Coexistence Of Eu3 Research Articles

Enhancement of luminescence in Sr9MgK(PO4)7:Eu2+ phosphor by doping Ce3+ ions for white LEDs

Sr9MgK(PO4)7:Eu2+ has been reported to be a potential orange-yellow phosphor for UV-based w-LEDs. Sr9MgK(PO4)7:Eu2+ prepared in reducing atmosphere exhibit both broadband emission of Eu2+ and line emission of Eu3+ under UV light excitation. The coexistence of Eu3+ ions in the Sr9MgK(PO4)7:Eu2+ phosphor suppresses the Eu2+ luminescence and largely limits the luminescence performance. In this study, the reduction of Eu3+ to Eu2+ is successfully facilitated via doping Ce3+ ions. The amount of Eu3+ is greatly minimized and the effective number of Eu2+ ions is obviously increased in the crystal lattice. Under the 393 nm radiation excitation, the integrated luminescent intensity of Sr9MgK(PO4)7:0.01Eu2+,0.06Ce3+ is found to be about 163% that of Sr9MgK(PO4)7:0.01Eu2+. The reduction mechanism of Eu3+ to Eu2+ is discussed based on the optical analyses and the incorporation of other optical−inert rare earth ions such as Y3+, La3+, Gd3+, and Lu3+. Therefore, Sr9MgK(PO4)7:Eu2+,Ln3+ (Ln = Ce, Y, La, Gd, Lu) phosphors may have potential application as orange-yellow-emitting phosphors for w-LEDs. The strategy proposed in this work should be also effective for various luminescent materials containing mixed valent Eu2+/Eu3+.

Luminescence properties and application of Eu2+/Eu3+-codoped NaCa2Si4O10F phosphor

NaCa2-2xEu2xSi4O10F (x = 0.01, 0.03, 0.04, 0.05, 0.06) ceramic phosphors were synthesized via the traditional solid-state reaction method. The framework of NaCaSi4O10F constructed by the double chains of optically inert [Si4O10] ensures its rigid lattice strength. The sample prepared in air atmosphere showed the red luminescence transitions of 5D0→7FJ (J = 0–4). The coexistence of Eu3+ and Eu2+ centers was observed in the samples prepared in a reducing atmosphere, that is, there are two broad emission bands peaked at 420 nm (EuI2+) and 535 nm (EuII2+) corresponding to the 4f65 d1 → 4f7(8S7/2) transitions of Eu2+ centers together with the characteristic 4f→4f luminescent lines (570–720 nm) from the Eu3+ centers. The number of EuI2+ center is overwhelming, which replace the 6-coordinated Ca2+ in octahedral CaO5F. The weak EuII2+ center occupies the 7-coordinating cation site in CaO5F2 polyhedra. The temperature-dependent luminescence intensities of the Eu2+ and Eu3+ centers were investigated. EuI2+ has a good thermal stability on temperature quenching. EuII2+ was suggested to a metastable Eu3+ ion binding an electron released from cation vacancies VCa′′ in the close vicinity, i.e., (Eu3++e) ([EuII2+]*), which can show band luminescence. Therefore, EuII2+ centre has a low temperature quenching. This is a potential single candidate used for white lighting application by combining contributions from both Eu3+ and Eu2+ ions.

Highly stabile ZnGa2O4:Eu nanocrystals as a fluorescence probe for bio-imaging

Acetylacetonate (acac) passivated and europium incorporated ZnGa2O4 nanocrystals (ZGO:Eu NCs) were successfully prepared via a facile one-step non-injection process in oleylamine at a relatively low reaction temperature of 300 °C under N2 ambient. The resultant ZGO:Eu NCs exhibited a well-dispersed morphology with an average particle size of 4 nm. The enhanced and broad absorption band from 250 to 450 nm is attributed to the synergistic effect between the acac as an antenna and the coexistence of Eu3+ and Eu2+. Under 405 nm excitation, a full spectrum emission covering from 400 to 720 nm was observed. The sharp red emission coming from Eu(III) 5D0 → 7FJ (J = 1–4) transitions can be enhanced by adjusting Eu concentrations and reaches the optimum at 20 at% Eu. Subsequently, the as-prepared NCs were transferred into an aqueous media via a ligand exchange by polyethyleneimine (PEI) to obtain the hydrophilic ZGO:Eu@PEI NCs. The initial fluorescent properties are preserved well. Finally, the strong red fluorescence emission, suitable excitation window, high photo-stability, excellent water solubility, and prominent bio-compatibility make ZGO:Eu@PEI NCs as high quality bio-imaging probes successfully identify living KB cells. The method mentioned in this paper overcomes the bottle-neck of the inorganic bulk phosphors as probes in bio-imaging, and as a result, expands their applications in optical detection and medical diagnosis fields.

Eu3+→Eu2+ unusual reduction and bond energy in MAlSi2O6:Eu (M = Li, Na, K, Rb, Cs)

Eu-doped MAlSi2O6 (M = Li, Na, K, Rb, Cs) phosphors were synthesized by solid-state reaction in air. The crystal structure and optical properties of the phosphors were investigated by X-ray powder diffraction and photoluminescence. Their emission spectra are composed of some broad bands coming from the 5d-4f transition of Eu2+ and some sharp emission peaks coming from the f-f transitions of Eu3+. The co-existence of Eu3+ and Eu2+ cations is detected in MAlSi2O6:2%Eu, which shows that the unusual Eu3+→Eu2+ reduction process occurs in MAlSi2O6. The preferential occupancy of Eu3+ or Eu2+ in MAlSi2O6 can be calculated to be M+ sites by the bond-energy method. The deviation of the Eu3+–O2- bond energy is closed to that of the Eu2+–O2- bond energy, which means that Eu3+ and Eu2+ can coexist in air. The abnormal reduction of Eu3+ to Eu2+ can be explained by charge compensation model. The doped Eu3+ was induced to M+ and the vacancy (VM) may be formed. The vacancy(VM) acted as a donor of electrons, and the EuM∙∙ defect acted as an acceptor of electrons. The theoretical result is consistent with the experimental phenomenon.

Bond energy, preferential occupancy and spontaneous reduction ability of Eu3+ doped in CaAl2Si2O8

A bond-energy method is used to determine the site occupancy preference of Eu3+ or Eu2+ and further firstly extended to study the spontaneous reduction of Eu3+ doped CaAl2Si2O8 based on the bond valence model. The phosphor CaAl2Si2O8: Eu was synthesized under air condition. The co-existence of Eu3+ and Eu2+ cations was detected through photoluminescence measurement. The occupancy of Eu3+ or Eu2+ in CaAl2Si2O8 can be determined by comparing the deviation of its bond energy in different locations at Ca2+, Si4+ and Al3+ sites. Theoretical calculation result indicates that the Eu3+ and Eu2+ would preferentially occupy the smaller energy variation sites Ca4, which can be assigned to Eu4 site. The more important result is that the energy variation of Eu2+ at Eu4 sites is smaller than that of Eu3+ at the same sites, which indicates that Eu2+ is more stable than Eu3+ at Eu4 sites, so the spontaneous Eu3+→Eu2+ reduction can be found in the CaAl2Si2O8. The same bond energy method can be extended to explain the reason why the Sm3+ doped CaAl2Si2O8 can't be reduced into Sm2+ under reduction condition. The theoretical result is consistent with the experimental phenomenon. This should help us to have a further understanding of spontaneous Eu3+→Eu2+ reduction in the inorganic material.

Versatile luminescence of Eu^2+,3+-activated fluorosilicate apatites M_2Y_3[SiO_4]_3F (M = Sr, Ba) suitable for white light emitting diodes

Eu-doped fluorosilicate apatites M2Y3[SiO4]3F (M = Sr, Ba) are prepared by solid state reaction. Unlike conventional Eu-doped materials, coexistence of Eu3+ and Eu2+ ions is found from the photoluminescence of Eu-doped apatites M2Y3[SiO4]3F (M = Sr, Ba) which were prepared in reducing atmosphere. Eu2+ ions are converted from Eu3+ ions in the reduction process. It is suggested that Eu2+ ions occupy A(I) (4f) or A(II) (6h) crystallographic site in the apatite lattices. Intense emission lines due to Eu3+ are observed at 600-630 nm, while broad emission band due to Eu2+ is observed at 450-650 nm. These emissions combined with blue emission from LED are suitable to obtain white light, i.e., white LEDs for lighting and display. Different luminescence characteristics are obtained between Sr2Y3[SiO4]3F:Eu and Ba2Y3[SiO4]3F:Eu, which were prepared in reducing atmosphere.

Microcrystal Growth in Heat-Treating Process for Europium Ions Doped Aluminosilicate Glasses

Eu doped YAG glass ceramics containing single phase of YAG micro-cryatals have been obtained by heat-treating the aluminosilicate precursor glasses. The primary crystallite size of YAG phase is calculated to be ~46 nm and the crystal grains grow with preffered orientation (444) in the internal network. The average secondary particle size is estimated to be ~8μm and the larger grain size is due to the particle aggregation. The pilotaxitic texture of YAG glass ceramics presented in SEM micrograph, can increase yield strength and tensile strength of the sample effectively. The emission spectrum of Eu doped YAG glass ceramics consists of a broad blue emission band and several narrow emission bands, indicating the coexistence of Eu3+ and Eu2+ ions in YAG glass ceramics.

In situ analysis for spontaneous reduction of Eu3+ in LiCl pyroprocessing media at 923 K

The spontaneous reduction of Eu3+ to Eu2+ was examined when EuCl3 was added into a pyroprocessing media of LiCl molten salt at 923 K. The amount of Eu2+ was calculated by measuring the total charge consumed to oxidize Eu2+ ions to Eu3+ ions. The concentration ratio of Eu2+ to Eu3+ was estimated to be about 0.40 in the media. In addition, it is confirmed that the reduction of Eu3+ to Eu2+ is caused by the oxidation power of Cl− to Cl2. The coexistence of Eu3+ and Eu2+ in the LiCl molten salt system was examined by UV–Visible and luminescence spectroscopy. The molar absorptivities of Eu3+ and Eu2+, calculated from UV–Visible absorption spectra, were 423 and 1954 M−1 cm−1, respectively.