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Abstract
The excitation and emission spectra of the octahedrally coordinated Eu2+ ion inCs2M2+P2O7, where M = Ca and Sr, is reported and discussed. The remarkable fea-tures of the Eu2+ luminescence in these double phosphate materials include (a) the verylarge Stokes shift of emission ( 1 eV), (b) the high luminescence quenching temperature and (c) the very low energy of the emitted photons, which is unusual for the luminescence of the Eu2+ ion in phosphate based materials. The broad emission bands of Eu2+ in Cs2CaP2O7 and Cs2SrP2O7 peak at 607 nm and 563 nm, respectively. The Stokes shift, crystal field splitting, the centroid shift and the red shift of the Eu2+ 4f65d1 electronic configurationhave been estimated from the relevant optical data. The radiative lifetime of the Eu2+emission in Cs2M2+P2O7 is ~ 1.2 us. The nature of the Eu2+ emission in Cs2M2+P2O7is discussed and arguments are presented to associate the luminescence with the normal4f65d1 --> 4f7[8S7/2] emission transition. A discussion of the characteristic properties of the "anomalous" emission of the Eu2+ ion in solids is also presented and compared with theproperties of the Eu2+ luminescence in Cs2M2+P2O7.