It has long been known that inorganic luminescent substances may be used to advantage to visualize invisible radiation images (e.g., in radiological diagnostics or display technology) and also for the purpose of general illumination (e.g., in fluorescent lamps or to produce white LEDs). Such luminescent substances usually have a host lattice doped with special elements. So far mostly sulfides, halides and oxides have been used as the host lattice for such luminophores in industrial applications but also to a particularly great extent, complex salts of oxygen-containing acids (borates, aluminates, silicates, phosphates, molybdates, tungstates, etc.) are used.
Only in recent years has it also been possible to develop nitridic materials (such as the red-emitting compounds of the type M2Si5N5:Eu2+ where M═Ca, Sr, Ba described by Hintzen et al. in EP 1 104 799 A1 and EP 1 238 041 B1, for example) and oxynitridic materials (examples include the blue, green and yellow-emitting europium- or cerium-doped MSi2O2N2 compounds according to Delsing et al. in WO 2004/030109 A1; M═Ca, Sr, Ba) as the host lattice for synthesis of efficient luminescent substances. The interest in such luminophores has since then grown to a great extent, especially in conjunction with their advantageous use as conversion luminescent substances for the production of white LEDs. This is attributed in particular to the fact that because of the high covalency of the chemical bonds and the proven marked rigidity of the basic lattice, a particularly high chemical and thermal stability is expected of materials of this type. The disadvantages of the mostly sulfidic and oxygen-dominated conversion luminescent substances is mainly that their luminescence efficiency usually declines very rapidly at temperatures above 100° C. However, for the production of more advanced white LEDs with a higher wattage, conversion luminescent substances with a greatly improved thermal stability are needed.
On the other hand, it should be pointed out in this context that all the inorganic conversion luminescent materials currently used industrially (yttrium aluminates, thiogallates, alkaline earth sulfides, alkaline earth silicates, nitrides, oxynitrides) which are used to produce white light in combination with blue-emitting LEDs, are without exception Eu2+ and/or Ce3+-activated systems with an extremely broadband emission. Electronic 5d-4f transitions which may easily be influenced by an external crystal field and thus naturally also by extinction centers that may be present are characteristic of such luminescent substances. The situation is fundamentally different than that when using luminophores in fluorescent lamps. In this case, the main substances used as red and green components are line-emitting luminescent substances in which the luminescence phenomena to be observed are attributed to transitions between the 4f electrons (4f-4f transitions) that are shielded with respect to the effects of external crystal fields.
High covalent bonding components are also characteristic of another class of compounds only recently discovered. These are the carbidonitridosilicates containing rare earth metals and/or alkaline earth metals. The first representatives of this class of materials (e.g., the compounds Ho2Si4N6C, Tb2Si4N6C (cf. Höppe et al., J. Mater. Chem 11 (2001) 3300) and (La,Y,Ca)2(Si,Al)4(N,C)7(cf. Lindel et al., J. Eur. Chem Soc. 25 (2005) 37) have been synthesized and described with respect to their basic physicochemical properties.
Information about the luminescence of such compounds has so far been completely unavailable in the technical literature. Now, however, SCHMIDT et al. have presented cerium-activated carbidonitridosilicate materials, in particular luminescent substances having the composition Y2Si4N6C:Ce with an activator concentration of 5% Ce in the patent WO 2005/083037 A1, which was published after the priority date of the present application. On excitation with UV radiation or with the light of blue-emitting LEDs, these materials luminesce in a broadband in the yellow spectral range and, according to the published information, have practically the same performance data with regard to quantum yield, absorption efficiency and temperature characteristics as the corresponding parameters of other known yellow-emitting conversion luminescent substances such as yttrium aluminum garnets, which are also doped with cerium, or Eu2+-activated alkaline earth orthosilicates.