On peculiarities of Eu3+ and Eu2+ luminescence in Sr2GeO4 host

Powders of Sr2GeO4:Eu were prepared in air or forming gas and were investigated for their luminescent properties in the 20-300 K range of temperatures. Oxidizing conditions produced a material with the orange-red luminescence of Eu3+. As many as eight Eu3+ symmetry sites active in luminescence were found at low temperatures. Four of them had the expected quite regular characteristics and were assigned to the four Sr sites offered by the host lattice. In addition, another four Eu3+ sites producing the D-5(0) -> F-7(0) emission at extraordinarily short wavelengths in the range of 574-577 nm were clearly revealed. Their CT excitation bands peaked at around 325 nm, which is an unusually low energy for the O2- -> Eu3+ CT transition. This unusual characteristic of Eu3+ luminescence was assigned to the presence of an interstitial oxygen not bound to Ge but strongly interacting with Eu3+ ions. The effect was justified using the Wybourne and Downer extension of the Judd-Ofelt theory. Reduced samples did not produce any Eu2+ emission at room temperature, but below about 70 K the 5d -> 4f luminescence of the ion came into view with an intense appearance at 20 K. No indications were found of Eu2+ luminescence from a few sites. A possible explanation of this observation as well as mechanism of the Eu2+ luminescence quenching are discussed.