Optical Properties of Divalent Thulium in Crystalline Strontium Tetraborate

Absorption and luminescence spectra from SrB₄0₇:Tm²⁺ prepared in air have been recorded at room temperature. Because thulium was introduced as Tm³⁺ from Tm₂O₃, a valence change from Tm³⁺ to Tm²⁺ was observed. Optimum production of Tm²⁺ ion occurs when the sample is heated in air at 650 °C. A broad emission band centered in the vicinity of 550 nm has been observed from the sample upon excitation at 457.9 nm. We suggest that it is due to Tm²⁺ ion emission from the 5d band into the ground-state 4f level (²F_7/2 Several conditions promoting the reduction of Tm³⁺ ion in the sample are discussed. In addition, to aid the reduction of Tm³⁺ ion, we have also prepared SrB₄0₇:Tm²⁺ in Ar/H₂ (4%) atmosphere and compared the optical characteristics of Tm²⁺ ion in these samples with those prepared in air.