Effects of Codoping on Site-Dependent Eu³⁺ Luminescence in Perovskite-Type Calcium Zirconate and Hafnate

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); Eu_Ca^• or Eu_Zr^'/Eu_Hf^') 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 (Ga_Zr^'/Ga_Hf^') achieved Eu³⁺(A) proportions of more than 90%, and La³⁺ codoping (La_Ca^•) 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.