The d-f luminescence of Eu2+, Ce3+ and Yb2+ ions in Cs2MP2O7 (M = Ca2+, Sr2+)

The efficient narrow band emission of Eu2+ in Cs2MP2O7 (M = Ca2+, Sr2+) is characterized by a large Stokes shift and a high quenching temperature which makes the material promising for application in warm white LEDs. The unusual Eu2+ luminescence properties were reported recently but an explanation for the peculiar behavior is lacking. In this paper we aim at providing new insights in the luminescence of the Eu2+ emission in Cs2MP2O7 through measurements at cryogenic temperatures (down to 4 K) and by comparison with the d-f luminescence of Ce3+ and Yb2+ in the same host. The results reveal a sharp onset of the Eu2+ emission and excitation bands at 4 K. Usually the sharp onset for narrow excitation and emission bands coincide at an energy corresponding to the zero-phonon (purely electronic) transition, but for Eu2+ in Cs2MP2O7 there is a large shift of 3500 cm-1 between the onsets, consistent with the large Stokes shift observed. The onset shift can be explained by emission from a lower energy distorted excited 4f65d1 state. For Ce3+, the f-d absorption bands are at energies expected based on the relation between the absorption energies for Eu2+ and Ce3+ reported by Dorenbos. Contrary to Eu2+, the emission for Ce3+ shows a normal Stokes shift and therefore the emission bands are at much higher energies than predicted from the energy of the Eu2+ emission and the Dorenbos relations. Based on the present results the unusually large Stokes shift for the Eu2+ emission in Cs2MP2O7 is assigned to a Jahn-Teller like deformation in the excited 4f65d1 state of Eu2+ that is not present in the 5d state of Ce3+