Eu3+-site occupations and peculiar reddish orange luminescence in aluminates Sr3RAl2O7.5:Eu3+ (R=Y, Lu)

A novel reddish orange-emitting phosphor of Eu3+-doped aluminate Sr3RAl2O7.5 (R=Lu, Y) was successfully synthesized by wet-chemistry sol-gel method. The samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). The luminescence properties such as photoluminescence excitation and emission spectra, decay curves, CIE color coordinates and luminescence quantum efficiency were investigated. This phosphor can be efficiently excited by UV and near-UV light and exhibits a bright reddish orange luminescence with the abnormally strongest 5D0→7F0 transition. Two distinct Eu3+ sites can be observed on the emission spectra under the excitation of UV light (excitation at charge transfer band of Eu3+ ions) or dye laser in the 5D0→7F0 wavelength region (selective excitation into each site). The site-selective luminescence decay curves have been investigated in the 5D0→7F0 region of each Eu3+ site by using a pulsed, tunable, and narrowband dye laser. Each Eu3+ site presents the distinct luminescence lifetime with different rise time. The Stark energy levels of two Eu3+ sites were assigned from the site-selective emission spectra. The energy transfer can happen between the closely neighbored Eu3+ sites in Sr3RAl2O7.5 (R=Lu, Y). The luminescence mechanism responsible for the abnormally strong 5D0→7F0 transition was discussed.