| Phosphors are the key raw material for white Light-emitting diodes (LED) to acquire high-quality light by lowering the correlated color temperature and improving color rening index. Rare earth activated alkaline silicate (or aluminate) can be used not only as phosphors, but also as precursors to synthesize new materials or as structure templates to develop new structures and properties. These silicate (or aluminate) host materials are abundant and possess various structures, however, other impurity phase is usually accompanied with the synthesis of the host materials, which is not favorable to improve the luminescent efficiency of phosphors. In addition, the luminous chromaticity of phosphors is dependent on the site occupation and symmetry of rare earth activators in crystal lattice, and not related to the impurity phase. In order to regulate luminescence and reveal the corresponding mechanism, the phase transformation, site occupation and controlled synthesis of orange-emitting M2SiO4:Eu (M=Sr,Ba,Ca) phosphors were firstly studied in this dissertation. Furthermore, we investigated the phase transition, the site occupation of Eu ion, tunable luminescence, band structure and luminescent mechanism of orange-emitting (Sr,Ba)3SiO5:Eu phosphors. Moreover, the luminescent properties of Eu2+ and Ce3+ in Li2SrSiO4 were also studied in detail, as well as the analysis of Li2SrSiO4 structure by using Eu3+ as spectra probe coupled with experimental characterization of X-ray diffraction, electron diffraction, EXAFS etc, and theoretical calculation. Finally, the red-emitting mechanism of Sr3Al3O6:Eu was discussed. The main results are listed as follows:(1) Sr2SiO4:Eu2+ phosphor has a low-temperature p-phase and a high-temperature?-phase. The increase of temperature within 1150-1250? promoted phase transition from a'-phase to ?-phase, thus the content of ?-phase increased; and the content of a'-phase would increase significantly when the temperature reached 1300?,however,little Sr3SiO5 impurity would presented simultaneously. In view of the situation, the nano-SiO2 was used to instead of conventional SiO2. The phase transition from a'-phase to p-phase was facilitated by increasing the content of nano-SiO2 within 1150-1250?,and the Sr3SiO5 impurity could be suppressed by the use of nano-SiO2 at 1300?,nevertheless, the material crystallinity decreased with increasing the content of nano-SiO2. Besides, a little nano-SiO2 was benefit to control the process of nucleation and growth, and enhance luminescence. Pure ?-phase was obtained when Sr was replaced with Ba, and pure ?-phase was obtained by the nano-Si3N4 partial substitution of conventional SiO2.(2) During the synthesis of Sr3SiO5:Eu, Sr2SiO4:Eu2+ impurity would appear, which was caused by the decomposition of materials in the cooling process. In Sr3SiO5:Eu phosphor, the Wyck.4c and 8f sites of Sr were randomly replaced by Eu. The mechanism, the emission of Sr3SiO5:Eu tunable by the dopping of Ba and Sm, was that the Eu valence state would be changed after dopping, or to be more exact, the dopping changed the depth of trap level. The orange-emitting of Sr3SiO5:Eu originated from the formation of Eu3+-alike quasi-particle state due to the combination of Eunu and holes.(3)The average long-range structure of Li2SrSi04 agreed with the symmetry of P3121 space group, while the short-range structure was consistent with the symmetry of C2 space group. Moreover, the hidden symmetry of the lithium compound was presented by using spectra probe.(4)The red-emitting mechanism of Sr3Al2O6:Eu was also originated from the Eu3+-alike quasi-particle state formed by the combination of Eu2+ and holes.In a word, all the aforementioned results established a solid base for the material structure and electronic structure. |
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