The single doping of Eu3+ and codoping with Ga3+ or La3+ into perovskite (ABO3)-type CaZrO3 and CaHfO3 were carried out to investigate the effect of codoping on the photoluminescence (PL) features of Eu3+, attempting the site-selective Eu3+ doping at either A or B sites. To comprehend the Eu3+ PL features, the accurate locations of Eu3+ were examined by X-ray absorption near edge structure (XANES) and the proportions of Eu3+ located at A or B sites (Eu3+(A) or Eu3+(B); EuCa • or EuZr ′/EuHf ′) were analyzed. No considerable differences in the XANES spectra and in the proportions of Eu3+(A) and Eu3+(B) were observed in the single-doped samples. On the other hand, the shape of the XANES spectra and the Eu3+ proportion markedly differed in the codoped samples. Ga3+ codoping (GaZr ′/GaHf ′) achieved Eu3+(A) proportions of more than 90%, and La3+ codoping (LaCa •) resulted in the largest Eu3+(B) proportions of approximately 40%. In both PL and PL excitation spectra, the site-dependent spectral features were changed depending on the proportion of Eu3+(A) and Eu3+(B), especially the PL peaks at 595 nm were intensified most in the La3+ codoped samples. Consequently, the site-dependent Eu3+ PL features in CZO and CHO were found to become more conspicuous by the effect of Ga3+ or La3+ codoping. The results also revealed that the codoped ions work not only to compensate for the charge of aliovalent Eu3+ but also to drive Eu3+ to the intentional doping sites.
To achieve efficient Eu 3+ luminescence, site-selective doping of Eu 3+ was attempted in perovskite-type (ABO 3 ) SrMO 3 (M = Zr, Hf). In contrast to CaMO 3 and BaMO 3 , it was found that the doping sites of Eu 3+ (Eu Sr• or Eu Zr ′ /Eu Hf ′ ) could be highly controlled in SrMO 3 by a codoping technique. This was mainly because the size of Sr 2+ was suitable for the site-selective doping of Eu 3+ in SrMO 3 . The codoping of small Ga 3+ at B sites (Ga Zr ′ /Ga Hf ′ ) was carried out for A-site doping of Eu 3+ , whereas the codoping of large La 3+ at A sites (La Sr• ) or small Nb 5+ at B sites (Nb Zr• /Nb Hf • ) was conducted for B-site doping of Eu 3+ . As a result, the proportion of Eu 3+ at A and B sites became more than 90% by the Asite and B-site doping, respectively, from the analysis of X-ray absorption near-edge structures for the Eu L 3 edge. Eu 3+ ions were driven to intentional sites by codoping due to the dual effects of charge compensation and ionic size balance. The high Eu 3+ proportion at B sites, viz. the almost complete B-site doping, led to the findings of significant enhancement of Eu 3+ luminescence, which was derived from the magnetic dipole transitions from 5 D 0 to 7 F 1 states. The quantum efficiencies of the enhanced luminescence from SrMO 3 doped with Eu 3+ at B sites exceeded 50% at room temperature.
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+.
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