Crystal Phosphors

inorganic crystalline luminophors.

Crystal phosphors luminesce on exposure to light, an electron stream, penetrating radiation, or electric current. The luminescence is due to the presence of a forbidden band in the energy spectrum of the crystal. Therefore, crystal phosphors can only be semiconductors or dielectrics. Low concentrations of admixtures called activators are also found in crystal phosphors. It is the activators, along with lattice defects (vacancies, internodal atoms), that form the centers of luminescence.

The luminescence mechanism is essentially of the recombination type. It can occur through the direct excitation of the luminescence centers or through the absorption of excitation energy by the crystal lattices and the transfer of the energy (by electrons, vacancies, excitrons) to the luminescence centers. The direct recombination of electrons and vacancies in crystal phosphors is also accompanied by luminescence (radiation recombination). The duration of the afterglow varies considerably (for example, from 10⁻⁹ sec to several hours). The luminescence spectrum of crystal phosphors ranges from ultraviolet to infrared, depending on the activator.

The basic constituents of crystal phosphors include zinc and cadmium sulfides, selenides, and tellurides, calcium and manganese oxides, and alkali halides. Metal ions (Cu, Co, Mn, Ag, Eu, Tu) serve as activators. Crystal phosphors are most often synthesized by roasting solid mixtures, although a number of them are prepared from a gas or melt. Combining the activators and base materials makes it possible to synthesize crystal phosphors that transform various kinds of energy into the visible light of primary colors with a high degree of efficiency (to tenths of a percent). For example, crystal phosphors have been formed that convert infrared radiation into visible radiation. Other crystal phosphors are characterized by increasing or decreasing luminosity under exposure to infrared radiation.

Crystal phosphors have found wide application (particularly crystal phosphors in which the width of the forbidden band is several electron volts) because of their broad possibilities for use, their high degree of luminosity, and their chemical and radiative stability. Powdery crystal phosphors are used in fluorescent lamps, television tubes, oscillographs, and electroluminescent panels. Crystal phosphors with a short period of afterglow (for example, Nal·T1) are used in scintillation counters to record high-speed elementary particles and gamma quanta. Certain crystal phosphors are used as an active medium in semiconductor lasers.