The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu and Dy

The role of B₂O₃ in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state MAS NMR spectra show the incorporation of boron as BO₄ in the AlO₄ framework of SrAl₂O₄ and Sr₄Al₁₄O₂₅. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy³⁺ forms substitutional defect complex with borate; [Dy-BO₄-V_Sr]²⁻, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO₃³⁻, i.e. [BO₃-V_O]³⁻. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO₃-V_O(e′)]⁴⁻. The holes are released from [Dy-BO₄-V_Sr(h)]¹⁻ under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu²⁺ to induce long phosphorescence.