Valence states and luminescence properties of ytterbium ions in strontium haloborates

The luminescence properties of ytterbium ions in strontium haloborates were studied under optical, X-ray and synchrotron excitation. The coexistence of Yb ions in two valence states (divalent and trivalent) was detected in the Sr2B5O9X:Yb (X = Cl, Br) powder materials prepared in a slightly reducing (H-2/N-2) or art oxidizing atmosphere. Under optical excitation, the 5d --> 4f Yb2+ luminescence at 420 nm is observed. Even under X-ray excitation, an emission band with a maximum of about 340 ran appears in the spectra. This broadband emission is attributed to charge transfer luminescence of Yb3+. The influence of the structural features of Sr2B5O9X and some preparative conditions of the samples on the luminescent behavior of Yb are discussed. (C) 2002 Elsevier Science (USA).