The Intermediate- and Strong-Coupling Cases

AR in arbitrary units, calculated by the Struck and Fongcr method [3]. AR is arbitrarily put at 10.0 for BajMgWOfi. 

There is a more impressive experiment to prove this simple model, viz. the luminescence in the ordered perovskites A2BWO6 where A and B are alkaline earth ions. Table 4.2 presents the quenching temperatures of the luminescence of the UO group in these lattices 18J. Those for the WOg group run parallel. These temperatures are used as a measure of the radiationless processes. The table shows that the radiationless rate does not depend on the nature of the ion A, whereas that of the ion B determines the value of this rate the smaller the B ion, the higher the quenching temperature. 

It is well known that luminescent materials with high quantum efficiencies and quenching temperatures usually have stiff lattices, so that expansion upon excitation is counteracted, i.e. AR is as small as possible. 

Part of solid stale chemistry is nowadays involved with what is called soft chemistry or soft materials. As a matter of fact these arc not expected to luminesce, at least not when the luminescent centers ate broad-band emitters. This has been shown to be the case, for example, for the isomorphous Al2(W04)3, Sc2(W04 3 and Zt2(P04)2S04. The Stokes shift of the tungstate and zirconate luminescence in these materials is enor- 

Thble 4.3. Stokes shift (10 cm ) of the band emission of some trivalent ions in the orthoborates MBO3 (M=Sc,Y,La) After Ref. [9] 

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We consider first the weak-coupling case (S 0), and subsequentfy the intermediate- and strong-coupling cases (S 0). 

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