The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu2+ and Dy3+

The role of B2O3 in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state 27Al 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–VSr]2−, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO33−, i.e. [BO3–VO]3−. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO3–VO(e′)]4−. The holes are released from [Dy–BO4–VSr(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.