Optical properties of europium ions in SrB2O4 crystal

We investigate absorption, emission and excitation properties of an SrB2O4:Eu crystal grown from the melt using the Czochralski method. Eu2+ absorption and Eu3+ emission spectra are obtained. The five broad absorption bands peaking at 344, 288, 259, 235 and 212 nm correspond to the transitions from the 4f(7) (S-8(7/2)) ground state to the excited states of the 4f(6)5d configuration of Eu2+. Several zero-phonon lines for the different 4f(6)(F-7(J)) levels of the lowest 4f(6)5d excited state are observed in the absorption spectrum measured at 10 K. The deep-orange luminescence in the 578-640 nm region is due to the transition from the D-5(0) excited state to the F-7(J) (J = 0, 1 and 2) states of Eu3+. Eu2+ does not give rise to luminescence, even at 10 K, because of the photoionization in the excited state, followed by a nonradiative return to the ground state of Eu2+ The strong emission due to the D-5(0) --> F-7(0) transition and the existence of different sites of Eu3+ for Sr2+ are explained by the charge compensation and structural characteristic of SrB2O4.