Synthesis, Local Structure And Optical Properties Of Tb³⁺ Doped 12CaO·7Al₂O₃

Tb-12CaO·7Al₂O₃ (C12A7) powders were prepared by chemical precipitation method. The X-ray diffraction (XRD), micro-Raman spectra, SEM were conducted to characterize the microstructure of C12A7:Tb powders. Absorption spectra, excitation spectra and emission spectra were used to study their optical properties.The results of XRD and Raman spectra indicated that the cage structural polycrystalline C12A7 powders of single cubic phase were obtained. It demonstrated that Tb³⁺ had been incorporated into C12A7 lattice site. SEM showed that C12A7:Tb³⁺ powders with the grain size of 200-300 nm were synthesized. Whereas one 100 nm-diameter hole in every grain of C12A7:Tb³⁺ powders could be observed with the concentration of 5 at.%. From absorption and excitation spectra, the absorption band of C12A7 and the peak formed typical 4f-5d transition of Tb³⁺ ions were observed.The photoluminescence of C12A7:Tb with different Tb³⁺ concentrations were observed under 280 nm UV excitation. For the lowest concentration of Tb³⁺, the spectra could be divided into two groups of lines: the blue emission below 470 nm was related to 5D3→7FJ transition whereas the green emission arose from 5D4→7FJ (J=6 to 3) transitions. At high Tb³⁺ concentration the green emission of Tb³⁺ due to 5D4→7FJ transitions dominated the emission spectra, which could be attributed to the cross-relaxation process. If two Tb³⁺ ions were closely located, a non-radiation energy transfer between these ions became possible. The cross-relaxation process produced the rapid population of the 5D4 level at the expense of 5D3, the PL originating from the 5D3 level (blue emission) was quenched and the PL originating from the 5D4 level (green emission) was enhanced.The study on the concentration quenching curve showed that the intensity of the emission from 5D3 decreased with increasing Tb³⁺ concentrations. However, the intensity of the emission from 5D4 had a maximum at a certain temperature. The energy transfers depopulating the 5D3 and 5D4 level were assigned by curve fitting. The electric dipole-dipole interaction was primary in 5D3→7F5 transition. For the 5D4→7F5 the energy transfer was an exchange interaction between two Tb³⁺ ions which were closed. The temperature dependence of the PL for Tb-doped C12A7 was studied under the excitation of a non-resonant 488 nm laser. It was observed that the emission intensity of the Tb³⁺ ion in C12A7 increased with the temperature increasing, indicating the high thermal stability of Tb-doped C12A7.Our results suggested that C12A7 was an appropriate candidate for Tb doping. Simultaneously, C12A7:Tb was a candidate for applications in display technology and lighting industry because of the strong green emission.