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

Abstract

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. Al'+, Fe4+ , 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. Basic Features of the MethodIonizing radiation can cause three types of permanent changes in a solid.a) The most drastic change is of course photolysis or radiolysis, and thus complete and irreversible decomposition. It is characteristic of certain compound classes, in particular of many organic compounds, which sometimes undergo characteristic changes utilized for preparative purposes''! b) Another possibility is a displacement of individual atoms from their equilibrium positions, e.g. with the formation of vacancies and interstitial atoms. Except for the alkali-metal halides, in which this takes place even with ultraviolet radiation['', particle irradiation of higher energy is necessary in other classes of compounds. c) Finally an electron can be transferred from a donor D(-) to an acceptor A(+). In accordance with the formal equation (I)[**]: this process usually leads to the formation of two paramagnetic defects, of which D is described as a hole center and A as an electron center. Whereas b) is a mainly physicomechanical process, which to a large extent obeys the laws of classical mechanics, c) comprises a photochemical redox process in which D(-) is oxidized and A(+) is reduced. The pioneering work of Pohl and his associates led to these color centers' being regarded in Germany as the domain of the physicist, and it has often been assumed that their formation is restricted to the alkali-metal halides, the classical representatives of ionic crystals. In reality, color centers can be produced in practically all crystalline or amorphous insulators. The color of many minerals is due to color centers. Among the quartzes alone we find three examples: the violet color of amethyst, Angew. Chem. Int. Ed. Engl. 17,89-97 (1978) the dark brown of smoky quartz, and the greenish yellow of natural citrine. The brown color of fired a m e t h y~t [~.~I is of different origin.The basis of the photochemical redox reaction (1) is kinetic in nature: electromagnetic radiation of sufficiently high energy liberates electrons from all types...

O−bound small polarons in oxide materials

“…A situation similar to that for citrine is also identified for the mineral brazilianite (NaAl 3 (OH) 4 (PO 4 ) 2 ). Here the O − ion, carrying the hole, is locked between Al 3+ and P 5+ and no equivalent oxygen ions are nearby [77]. Therefore the characteristic bound polaron absorption does not occur.…”

A naturally induced hole center in the mineral hypersthene

“…Charge compensation after the incorporation of impu¡ if required, can occur in several ways. In the case of incorporation of lower valence cations, anion vacancies are formed and an O-adjacent to a lower valence impurity is by far the most frequent center in oxides [2].…”