Anomalous Trapped Exciton and d–f Emission in Sr₄Al₁₄O₂₅:Eu²⁺

Abstract

The photoluminescence and time-resolved emission for Eu(2+) in Sr4Al14O25 has been investigated in the temperature range 4 to 500 K. The Eu(2+) 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 Eu(2+) 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 Eu(2+) 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 Eu(2+) trapped exciton state is at a slightly lower energy than the lowest energy 4f(6)5d state. Upon raising the temperature, the 4f(6)5d 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.

Evidence of three different Eu2+ sites and their luminescence quenching processes in CaAl2O4:Eu2+

“…Consequently, the ETE luminescence was observed only in the Ca I site. This relationship between the relative energy location of ETE and 5d states and optical properties in other Eu 2+ -doped compounds is already explained by many researchers [32][33][34][35][36][37][38].…”

Eu2+luminescence in strontium aluminates

“…The origin of the shift was discussed by the authors elsewhere. 46 The emission at 400 and 490 nm were assigned to 'normal' d-f emission from Eu 2+ on the 10-and 7 coordinated sites, respectively, while the 425 nm emission was assigned to 'anomalous' Eu 2+ trapped exciton emission (ETE were recorded at different emission wavelengths for the Eu(1) center and Eu(2) centre (Fig. 15).…”

Insights into the complexity of the excited states of Eu-doped luminescent materials

“…It is then not surprising that different -often contradictory -analyses of experimental 4f 7 →4f 6 5d spectra exist [17][18][19][20][21][22][23][24][25][26][27] and it is not clear which model correctly grasps the nature of the electronic levels underlying the broad absorption or excitation bands. Furthermore, emission bands are often present in photoluminescence spectra that seem to be related to the presence of Eu, but do not correspond to the expected 4f 7 →4f 7 or 4f 6 5d 1 →4f 7 emissions [28][29][30][31][32] . Because no convincing systematic behavior is found in terms of its occurrence or properties (such as lifetime, thermal behavior, etc.…”