Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2M2+P2O7 (M2+=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu2+) ions in Cs2M2+P2O7 (M2+=Ca, Sr) are reported and discussed. The remarkable features of the Eu2+ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu2+ luminescence in phosphate-based materials. The broad emission bands of Eu2+ in Cs2CaP2O7 and Cs2SrP2O7 peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu2+ 4f65d1 electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu2+ emission in Cs2M2+P2O7 is ∼1.2 μs. The nature of the Eu2+ emission in Cs2M2+P2O7 is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f65d1→4f7[8S7/2] emission.