The 4f-5d transitions of Er3+ ions doped in crystals were widely studi ed due to their potential applications in quantum cutting phosphors and VUV lase rs,etc.The theory to do the calculations of 4f-5d transitions and various rela ted aspects,such as the ways to determining various parameters,were summarized .The impacts of various interactions on the spectra were also demonstrated clea rly with Er3+ ions in crystals CaF2 and LiYF4.Predicted results were compared w ith measured spectra.
Under the excitation of 980 nm diode laser, intense green emission （5F4＋5S2–5I8） of Ho3＋ was observed in Ho3＋ and Yb3＋ co-doped cubic Y2O3. The doping concentration and laser power dependence of the upconverted emission were studied. The decay curves of 5F4＋5S2 emission of Ho3＋ under the excitation of 355 nm pulse laser were measured to investigate the energy transfer process between Ho3＋ and Yb3＋. The results indicated that two-photon process was responsible for the upconversion （UC） emission. The Ho3＋ concentration of 0.04 mol.% and the Yb3＋ concentration of 5 mol.% were determined to be the best value for the strongest Ho3＋ emission under the excitation of 980 nm light. The cross-relaxation between two neighboring Ho3＋ ions and the back energy transfer from Ho3＋ to Yb3＋ were important factors for determin- ing the optimal doping concentration. This material was a promising candidate for the application in biomedical fluorescent labels for the intense green emission upon excitation of near-infrared （NIR） light.
Tb3＋ and Yb3＋ codoped Lu2O3 nanophosphors were synthesized by the reverse-strike co-precipitation method. The obtained Lu2O3：Tb3＋,Yb3＋ nanophosphors were characterized by X-ray diffraction （XRD） and photoluminescence （PL） spectra. The XRD results showed that all the prepared nanophosphors could be readily indexed to pure cubic phase of Lu2O3 and indicated good crystallinity. The Tb3＋→Yb3＋ energy transfer mechanisms in the UV-blue region in Lu2O3 nanophosphors were investigated. The experimental results showed that the strong visible emission around 543 nm from Tb3＋ （5D4→7F5） and near-infrared （NIR） emission around 973 nm from Yb3＋ （2F5/2→2F7/2） of Lu2O3：Tb3＋,Yb3＋ nanophosphors were observed under ultraviolet light excitation, respectively. Tb3＋ could be effectively excited up to its 4f75d1 state and relaxed down to the 5D4 level, from which the energy was transferred cooperatively to two neighboring Yb3＋. The Yb3＋ concentration dependent luminescent properties and lifetimes of both the visible and NIR emissions were also studied. The lifetime of the visible emission decreased with the increase of Yb3＋ concentration, verifying the efficient energy transfer from the Tb3＋ to the Yb3＋. Cooperative energy transfer （CET） from Tb3＋ to Yb3＋ was discussed as a possible mechanism for the near-infrared emission. When doped concentrations were 1 mol.% Tb3＋ and 2 mol.% Yb3＋, the intensity of NIR emission was the strongest.
We constructed an effective one-electron Hamiltonian by using the 4f/5d energies and eigenvectors obtained from the first-principles calculation with the relativistic self-consistent discrete variational Slater software package （DV-Xα）. From the effective Hamiltonian, we obtained the crystal-field and spin-orbit interaction parameters for the 4f and 5d electrons of lanthanide ions （Ce^3＋, Pr^3＋, Nd^3＋ and Eu^3＋） doped in YPO4, and these parameters were used to calculate the 4fN-4fN- 15d transition. Comparison with experiments shows that the obtained parameters are reasonable and the excitation spectra can be well predicted.
Tb and Yb codoped LUO nanophosphors were synthesized by the reverse-strike co-precipitation method.The obtaine...
Li Li~(1,2),Wei Xiantao~1,Chen Yonghu~1,Guo Changxin~1,Yin Min~(1*) (1.Department of Physics,University of Science and Technology of China,Hefei 230026, P R China) (2.College of Mathematics and Physics,Chongqing University of Posts and Telecommunications, Chongqing 400065,P R China)
The f-d transition of Ce3+ and Tb3+ in BaBPO5 was studied theoretically using the parametric Hamiltonian model. In order to overcome the difficulty in determining many of the parameter values, we adopted the model-space effective Hamiltonian method to determine the crystal-field parameters and spin-orbit parameters values. The method made use of the energies and eigenvectors, which were obtained from an ab initio calculation using the relativistic self-consistent discrete variational Slater software package (DV-Xα). Other parameters, which were less dependent on host crystals, were taken from published data. The calculated values of parameters were reasonable, and the energy-levels and f-d transition spectra agreed reasonably well with the measured excitation spectra of 5d-4f emission.
Nanocrystalline cubic Y2O3：Eu were prepared by combustion reaction.The crystal structure and morphology were analyzed by means of X-ray diffraction（XRD） and transmission electron microscopy（TEM）.The luminescent properties of the powder were investigated.The charge transfer band position showed redshift from 241 to 251 nm,which was related to the change of the local surroundings of Eu3＋ ions in nanocrystalline Y2O3：Eu.The ground-state electronic structure and charge transfer transition of both the bulk and nanocrystalline cubic Y2O3：Eu crystals were calculated by the ab initio self-consistent relativistic DV-Xα（discrete variational Xα） method.A complete 35-ion cluster was selected to simulate the local coordination surroundings of Eu doped in Y2O3 bulk crystals while five additional incomplete clusters were also selected to simulate the local surroundings of Eu ions in nanocrystals.It could be found that the charge transfer energies of the nanocrystalline Y2O3：Eu were less than that of the bulk counterpart,which was consistent with the redshift phenomenon of the CT band in the excitation spectrum of the nanocrystalline Y2O3：Eu.
All of the samples were synthesized by sol-gel methods.Two approaches to charge compensation,（i） 2Ca2＋→Yb3＋＋M＋,where M＋ is an alkali ion like Li＋,Na＋ and K＋,and（ii） indirect charge compensation：3Ca2＋→2Yb3＋＋vacancy,were studied in detail.It was found that charge compensation would be very beneficial for the growth of the grains,especially in Li＋ ions added samples.All the grains were homogeneously spherical with less boundaries;in addition,a great variety of the absorption ability in different charge compensation samples were observed：in comparison with the phosphors without charge compensation,indirectly charge compensated and Li＋ ions added phosphors showed much stronger absorption strength in the ultraviolet（UV） region whereas that of Na＋ and K＋ ions added samples was much weaker;moreover,measurements of the emission intensities showed that：in comparison with the phosphors without charge compensation,the visible emission intensity from MoO42-decreased a lot in indirectly charge compensated and Li＋ ions added phosphors,whereas there was a remarkable increase of the near infrared（NIR） emission intensity from Yb3＋ ions in the two types of samples under 266 nm excitation,implying more efficient energy transfer（ET） from MoO42-to Yb3＋ ions;at last,measurements and analysis of the decay curves of the visible 495 nm emission were carried out,and it was found that the energy transfer from MoO42-to Yb3＋ ions were more efficient in the two above types of phosphors.The theoretical quantum cutting（QC） efficiency was also improved greatly.Overall,the addition of Li＋ ions would be very beneficial for the morphology of the powders in addition to the growth of the grains.It was advantageous to increase the downconversion（DC） quantum efficiency;however,indirect charge compensation would enhance the NIR emission intensity to the most for its strongest absorption ability in the UV region.
