The valence change from RE3+ to RE2+ (REEu, Sm, Yb) in SrB4O7: RE prepared in air and the spectral properties of RE2+

A valence change from RE3+ to RE2+ (REEu, Sm, Yb) was observed in samples of SrB4O7: RE prepared by solid state reaction at high temperature in air. The phosphors SrB4O7: RE2+ show efficient luminescence at room temperature. The broad band d-f emissions of Eu2+ and Yb2+ are at 367 and 360 nm respectively. The sharp line f-f emissions of the Sm2+ ion (5DO-7FJ) are in the range 680–780 nm. The probable reasons for the valence change of Eu, Sm and Yb in this host are discussed.