Anomalous trapped exciton and D-F emission in Sr4Al 14O25:Eu2+

The photoluminescence and time-resolved emission for Eu2+ in Sr4Al14O25 has been investigated in the temperature range 4 to 500 K. The Eu2+ emission changes in a peculiar way with temperature. At low temperature two emission bands are observed at 490 and 425 nm, which are attributed to emission from Eu2+ on the 7- and 10-coordinated sites. Upon raising the temperature, an unexpectedly large blue shift to 400 nm is observed for the 425 nm emission band. To explain these observations, the 400 and 425 nm emission bands are assigned to d-f and trapped exciton emission, for Eu2+ on the 10-coordinated site. The trapped exciton emission is characterized by a short (0.5 μs) decay time. The temperature dependence of the emission is explained by a configurational coordinate diagram in which the Eu2+ trapped exciton state is at a slightly lower energy than the lowest energy 4f65d state. Upon raising the temperature, the 4f65d state is thermally populated and emission from this state is observed, and because of the smaller lattice relaxation (smaller Stokes shift), a large blue shift from 425 to 400 nm is observed