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.     

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.     

A new BaB2Si2O8:Eu/Eu, Tb phosphor - Synthesis and photoluminescence properties

In the present work, we have synthesized maleevite mineral phase BaB₂Si₂O₈ for the first time, which is isostructural with the pekovite mineral SrB₂Si₂O₈. In these europium doped host lattices, we observed the partial reduction of Eu³⁺ to Eu²⁺ at high temperature during the synthesis in air. Tb³⁺ co-doping in MB₂Si₂O₈:0.01(Eu³⁺/Eu²⁺) [M=Sr, Ba] improves the emission properties towards white light. The emission color varies from bluish white to greenish white under UV lamp excitation when the host cation changes from Sr to Ba.     

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.     

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.     

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.     

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。     

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO₂NO₃, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2) pm for M=Pb; a=396.337(5), c=1412.83(3) pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1) pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3) pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO₂]⁺ and [NO₃]⁻ layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M²⁺ cations in BiMO₂NO₃ are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)₂Sr₂Ca_n−1Cu_nO_x.     

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.     

Sensitized red luminescence from Ce, Mn-doped glaserite-type alkaline-earth silicates

Bright red luminescence is observed from Ce, Mn-doped glaserite-type alkaline-earth silicates with M₂BaMgSi₂O₈ (M: Ba, Sr, Ca) chemical composition. Under UV excitation, Ce-doped M₂BaMgSi₂O₈ exhibits strong near-UV emission with asymmetric peak shape. UV-excited Mn-doped M₂BaMgSi₂O₈ compounds show visible red emission only when Ce³⁺ ions are doped together. These results indicate that Mn²⁺-derived red emission is caused by an efficient energy transfer from Ce³⁺ to Mn²⁺. The red emission becomes intense with an increase in Ba-amount. This trend originates from the relaxation of the selection rule for 3d-3d transition in Mn²⁺ ions, which is caused by the structural deformation due to Ba²⁺ occupation for layer-pockets.     

Structure and photoluminescence properties of Ce-doped novel silicon-oxynitride Ba4−zMzSi8O20−3xN2x (M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce³⁺-doped novel silicon-oxynitride phosphors of Ba_4−zM_zSi₈O_20−3xN_2x (M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba_4−zM_zSi₈O_20−3xN_2x oxynitride were synthesized by partial substitutions of 3O²⁻→2N³⁻ and Ba→M (M=Mg, Ca, Sr) in orthorhombic Ba₂Si₄O₁₀. The influences of the type of alkaline earth ions of M, the Ce³⁺ concentration on the photoluminescence properties and thermal quenching behaviors of Ba₃MSi₈O_20−3xN_2x (M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba₃MSi₈O_20−3xN_2x:Ce³⁺ (x=0.5) exhibits efficient blue emission centered at 400-450 nm in the range of 350-650 nm owing to the 5d→4f transition of Ce³⁺. The emission band of Ce³⁺ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce³⁺ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce³⁺ ions. Among the silicon-oxynitride phosphors of Ba₃MSi₈O_18.5N:Ce³⁺, M=Sr_0.6Ca_0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce³⁺.     

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.     

Structural investigation of Eu emissions from alkaline earth zirconium phosphate

Eu²⁺ doped A_0.5Zr₂(PO₄)₃ (A=Ca, Sr, Ba) phosphors with the NASICON structure were synthesized by a co-precipitation method. Their photoluminescent and structural properties were investigated by photoluminescent spectroscopy and powder X-ray Rietveld analysis, which determined two sites for Eu²⁺ ions in the host structure, 3a and 3b. The Eu-O bond lengths were increased by changing alkaline earth ions from Ca to Ba, causing Eu²⁺ emission bands to shift from blue-green to blue. A correlation was observed between the peak wavelength positions and the Eu-O bond length. The photoluminescent properties are discussed in terms of crystal field strength and nephelauxetic effect, and a schematic diagram of Eu²⁺ emissions is proposed for the Eu²⁺ doped NASICON phosphor.     

Crystal structure and properties of the new vanadyl(IV)phosphates Na2MVO(PO4)2, M=Ca and Sr

Two new complex vanadyl(IV)phosphates Na₂MVO(PO₄)₂ (M=Ca, Sr) were synthesized in evacuated quartz ampoules and investigated by means of X-ray diffraction, electron microscopy, DTA, ESR and magnetic susceptibility measurements. The crystal structure of Na₂SrVO(PO₄)₂ was solved ab initio from X-ray powder diffraction data. Both compounds are isostructural: a=10.5233(3) Å, b=6.5578(2) Å, c=10.0536(3) Å and a=10.6476(3) Å, b=6.6224(2) Å, c=10.2537(3) Å for Ca and Sr, respectively; S.G. Pnma, Z=4. The compounds have a three-dimensional structure consisting of V⁴⁺O₆ octahedra connected by PO₄ tetrahedra via five of the six vertexes forming a framework with cross-like channels. The strontium and sodium atoms are located in the channels in an ordered manner. Electron diffraction as well as high-resolution electron microscopy confirmed the structure solution. The new vanadylphosphates are Curie-Weiss paramagnets in a wide temperature range down to 2 K with θ=12 and 5 K for Ca and Sr phases, respectively.     

Structural studies of five layer Aurivillius oxides: A2Bi4Ti5O18 (A=Ca, Sr, Ba and Pb)

The room temperature structures of the five layer Aurivillius phases A₂Bi₄Ti₅O₁₈ (A=Ca, Sr, Ba and Pb) have been refined from powder neutron diffraction data using the Rietveld method. The structures consist of [Bi₂O₂]²⁺ layers interleaved with perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks. The structures were refined in the orthorhombic space group B2eb (SG. No. 41), Z=4, and the unit cell parameters of the oxides are a=5.4251(2), b=5.4034(1), c=48.486(1); a=5.4650(2), b=5.4625(3), c=48.852(1); a=5.4988(3), b=5.4980(4), c=50.352(1); a=5.4701(2), b=5.4577(2), c=49.643(1) for A=Ca, Sr, Ba and Pb, respectively. The structural features of the compounds were found similar to n=2-4 layers bismuth oxides. The strain caused by mismatch of cell parameter requirements for the [Bi₂O₂]²⁺ layers and perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks were relieved by tilting of the TiO₆ octahedra. Variable temperature synchrotron X-ray studies for Ca and Pb compounds showed that the orthorhombic structure persisted up to 675 and 475 K, respectively. Raman spectra of the compounds are also presented.     

A large series of isotypic Mo(V) diphosphates with a tunnel structure: From A(MoO)10(P2O7)8 with A=Ba, Sr, Ca, Cd, Pb to A(MoO)5(P2O7)4 with A=Ag, Li, Na, K

The single crystal structure of a series of nine isotypic Mo(V) diphosphates was determined from crystals with composition A²⁺(MoO)₁₀(P₂O₇)₈ (A=Ba, Sr, Ca, Cd, Pb) and A⁺(MoO)₅(P₂O₇)₄ (A=Ag, Li, Na, K). The structure of those phosphates, built up of corner sharing MoO₆ octahedra, MoO₅ tetragonal pyramids and P₂O₇ diphosphates groups, forms eight-sided tunnels as described by Lii et al. for A=Ag. New features are evidenced: (1) existence of two orientations, up and down along b for the MoO₅ pyramids; (2) maximum insertion rate of the divalent cations which is twice less than that of the univalent cations; (3) different behavior of the series “Pb, Sr, Ba, Li, Na, K” which exhibits only one kind of site for the inserted cation, compared to the “Cd, Ca, Ag” series for which two kinds of sites are observed; (4) off-centering of the A-site cations with respect to the tunnel axis; and (5) unusually high thermal factors along the tunnel axis, but absence of ionic conductivity.     

Synthesis, crystal structure and optical investigation of the new phosphates: Na7Mg13Ln(PO4)12 (Ln=La, Eu)

Two new isostructural rare earth phosphates Na₇Mg₁₃Ln(PO₄)₁₂ (Ln=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the Cmc2₁ space group (Z=4). The crystal structure exhibits a new type of framework built up from LnO₈ (Ln=La, Eu), MO₆ (M=0.5Mg+0.5Na) and MgO_x (x=5, 6) polyhedra and PO₄ tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg₄MP₄O₂₂]_∞ layers stacked along the a direction. These layers are interconnected by [Mg₄LnP₄O₃₆]_∞ undulating chains spreading along the b direction. This framework delimits 6 distinct cavities occupied by Na⁺ cations. The results of the optical study of Na₇Mg₁₃La_1−xEu_x(PO₄)₁₂ (x=0, 0.02, 0.1, 1) reveal the presence of two different Eu³⁺ ion environments whereas the X-ray study predicts the existence of only one Eu site. This difference can be explained by the possible presence of the europium element in the sodium sites with small occupancies which cannot be detected by the X-ray structural determination.     

Photoluminescence properties of a novel phosphor, Na3La9O3(BO3)8:RE(RE=Eu,Tb)

The new oxyborate phosphors, Na₃La₉O₃(BO₃)₈:Eu³⁺ (NLBO:Eu) and Na₃La₉O₃(BO₃)₈:Tb³⁺ (NLBO:Tb) were prepared by solid-state reactions. The photoluminescence characteristics under UV excitation were investigated. The dominated emission of Eu³⁺ corresponding to the electric dipole transition ⁵D₀→⁷F₂ is located at 613 nm and bright green luminescence of NLBO:Tb attributed to the transition ⁵D₄→⁷F₅ is centered at 544 nm. The concentration dependence of the emission intensity showed that the optimum doping concentration of Eu and Tb is 30% and 10%, respectively.     

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.     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.