Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

Fluorescence and phosphorescence properties of the low temperature forms of the MAl2Si2O8:Eu (M=Ca, Sr, Ba) compounds

The fluorescence and phosphorescence properties of Europium-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) are reinvestigated and discussed on the basis of the propensity of an activator to agglomerate with an oxygen vacancy. Due to a stronger attraction of the anion vacancy towards Eu²⁺ cations going from BaAl₂Si₂O₈ to SrAl₂Si₂O₈ and CaAl₂Si₂O₈ host lattices, the interpretation of the fluorescence spectra turns out to be less trivial in the Ca and Sr host lattices than in the Ba one and requests the account for Eu²⁺ cations lying at alkaline-earth sites with or without vacancy in their neighborhood. Phosphorescence in these compounds is highlighted.     

Crystal structure of Na2MMgP2O8 (M: Ba, Sr, Ca) orthophosphates and their luminescence properties activated by Eu; analogous structural behaviors of glaserite-type phosphates and silicates

Rietveld refinements of X-ray powder diffraction data and vibrational spectroscopy have confirmed the crystal structure of Na₂MMgP₂O₈ (M: Ba, Sr, Ca) prepared by a standard solid state reaction. They have glaserite-type layered structure. Na₂MMgP₂O₈ has a trigonal P3? form for M=Ba, and monoclinic P2₁/c forms for M=Sr and Ca. The observed structural transition is analogous to the corresponding layered orthosilicate M₃MgSi₂O₈.Eu²⁺-doped Na₂MMgP₂O₈ exhibits an intense blue to violet emission under ultraviolet excitation, based on 5d-4f electron transition of Eu²⁺ ions. The emission character is very sensitive to the structural transition induced by M²⁺ and the subsequent site symmetry changes.     

Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

Electronic properties and rare-earth ions photoluminescence behaviors in borosilicate: SrB2Si2O8

Undoped and RE ions doped SrB₂Si₂O₈ were successfully synthesized. After the application of UV and VUV spectroscopy measurements, we made a novel discovery that the emission of SrB₂Si₂O₈:Eu prepared in air can be switched between red and blue by the different excitations. The information is that quite a part of Eu³⁺ was spontaneously reduced to Eu²⁺ in air. The PL properties of Eu²⁺ in VUV and Eu³⁺, Ce³⁺ and Tb³⁺ in UV-VUV region in SrB₂Si₂O₈ were evaluated for the first time. The excitation mechanisms of the O²⁻-Eu³⁺ CT, Ce³⁺f-d and Tb³⁺f-d transitions in UV region as well as the Eu³⁺f-d, O²⁻-Ce³⁺ CT, O²⁻-Tb³⁺ CT transitions and the host lattice absorption in VUV region were established. In addition, first principles calculation within the LDA of the DFT was applied to calculate the electronic structure and linear optical properties of SrB₂Si₂O₈ and the results were compared with the experimental data.     

Preparation and photoluminescence properties of Mn-activated M2Si5N8 (M=Ca, Sr, Ba) phosphors

Mn²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) phosphors have been prepared by a solid-state reaction method at high temperature and their photoluminescence properties were investigated. The Mn²⁺-activated M₂Si₅N₈ phosphors exhibit narrow emission bands in the wavelength range of 500-700 nm with peak center at about 599, 606 and 567 nm for M=Ca, Sr, Ba, respectively, due to the ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Mn²⁺. The long-wavelength emission of Mn²⁺ ion in the host of M₂Si₅N₈ is attributed to the effect of a strong crystal-field of Mn²⁺ in the nitrogen coordination environment. Also it is observed that there exists energy transfer between M₂Si₅N₈ host lattice and activator (Mn²⁺). The potential applications of these phosphors have been pointed out.     

Crystal structure of BaMg2Si2O7 and Eu luminescence

Crystal structure of BaMg₂Si₂O₇ was determined and refined by a combined powder X-ray and neutron Rietveld method (monoclinic, C2/c, no. 15, Z=8, a=7.24553(8) Å, b=12.71376(14) Å, c=13.74813(15) Å, β=90.2107(8)°, V=1266.44(2) Å³; R_p/R_wp=3.38%/4.77%). The structure contains a single crystallographic type of Ba atom coordinated to eight O atoms with C₁ (1) site symmetry. Under 325-nm excitation Ba_0.98Eu_0.02Mg₂Si₂O₇ exhibits an asymmetric emission band around 402 nm. The asymmetric shape of the emission band is likely associated with a small electron-phonon coupling in BaMg₂Si₂O₇. The integrated intensity of the emission band was observed to remain constant over the temperature range 4.2-300 K.     

The effect of replacement of Sr by Ca on the structural and luminescence properties of the red-emitting Sr2Si5N8:Eu LED conversion phosphor

The influence of the replacement of Sr by Ca on structural and luminescence properties of Eu²⁺-doped Sr₂Si₅N₈ is reported. The Rietveld refinement of the powder X-ray diffraction data shows that the Ca²⁺ ion preferentially occupies the larger Sr site in Sr₂Si₅N₈:Eu²⁺. Although the excitation spectrum is hardly modified, the position of the emission band of Eu²⁺ can be tailored through partial replacement of Sr by Ca in Sr₂Si₅N₈:Eu²⁺, resulting in red-emission shifting from 620 to 643 nm. Furthermore, (Sr, Ca)₂Si₅N₈:Eu²⁺ shows high potential as a conversion phosphor for white-light LED applications due to similar absorption, conversion efficiency and thermal quenching behaviour for 465 nm excitation after the introduction of the Ca ion.     

Eu luminescence in La5Si2BO13 with apatite related structure and magnetic studies in Ln5Si2BO13 (Ln=Gd, Dy)

Eu³⁺ photoluminescence is studied in La₅Si₂BO₁₃ with apatite related structure. La_5−xEu_xSi₂BO₁₃ [x=0.05, 0.1, 0.3, 0.5, 0.7, 1.0, 2.0] compositions are synthesized. The emission results shows that Eu³⁺ ions occupy two different cationic sites viz., La(1) and La(2). The increase in the intensity of ⁵D₀-⁷F₀ line with increasing Eu³⁺ content shows the preferential occupancy of Eu³⁺ in La(2) site due to the existence of short La(2)-O(4) (free oxide ion) bond. The observation of antiferromagnetic interactions in Gd and Dy analogues supports the structural features elucidates from photoluminescence studies.     

Host-sensitized emission of LiInW2O8 wolframites: From red-Eu to white-Dy phosphors

The LiInW₂O₈:Eu³⁺, LiInW₂O₈:Dy³⁺ and LiInW₂O₈:Eu³⁺/Dy³⁺ phosphors were synthesized by solid-state reaction and their photoluminescence properties were studied. Under UV excitation, the LiInW₂O₈:Eu³⁺ phosphor exhibits an intense red emission whereas the LiInW₂O₈:Dy³⁺ and LiInW₂O₈:Dy³⁺/Eu³⁺ phosphors show a white emission. The WO₆ octahedra play a major role in the luminescence of the host lattice, characterized by a blue emission under UV excitation. The emission of activator ion results from an efficient energy transfer from the LiInW₂O₈ host lattice to the Eu³⁺ and Dy³⁺ ions. The LiIn_0.97Dy³⁺_0.03W₂O₈ and LiIn_0.965 Dy³⁺_0.03Eu³⁺_0.005W₂O₈ samples, optimized for white emission, are interesting candidates for solid-state lighting applications.     

A study of magnetic interactions in M2EuRuO6 (M = Ca,Sr, Ba) by 151Eu Mössbauer spectroscopy

The series of compounds M₂EuRuO₆ (M = Ca, Sr, Ba) has been studied by ¹⁵¹Eu Mössbauer spectroscopy. X-Ray data show them to be structurally derived from the ABO₃ perovskite lattice, but only the Ba compound gives positive evidence to suggest ordering of the $\text{Eu}{}^{\mathrm{3+}}\text{Ru}{}^{\mathrm{5+}}$ cations. The ¹⁵¹Eu resonance shows magnetic hyperfine splitting at 4.2 K. The Ru⁵⁺OEu³⁺ORu⁵⁺ exchange takes place by admixture of low-lying excited states into the diamagnetic J = 0 ground-state of the Eu³⁺. The Curie temperatures are approximately 18, 31, and 42 K for the Ca, Sr, and Ba compounds. Detailed analysis shows that substantial disorder of cations occurs, being quite large for Ca, <8% for Sr, and <5% for Ba. However, it appears that considerable canting of the Ru⁵⁺ spins takes place in the Ba compound immediately below the Curie temperature as a result of the disorder and low anisotropy at the Ru sites. This effect is much reduced in the more distorted Sr compound.     

Role of crystallite size on the photoluminescence properties of SrIn2O4:Eu phosphor synthesized by different methods

Photoluminescence (PL) of Eu³⁺ was studied in SrIn₂O₄ host lattice. A complete solid solubility of Eu³⁺ has been found in the series SrIn_2−xEu_xO₄ [x=0-2.0]. The phase formation at a relatively low temperature and in a very short duration was achieved by combustion synthesis (CS). Concentration quenching of luminescence has been observed in SrIn_2−xEu_xO₄ [x=0.1-2.0] and the critical concentration for maximum emission was found to be with x=0.3. In order to find the role of crystallite size on the PL properties of SrIn₂O₄:Eu³⁺, the results obtained with phosphors synthesized by solid state reaction (SSR) and CS methods were compared.     

A re-investigation of the crystal structure and luminescence of BaCa2MgSi2O8:Eu

Barium calcium magnesium silicate (BaCa₂MgSi₂O₈), a compound whose space group was obtained via X-ray diffraction data, was re-investigated using neutron diffraction techniques. A combined powder X-ray and neutron Rietveld method revealed that BaCa₂MgSi₂O₈ crystallizes in the trigonal space group P3? (Z=1, a=5.42708(5) Å, c=6.79455(7) Å, V=173.310(4) Å³; R_p/R_wp=5.52%/7.63%), instead of the previously believed space group P3?m1. The difference in the two structures arises from the displacement of the O2 atom. Blue emission from Ba_0.98Eu_0.02Ca₂MgSi₂O₈ under 325-nm excitation is ascribed to the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ ions at Ba sites and Ca sites. Site assignment of Eu²⁺ ions in the titled compound was performed by analysis of emission spectra at temperatures in the range of 4.2-300 K.     

Defect chemistry and VUV optical properties of the BaMgAl10O17:Eu-Ba0.75Al11O17.25:Eu solid solution

The optical properties of the BaMgAl₁₀O₁₇:Eu²⁺ (BAM)-Ba_0.75Al₁₁O_17.25:Eu²⁺ (BAL) solid solution have been studied using VUV excitation, emission and reflectance spectroscopy. Three unique Eu²⁺ emission centers are observed in a ratio that depends on the composition of the host and the dopant concentration. Two of the emission centers are assigned to Eu on normal Beevers-Ross sites and Eu on anti Beevers-Ross sites. The defect chemistry of this system is modeled based on the known behavior of the spinel (MgO·nAl₂O₃) system. Based on this model, the third Eu center can be assigned either to Eu near Al vacancies or to Eu associated with O atoms in the cation layer. In undoped materials exciton emission is observed, peaking at 263 nm in BAM and 285 nm in BAL. This emission may be the mechanism of host-to-activator energy transfer in these phosphors.     

M5(PO4)3F (M=Ca, Sr, Ba)中Sm3+的电荷迁移态及Sm3+和Eu3+的电荷迁移能量关系

采用高温固相反应合成了M_5-2xSm_xNa_x(PO₄)₃F(M=Ca,Sr,Ba)荧光体,研究了其在真空紫外-可见光范围的发光特性。发现在Ca₅(PO₄)₃F中Sm³⁺的电荷迁移带约在191 nm,在Sr₅(PO₄)₃F中约在199 nm,而在Ba₅(PO₄)₃F中约在204 nm,随着被取代碱土离子半径的增大电荷迁移能量逐渐减小。比较了M₅(PO₄)₃F (M=Ca,Sr,Ba)中Sm³⁺和Eu³⁺电荷迁移能量的关系。     

(Ca,Me)La4Si3O13:Eu3+(Me=Sr,Ba)荧光粉的结构和发光特性

采用高温固相法制备了(Ca,Me)La4Si3O13∶Eu3+(Me=Sr,Ba)系列红色荧光体,考察了Eu3+掺杂浓度和Sr2+,Ba2+置换对荧光体结构和发光特性的影响。Eu3+最佳掺杂浓度为nEu3+∶nLa3+=1∶7,5D0-7F2与5D0-7F1跃迁发射强度比为2.55。Eu3+掺杂使晶胞参数a和c呈线性变小,对c的影响大于a,使a/c比增大。Sr2+和Ba2+分别置换基质中的Ca2+可以形成完全固溶体,晶胞参数随Sr2+或Ba2+的置换量增加呈线性增大,使a/c比减小。各发射峰强度在Sr2+置换量为0.4 mol时出现极大值,但随Ba2+置换量的增加而不断增强,全置换后荧光强度最大。荧光体的色坐标为(0.638 5,0.353 0)。     

Vacuum Ultraviolet Excited Photoluminescence Properties of Y2O2S：Eu^3＋,Bi^3＋ Phosphor

As an Hg-free lamp using phosphor, the Bi^3＋ and EH^3＋ co-doped Y2O2S phosphors were prepared and their luminescence properties under vacuum ultraviolet（VUV） excitation were investigated. The VUV photoluminescent intensity of Y2O2S：Eu^3＋ was weak, however, considerably stronger red emission at 626 nm with good color purity was observed in Y2O2S：Eu^3＋,Bi^3＋ systems. Investigation on the photoluminescence reveals that the strong VUV luminescence of Y2O2S：Eu^3＋,Bi^3＋ at 147 nm is mainly because the Bi^3＋ acts as a medium and effectively performs the energy transfer process： Y^3＋-O^2-→Bi^3＋→Eu^3＋, while the intense emission band at 172 nm is attributed to the absorption of the characteristic ^1So-^1P1 transition of Bi^3＋ and the direct energy transfer from Bi^3＋ to Eu^3＋. The Y2O2S：Eu^3＋,Bi^3＋ shows excellent VUV optical properties compared with the commercial （Y,Gd）BO3：Eu^3＋. Thus, the Y2O2S：Eu^3＋,Bi^3＋ can be a potential red VUV-excited candidate applied in Hg-free lamps for backlight of liquid crystal display.     

Research on Molecular Structure and Electronic Properties of Ln3+ (Ce3+, Tb3+, Pr3+)/Li+ and Eu2+ Co-Doped Sr2Si5N8 via DFT Calculation

We use density functional theory (DFT) to study the molecular structure and electronic band structure of Sr₂Si₅N₈:Eu²⁺ doped with trivalent lanthanides (Ln³⁺ = Ce³⁺, Tb³⁺, Pr³⁺). Li⁺ was used as a charge compensator for the charge imbalance caused by the partial replacement of Sr²⁺ by Ln³⁺. The doping of Ln lanthanide atom causes the structure of Sr₂Si₅N₈ lattice to shrink due to the smaller atomic radius of Ln³⁺ and Li⁺ compared to Sr²⁺. The doped structure’s formation energy indicates that the formation energy of Li⁺, which is used to compensate for the charge imbalance, is the lowest when the Sr2 site is doped. Thus, a suitable Li⁺ doping site for double-doped lanthanide ions can be provided. In Sr₂Si₅N₈:Eu²⁺, the doped Ce³⁺ can occupy partly the site of Sr₁²⁺ ([SrN₈]), while Eu²⁺ accounts for Sr₁²⁺ and Sr₂²⁺ ([SrN₁₀]). When the Pr³⁺ ion is selected as the dopant in Sr₂Si₅N₈:Eu²⁺, Pr³⁺ and Eu²⁺ would replace Sr₂²⁺ simultaneously. In this theoretical model, the replacement of Sr²⁺ by Tb³⁺ cannot exist reasonably. For the electronic structure, the energy level of Sr₂Si₅N₈:Eu²⁺/Li⁺ doped with Ce³⁺ and Pr³⁺ appears at the bottom of the conduction band or in the forbidden band, which reduces the energy bandgap of Sr₂Si₅N₈. We use DFT+U to adjust the lanthanide ion 4f energy level. The adjusted 4f-CBM of Ce_Sr1Li_Sr1-Sr₂Si₅N₈ is from 2.42 to 2.85 eV. The energy range of 4f-CBM in Pr_Sr1Li_Sr1-Sr₂Si₅N₈ is 2.75–2.99 eV and its peak is 2.90 eV; the addition of Ce³⁺ in Eu_Sr1Ce_Sr1Li_Sr1 made the 4f energy level of Eu²⁺ blue shift. The addition of Pr³⁺ in Eu_Sr2Pr_Sr2Li_Sr1 makes part of the Eu²⁺ 4f energy level blue shift. Eu²⁺ 4f energy level in Eu_Sr2Ce_Sr1Li_Sr1 is not in the forbidden band, so Eu²⁺ is not used as the emission center.     

Photoluminescence of new red phosphor SrZnO2:Eu

SrZnO₂:Eu³⁺ has been synthesized by solid-state reaction and its photoluminescence in ultraviolet (UV)-vacuum ultraviolet (VUV) range was investigated. The broad bands around 254 nm are assigned to CT band of Eu³⁺-O²⁻. With the increasing of Eu³⁺ concentration, Eu³⁺ could occupy different sites, which leads to the broadening of CT band. A sharp band is observed in the region of 110-130 nm, which is related to the host absorption. The phosphors emit red luminescence centered at about 616 nm due to Eu³⁺⁵D₀→⁷F₂ both under 254 and 147 nm, but none of Eu²⁺ blue emission can be observed.     

Multiband orange-red photoluminescence of Eu ions in new “114” LnBaZn3GaO7 and LnBaZn3AlO7 oxides

A new series of gallozincates LnBaZn₃GaO₇ (Ln=La, Nd, Sm, Eu, Gd, Dy, Y) and new aluminozincates LnBaZn₃AlO₇ (Ln=Y, Eu, Dy) have been synthesized. Their structure refinements show that these phases belong to the “114” series, with hexagonal P6₃mc space group previously described for SmBaZn₃AlO₇. The photoluminescence study of these oxides shows that the Eu³⁺ activated LnBaZn₃MO₇ oxides with Ln=Y, La, Gd; and M=Al, Ga exhibit strong magnetic and electric dipole transitions (multiband emission) which is of interest for white light production. These results also confirm that the site occupied by Eu³⁺ is not strictly centrosymmetric. The electric dipole transition intensity is the highest in GdBaZn₃MO₇ [M=Al, Ga]: 0.05Eu³⁺ as compared with other Eu³⁺ activated compositions. This is due to the layer distortion around GdO₆ octahedra when compared with YO₆ and LaO₆ octahedra.