Optical spectroscopy, thermal quenching and electron-vibrational interaction of the octahedrally coordinated Eu²⁺ ion in CaAl₂B₂O₇ and BaAl₂B₂O₇

The spectroscopic properties of the six-fold coordinated Eu²⁺ in CaAl₂B₂O₇ and BaAl₂B₂O₇ are investigated and reported in this paper. Relevant optical parameters such as the Stokes shift, crystal-field splitting, the centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration are calculated from the experimental data. Further, the Huang-Rhys factor and the effective phonon energy, which are responsible for the electron-vibrational interaction, are theoretically estimated for the Eu²⁺ ion in CaAl₂B₂O₇ and BaAl₂B₂O₇. The low quenching temperature of the luminescence in these materials is tentatively connected with the thermally induced promotion of the Eu²⁺ 4f⁶5d¹ electron to the host lattice conduction band (photoionization). Evidence for the limited solubility of Eu²⁺ in BaAl₂B₂O₇ is presented. A comparative study of the Eu²⁺ luminescence in M²⁺Al₂B₂O₇ (M²⁺ = Ca²⁺, Sr²⁺, Ba²⁺) family of materials is also presented.