Luminescence from samples: substitutional ions are `silent'

Through this work it is shown that the luminescence observed in samples is related to the formation of microcrystalline precipitates. This conclusion is supported by the experimental emission and excitation spectra as well as by the temperature dependence displayed by the emission intensity. Also the EPR spectrum of most doped samples clearly shows the presence of a signal (superimposed on that coming from isolated ions) whose temperature dependence is similar to that displayed by crystals. The absence of luminescence associated with isolated ions in the host lattice is discussed. It is pointed out that luminescence quenching is probably related to a multiphonon non-radiative process rather than to a (G) excited state close to the bottom of the conduction band.

Although luminescence quenching appears for a Dexter - Klick - Russell parameter probably close to 0.1, this is qualitatively related to a high radiative lifetime ( ms). The analysis made also shows that firstly increases by a factor of about 1.7 on going from -doped fluoroperovskites to in fluoride-type crystals, and secondly increases on going from to . Both variations mainly reflect a parallel increase in the Stokes shift.