Development of rare-earth ion-doped oxide phosphors upon band gap excitation and elucidation of their emission mechanism

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(1) Photoluminescence of Pr^3+-doped Ruddlesden-Popper-type titanates, Sr_<n+1>Ti_nO_<3n+1>:Pr^<3+> (n=1, 2, ∞) and Ca_<n+1>Ti_nO_<3n+1>:Pr^<3+> (n=2)Perovskite-related Ruddlesden-Popper-type compounds, Sr_<n+1>Ti_nO_<3n+1>:Pr^<3+> (n=1, 2, ∞) and Ca_<n+1>Ti_nO_<3n+1>:Pr^<3+> (n=2) were prepared by a Pechini-type polymerizable complex method using a citric metal complex or conventional solid state reaction, and their photoluminescence properties were investigated. The compounds show distinct emission corresponding to the f-f transitions of Pr^<3+>. The excitation process is consistent with the transition between the valence band and the conduction band in host materials. Ca_3Ti_2O_7:Pr^<3+> exhibits red emission more than ten times stronger than Sr_3Ti_2O_7:Pr^<3+>. The differences in luminescent properties among the compounds originate from the structure dependence of the energy transfer from host to activator Pr^<3+>, and the site symmetry of Pr^<3+>.(2) Photoluminescence upon band g … More ap excitation in perovskite-type titanates R_<1/2>Na_<l/2>TiO_3:Pr (R=La, Gd, Lu, and Y)Pr^<3+>-doped perovskites R_<1/2>Na_<1/2>TiO3:Pr (R=La, Gd, Lu, and Y) were synthesized, and their structures, optical absorption and luminescent properties were investigated, and the relationship between structures and optical properties was discussed. Optical band gap of R_<1/2>Na_<1/2>TiO3 increases in the order R=La, Gd, Y, and Lu, which is primarily due to a decrease in band width accompanied by a decrease in Ti-O-Ti bond angle. Intense red emission assigned to f-f transition of Pr^<3+> from the excited ^1D_2 level to the ground ^3H_4 state upon the band gap photo-excitaion (UV) was observed for all compounds. The wavelength of emission peaks was red-shifted in the order R=La, Gd, Y, and Lu, which originates from the increase in crystal field splitting of Pr^<3+>. This is attributed to the decrease in inter-atomic distances of Pr-O together with the inter-atomic distances (R, Na)-O, i.e., increase in covalency between Pr and O. The results indicate that the luminescent properties in R_<1/2>Na_<1/2>TiO_3:Pr are governed by the relative energy level between the ground and excited state of 4f^2 for Pr^<3+>, and the conduction and valence band, which is primarily dependent on the structure, e.g., the tilt of TiO6 octahedra and the Pr-Ti inter-atomic distance and the site symmetry of Pr ion.(3) Occupation site of Pr and luminescent properties in Pr-doped perovskite-type lithium ion-conducting oxides, A_0.6Li_0.3Ti_0.5Ta_0.5O_3 : Pr (A=Ca, Sr)We investigated the lattice parameters and luminescent properties of Pr-doped perovskite-type lithium ion conductors, A_<0.6-3x/2>Pr_xLi_0.3Ti_0.5Ta_0.5O_3 (A=Ca and Sr, 0≦x≦0.05). Ca_<0.6>Li_0.3Ti_0.5Ta_0.5O_3:Pr shows the intense red emission corresponding to the f-f transition of Pr^<3+> (^1D_2 → ^3H_4) on the ultraviolet photo-excitation with the energy corresponding to the band-gap of host. In contrast, Sr_<0.6>Ti_<0.5>Ta_0.5O_3:Pr exhibites blue-green and red emission on the excitation with blue light corresponding to the f-f transition of Pr^<3+> (^3H_4 →^3P_4) . It was found that these differences in luminescent properties primarily originate from the occupation site of Pr in these perovskite-type oxides. Less