusions to be drawn about the defect structure of real crystals, which could hardly be clarified in other ways.