Phosphor hosts oxides

Generalized composition of oxidic phosphor hosts in which XEOL of rare earths has been observed. 

The phosphors described in this chapter are usually based on metal oxide, metal oxysulfide, and metal sulfide lattices, which act as hosts for light-emitting centers that are doped into them. This is denoted by, for example, ZnS Pb (meaning a trace of Pb doped into a ZnS host lattice) or LiGdF4 Er3+,Tb3+ (meaning a mixture of Er3+ and Tb3+ ions doped into a LiGdF4 host lattice). 

Most of the successful rare earth activated phosphors comprise host lattices in which the host cation is also a rare earth. A principal reason for this relates to the optical inertness of La, Gd, Y, and Lu this is essential to avoid interference with activator emission spectra. Close chemical compatibility including amenability to substitutional Incorporation of rare earth activators are also essential features. Rare earth hosts such as oxides, oxysulfides, phosphates, vanadates and silicates also tend to be rugged materials compatible with high temperature tube processing operations and salvage. 

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

Bril and Wanmaker (149) examined the fluorescent properties of some europium-activated phosphors. In particular, they studied gadolinium oxide, gadolinium borate, gadolinium phosphate, lanthanum borate, and europium phosphate. The phosphors all show an emission spectrum characteristic of europium. From the excitation spectra one can deduce that, in the gadolinium containing host energy absorbed by the gadolinium, ions can be transferred to the europium ions. 

Yttrium compounds are frequently useful host materials for later Ln + ions, as mentioned in Section 5.4.4 Eu Y2O2S is the standard material for the red phosphor in virtually all colour and television cathode ray tubes, whilst Eu Y203 is used for energy-efficient fluorescent tubes. Yttrium oxide is used to stabilize zirconia (YSZ), yttrium iron garnets (YIG) are used in microwave devices, and of course YBa2Cu307 is the classic warm superconductor. Yttrium, like scandium, is naturally monoisotopic. Y has I = 1/2 though signals can be difficult to observe, valuable information can be obtained from NMR studies. 

Some reactions require a reducing atmosphere to incorporate activator ions in an oxidation state which is lower than the maximum oxidation state, e.g. Eu, or to prevent oxidation of the host lattice, e.g. in the case of the preparation of ZnS phosphors. This can be achieved by executing the reaction in diluted H2 or in CO. A CO atmosphere is easily obtained by heating graphite grains in a closed vessel, in which a second, smaller, vessel containing the reaction mixture is placed. 

However, there is a very usefiil reason for determining the type of luminescence decay curve present. Confirming the presence of an exponential decay curve means that only one type of emitter is jn sent. If a logarithmic decay process is found, it usually means that more than one type of emitting center is present, or that two or more decay processes are operative. While this does not occur very often, it is useful to know if such is present. This phenomenon occurs more in cathode-ray phosphors than in lamp phosphors, i.e.- sulfides vs oxide- hosts. 

In order to obtain homogeneous phosphors it is often necessary to leave the simpie soiid state technique. Coprecipitation may be of importance, especiaily if the activator and the host iatiice ions are chemicaily similar. This is, of course, the case with rare-earth activated phosphors. For example, Y2O3 Eu " can be prepared profitabiy by coprecipitating the mixed oxaiates from soiution and firing the precipitate [S]. Actually the mixed oxides have become available commercially. 

A new class of phosphors based on ceramic materials has been introduced by several groups [26,27]. These materials are derivatives of the well-known lare-eaith phosphor systems of oxides and oxysulfides. The most promising ceramic host lattices are (Y,Gd)203, Gd202S and Gd3Ga50 2. 

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