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The fluorescence of a number of new Ce3+-activated phosphors is described and discussed. Only host lattices with a sublattice consisting of trivalent lanthanide ions are used to avoid charge compensation of the Ce3+ ion. Usually the Ce3+ emission is in the near-ultraviolet region. Y3Al5O12–Ce and SrY2O4–Ce, however, show emission in the visible region with a maximum in the green. Conditions for visible Ce3+ emission are indicated, viz., large crystal-field splittings (Y3Al5O12–Ce) or a large Stokes shift (SrY2O4–Ce). In a number of cases we were able to observe all crystal-field components of the excited 5d level of Ce3+. The cubic crystal-field splitting of the 5d level varies strongly with host lattice from 7000 to 14 000 cm−1. The position of the center of the 5d levels in oxides is about 30% lower than in the free ion. For some phosphors we observed more than one emission band at room temperature. This is due to fluorescence from higher excited levels. Efficient energy transfer from Ce3+ to Cr3+ was observed in Y3Al5O12.

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This chapter addresses those readers for who luminescent materials are a new challenge. Of course you are familiar with luminescent materials: you meet them everyday in your laboratory and in your home. If this should come as a surprise, switch on your fluorescent lighting, relax in front of your television set, or take a look at the screen of your computer. Perhaps you would like something more specialized. Remember then your visit to the hospital for X-ray photography. Or the laser in your institute; the heart of this instrument consists of a luminescent material. However, such a high degree of specialization is not necessary. The packet of washing powder in your supermarket also contains luminescent material. Now that we have been reminded of how full of luminescent materials our society is, the question arises "How do we define a luminescent material?" The answer is as follows: A luminescent material, also called a phosphor, is a solid which converts certain types of energy into electromagnetic radiation over and above thermal radiation. When you heat a solid to a temperature in excess of about 600"C, it emits (infra)red radiation. This is thermal radiation (and not luminescence). The electromagnetic radiation emitted by a luminescent material is usually in the visible range, but can also be in other spectral regions, such as the ultraviolet or infrared.Luminescence can be excited by many types of energy. Photoluminescence is excited by electromagnetic (often ultraviolet) radiation, cathodoluminescence by a beam of energetic electrons, electroluminescence by an electric voltage, triboluminescence by mechanical energy (e.g. grinding), X-ray luminescence by X rays, chemiluminescence by the energy of a chemical reaction, and so on. Note that thermoluminescence does not refer to thermal excitation, but to stimulation of luminescence which was excited in a different way. Fig. 1.1. A luminescent ion A in its host lattice. Exe: excitation; EM: emission (radiative return to the ground state); HEAT: nonradiative return to the ground state G. Blasse et al., Luminescent Materials

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The fluorescence of Y3Al5O12–Ce3+ under cathode-ray excitation consists of an emission band peaking at 550 nm. The decay time is 0.07–0.08 μsec. In view of these properties this phosphor is very suitable for flying-spot cathoderay tubes for color television.

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