Synthesis, Local Structure And Optical Properties Of Eu³⁺ Ions In 12CaO·7Al₂O₃

The objective of this work was to develop a kind of inorganic compounds, which may be used as host material for broadband europium-doped red phosphors. In this thesis, analysis had been made about the relationships of composition, strcture and optical properties within 12CaO·7Al₂O₃ (C12A7) system.Eu-12CaO·7Al₂O₃ (C12A7) powders were prepared by chemical precipitation method. The X-ray diffraction (XRD), micro-Raman spectra, excitation sepctra,absorption spectra and photoluminescence (PL) were conducted to characterize the microstructure of C12A7:Eu powders. The result of XRD indicated that polycrystalline C12A7 powders of single cubic phase were obtained. The Raman spectra showed that the cage structural C12A7:Eu had been obtained. It further demonstrated that Eu3+ had been incorporated into C12A7 lattice site but not destroyed the lattice structure. From excitation spectra and absorption spectra, it revealed that Eu3+ existed in the host of C12A7 and caused the peaks which were fromed typical f-f transition of Eu3+ ion.The photoluminescence of C12A7: Eu in the visible region was observed by using 488 nm (2.54 eV) Ar+ line and 325 nm (3.82 eV) He-Cd line as excitation sources at room temperature. Intra-4f transitions of Eu3+ ions were investigated, the photoluminescence (PL) peaks around 578, 588, 614 nm were ascribed to the transition from the excited states 5D0 to the ground state 7FJ (J=0,1,2) transitions of Eu3+, respectively. The sharp and intense red emission bands with multi-peaks around 614 nm were corresponded to the 5D0→7F2 transition. Because two sharp lines was observed in the region for 5D0→7F0 transition in C12A7:Eu, this further indicated the presence of two different Eu3+ sites. By analyzing the structure of Ca atoms in C12A7, we could infer that Eu3+ in C12A7: Eu occupied the site of Ca2+. The forbidden transitions of intra-4f shell electrons in free Eu3+ ions have been allowed in C12A7 due to lack of the inversion symmetry in the Eu3+ position in C12A7 crystal field. It revealed that cation sites with low symmetry of the host were beneficial to the photoluminescence of Eu3+ ions.The temperature dependence of the PL for Eu-doped C12A7 was measured from 100 to 300 K, by the excitation of a non-resonant 488 nm laser. It was observed that the emission intensity of the Eu3+ ion in C12A7 had a maximum at a certain temperature. A unified theoretical model was given considering thermal activation and nonradiative energy transfer processes. Based on the model, all of the experimental data were well fitted. The luminescent intensity of Eu3+ ions in C12A7 excited at 325 nm also had a maximum, which might be attributed to the energy transfer process from defect states in C12A7 to Eu3+ ion. Our results suggested that C12A7 was an appropriate candidate for Eu doping. Simultaneously, the results suggested that C12A7: Eu was a candidate for applications in emissive display technology and lighting industry.