Site-Dependent Luminescence from Pr3+ in Double-Perovskite-Type Alkaline Earth Lanthanum Tantalates

Site-dependent photoluminescence (PL) from Pr3+ was investigated in Pr3+-doped alkaline earth lanthanum tantalates with a double-perovskite-type structure (AA')[BB']O6. Pr3+ was partly substituted for La3+ in both (CaLa)[CaTa]O6 (CLTO) and (Ba2)[LaTa]O6 (BLTO). The results of X-ray absorption near-edge structure of the Pr L3 absorption edge strongly supported that Pr3+ ions were located at A sites in CLTO:Pr3+ and at B sites in BLTO:Pr3+. In the PL excitation (PLE) spectra, both CLTO:Pr3+ and BLTO:Pr3+ showed intense peaks in the UV region, which were assignable to the 4f–5d transitions in Pr3+. The energy for the 4f–5d excitation peak in BLTO:Pr3+ was found to be lower than that in CLTO:Pr3+, which was primarily attributed to the low energy of Pr3+ 5d states resulted from a large crystal field at B sites. On the Pr3+ 4f–5d excitation by UV light, CLTO:Pr3+ showed white PL due to the transitions from both 3PJ and 1D2 levels, whereas BLTO:Pr3+ exhibited red PL caused by the transitions from only 1D2 levels. The low energy of Pr3+ 5d states at B sites was responsible for the quenching of the blue-green luminescence from 3PJ levels in BLTO:Pr3+.