Local structure of Gd3+ impurity center at cubic sites in fluorites

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of¹⁹F nuclei in four anionic spheres of cubic MeF₂:Gd³⁺ (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd³⁺ cubic impurity center in crystals MeF₂ (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd³⁺-F^?. For the first time not only radial displacements but also angular displacements of F^? ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.     

Local structure of bivalent copper centers in SrF2 crystals: An ENDOR spectroscopic study

Structural distortions of the SrF₂ crystal lattice near the bivalent copper impurity Jahn-Teller center are investigated by the ENDOR method (v=9.3 GHz, T=4.2 K). The approximate directions and the magnitudes of displacements of a Cu²⁺ impurity ion and its surrounding F^? ions are determined with respect to one of the anionic networks in the crystal matrix. The tensor components for the ligand hyperfine interaction (LHFI) with fluorine ions separated from the impurity by a distance R≤6.2 Å are obtained from the angular dependences of the location of the ENDOR resonance lines. It is found that the parameters of magnetic interactions between the impurity and these ligands contain the contributions determined by the covalence of bonds in the impurity complex and the polarization of electron shells of the ligands.     

Ligand hyperfine Yb3+ interaction in CsCaF3 and Cs2NaYF6 crystals

The operator, which determines the contribution of processes including the polarization of central ion core to the ligand hyperfine interaction (LHFI), has been obtained in the second quantization representation in the basis of partially nonorthogonal orbitals. The contributions of the impurity Yb³⁺ ion to the LHFI parameters have been also calculated in Cs₂NaYF₆ and CsCaF₃ crystals, which are determined by the mechanisms studied earlier. The single-particle orbital basis has been extended as compared to the previous work. There is a sufficiently good agreement with the experiment.     

Ligand electron-nuclear double resonance of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> Trigonal Gd<Superscript>3+</Superscript> centers in CaF<Subscript>2</Subscript> with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T₁ trigonal ¹⁵⁷Gd³⁺ centers in the CaF₂ compound. It is experimentally found that the nearest environment of an impurity center contains only one ¹⁹F ion. Anions in the other coordination shells are identical to those in the pure CaF₂ crystal. However, ¹⁹F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for ¹⁹F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF₂ crystal are determined. It is demonstrated that the unusual isotropic LHFI constant A _s >0 for Gd³⁺ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T₁ centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for ¹⁵⁷Gd³⁺ centers.     

Preparation and luminescent properties of phosphor MGd2(MoO4)4: Eu (M=Ca, Sr and Ba)

Intense red emitting phosphors MGd₂(MoO₄)₄: Eu³⁺ (M=Ca, Sr and Ba) have been synthesized by the simple sol-gel technique. The formation processes and the phase impurity of phosphors are characterized by thermogravimetry-differential thermal analysis (TG-DTA) and power X-ray diffraction (XRD). The narrower size distribution and the regular shape of the phosphor particles are also measured by Field emission scanning electronic microscopy (FE-SEM). Photo-luminescent properties of the phosphors are performed at room temperature. Their excitation spectra present strong absorption at 395 nm near-UV light and 465 nm blue light, which match well with commercial LED chips. The phosphors exhibit satisfactory and excellent red light dominated by 616 nm and their photoluminescence intensity is about 3-4 times stronger than that of phosphor YAG under the 465 nm excitation. In addition, the optimal concentrations of Eu³⁺ for phosphors MGd₂(MoO₄)₄ (M=Ca, Sr and Ba) have also been determined.     

Sr和Ba替代对Eu掺杂Ca2.955Si2O7的结构和发光特性的影响研究

利用固相反应法制备了Sr和Ba替代的Ca_2.955-xM_xSi₂O₇: 0.045Eu²⁺ (M= Sr, Ba, x= 0.1-0.5)系列荧光粉, 利用较大离子半径的Sr和Ba元素替代Eu掺杂Ca_2.955-xM_xSi₂O₇ 中的Ca元素,研究Sr和Ba替代对样品结构和发光特性的影响. X射线衍射测试结果表明,少量Sr和Ba替代不会改变基质的晶体结构, 样品仍然为单斜晶系.未替代前, Ca_2.955Si₂O₇: 0.045Eu²⁺ 样品的发射峰在574 nm左右,随着Sr含量的增加,样品的发射峰发生蓝移; 而Ba含量在x= 0.1-0.4时不会引起发射峰位置的移动, 但x= 0.5样品的发射峰发生蓝移.同等含量的Sr和Ba部分替代样品中的Ca元素, Ba替代样品的光谱强度较强.     

Luminescence of Eu<Superscript>2+</Superscript> ions in alkaline earth dilithiosilicates

Spectral-luminescent properties of Eu²⁺ ions in alkaline earth dilithiosilicates of composition MLi₂SiO₄ (M = Ca, Sr, Ba) have been studied at 77 K. The reasons for the different positions of the Eu²⁺ 4f ⁶5 d → 4f ⁷ emission band maximum in spectra of MLi₂SiO₄ were found based on the obtained results. It was shown that the increase in the decay time of the Eu²⁺ emission on going from CaLi₂SiO₄ (0.41μs) through BaLi₂SiO₄ (0.64 μs) to Sr-silicate (1.11μs) correlates with the shift of the emission maximum to longer wavelengths.     

Single-phased silicate-hosted phosphor with 660 nm-featured band emission for biological light-emitting diodes

A single-phased (Ba,Sr)₃MgSi₂O₈:Eu²⁺, Mn²⁺ phosphor with 660 nm-featured dual band-emission is investigated upon optimizing composition to simulate the artificial photosynthetic action spectrum (PAS) for near-ultraviolet (NUV) biological light-emitting diodes (bio-LEDs). A specific composition range in Ba–Sr binary solid solution of (Ba,Sr)₃MgSi₂O₈ is found to be capable of obtaining single-phased host in the absence of an easily formed orthosilicate impurity, leading to a 660 nm-featured red band emission of Mn²⁺ induced by an efficient energy transfer from a co-doped blue-emitting Eu²⁺ sensitizer. This dual broad band emission phosphor has a 72 nm full width at half maximum (FWHM) for red band that covers fairly well to the absorption spectrum of chlorophyll and PAS for most plants, enabling a flexible option in the application of bio-illumination for artificial photosynthesis.     

Phase equilibria in the La-Me-Co-O (Me=Ca, Sr, Ba) systems

The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 °C. Two types of solid solution of general composition La_1−xMe_xCoO_3−δ and (La_1−y Me_y)₂CoO₄ were found to exist in the systems. The limiting composition of La_1−xMe_xCoO_3−δ lies at x=0.8 for Me = Sr, Ba and between 0.3–0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type La_1−xMe_xCoO_3−y decreases while x increases. La_1−xMe_xCoO_3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La_1−yMe_y)₂CoO₄ was found to be 0.25≤y≤0.35 for Me = Ca, 0.3≤y≤0.55 for Me = Sr and 0.3≤y≤0.375 for Me = Ba. All phases have tetragonal K₂NiF₄-type crystal structure. Based on the XRD and neutron diffraction patterns of quenched samples, the phase diagrams (Gibbs triangles) are constructed for all systems. The phase equilibrium at low oxygen pressure is shown for the example of the La-Sr-Co-O system. The decomposition mechanism of La_1−xSr_xCoO_3−δ at 1100 °C for the samples with 0.5<x<0.8 within the oxygen pressure range −0.678>log(Po₂)>−2.25 can be written as follows: La_1−x′ Sr_x′CoO_3−δ′=n La_1−x″Sr_x″CoO_3−δ″+m SrCoO_2.5+q/2 O₂ where x′>x″. The decomposition mechanism of La_1−xSr_xCoO_3−δ for the samples with x < 0.5 within the oxygen pressure range −2.25>log(Po₂)>−3.55 changes and can be written as follows: La_1−xSr_xCoO_3−δ′=r La_1−x′Sr_x′CoO_3−δ″+w (La_1−y′Sr_y′)₂CoO₄+v CoO+f/2 O₂. The results are shown in “logPo₂-composition” diagrams. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Luminescence properties and energy transfer in Eu2+, Mn2+ codoped Na(Sr,Ba)PO4 phosphor

Eu²⁺ and Mn²⁺ singly doped and codoped Na(Sr,Ba)PO₄ phosphors were synthesized, and their luminescent properties were investigated. A broad blue emission and a broad orange emission band were observed in Na(Sr,Ba)PO₄:Eu²⁺, Mn²⁺ phosphor. The resonant-type energy transfer from Eu²⁺ to Mn²⁺ was demonstrated, and the energy transfer efficiency was also calculated according to their emission spectra. Based on the principle of energy transfer, the emission intensity ration of Eu²⁺ and Mn²⁺ could be appropriately tuned by adjusting the contents of activators. Due to the strong absorption in the 250–400 nm range, Na(Sr,Ba)PO₄:Eu²⁺, Mn²⁺ phosphor could be used as a potential candidate for near-UV white light-emitting diodes (LEDs).     

The structure of mixed fluorides Ca1?xSrxF2 and Sr1?xBaxF2 and the luminescence of Eu2+ in these crystals

The structure of the mixed fluorites Ca_1−x Sr_xF₂ and Sr_1−x Ba_xF₂, as well as the structure of the Eu²⁺ impurity center in these crystals, is calculated within the framework of the virtual-crystal method realized in the shell model and pair-potential approximation. The phenomenological dependence of the position of the lower level of the 4f ⁶5d configuration of the Eu²⁺ ion on distance to the Eu²⁺-ligand is derived. The dependences of the Stokes shift and the Huang-Rhys factor on x are calculated for the yellow luminescence in Sr_1−x Ba_xF₂:Eu²⁺. The value of x at which the lower level of the 4f ⁶5d configuration of the Eu²⁺ ion in Sr_1−x Ba_xF₂:Eu²⁺ falls within the conduction band is found. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 5, 2003, pp. 823–826. Original Russian Text Copyright ? 2003 by Nikiforov, Zakharov, Chernyshev, Ugryumov, Kotomanov.     

Eu2+, Cr3+共掺杂的MAl12O19 (M=Ca, Sr, Ba) 的发光性质及能量传递

三基色荧光粉中, 红色荧光粉性能较差, 为获得性能优良的红色荧光粉, 本文采用高温固相法合成了Eu²⁺, Cr³⁺单掺杂及共掺杂的碱土金属多铝酸盐MAl₁₂O₁₉ (M =Ca, Sr, Ba) 发光体. 实验表明, 在以上三种基质中均存在Eu²⁺→Cr³⁺的能量传递, 利用能量传递可以有效将Eu²⁺的蓝光或绿光转换为红光. 三种碱土金属多铝酸盐基质的晶体结构相似,但Eu²⁺, Cr³⁺发光受晶体场影响,导致在不同的基质中Eu²⁺, Cr³⁺间能量传递效率不同.通过光谱分析及能量传递效率计算发现, 相同掺杂浓度下,CaAl₁₂O₁₉中Eu²⁺→Cr³⁺的能量传递效率最高,SrAl₁₂O₁₉次之, BaAl₁₂O₁₉最低.红光转换率在CaAl₁₂O₁₉中最高.     

Synthesis and photoluminescence properties of?novel red-emitting phosphors for light emitting diodes

The novel red-emitting phosphors K₂Ba_1−x (MoO₄)₂: xEu³⁺(0.02≤x≤0.15) phosphors were prepared by solid-state reaction and their crystal structures, photo luminescent characteristics were investigated. The results show that all samples can be efficiently excited by UV (396 nm) and blue (466 nm) light, which are coupled well with the characteristic emission from UVLED and blue LED, respectively. Their emission spectra show intense red emission at 616 nm with line spectra due to the ⁵D₀–⁷F₂ transition of Eu³⁺. The XRD and photoluminescence experimental results indicate that the K₂Ba(MoO₄)₂: Eu³⁺ phosphor crystallization optimum annealing temperature occurs at about 800°C. The optimum doping concentration of Eu³⁺ is 0.10 mol, and the critical transfer distance (Rc) among Eu³⁺ ions is calculated to be about 11.126 ?. The approach to charge compensation was used: Ba²⁺→Eu³⁺+X⁻ (X=F, Cl, Br), and the charge compensation influence on the luminescent intensity of phosphors is investigated.     

Effect of isoelectronic impurities K<Superscript>+</Superscript> and I<Superscript>−</Superscript> on luminescence of CsBr:Eu<Superscript>2+</Superscript> crystals

We have investigated the photoluminescence (PL) and photostimulated luminescence (PSL) spectra at 300 K to study the effect of isoelectronic impurities K⁺ and I⁻ on the formation and energy structure of Eu²⁺-V_Cs isolated dipole centers and aggregate centers in the form of single crystals of CsEuBr₃ in CsBr:Eu²⁺ single crystals. We have shown that K⁺ and I⁻ impurities in a concentration of 5 mol% do not have a substantial effect on the energy spectrum of isolated dipole centers in CsBr:Eu²⁺ single crystals and the processes for the formation of such centers during growth of CsBr:Eu single crystals from the melt by the Bridgman method. We have established that in Cs_0.95K^0.05Br:Eu²⁺, more favorable conditions are realized for the formation of aggregate centers than in CsBr:Eu²⁺ and CsBr^0.95K^0.05Br:Eu²⁺ single crystals. So in order to improve the storage properties of phosphors based on CsBr:Eu²⁺, in particular for increasing the efficiency of PSL excitation, it is expedient to dope them with K⁺ impurity in a concentration up to 5 mol%. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 627–630, September–October, 2007.     

Quantum cutting in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript>: Eu<Superscript>3+</Superscript> by VUV excitation

The emission and excitation spectra of Gd₂SiO₅∶Eu³⁺ were investigated using the VUV beam line of the Beijing Synchrotron Radiation Facility (BSRF). The experimental results were discussed in the frame of visible quantum cutting process involved in Gd³⁺−Eu³⁺ system. Upon direct excitation into the⁶G _J states of Gd³⁺, two visible photon emissions from Eu³⁺ were observed. Cursory evaluation proved that Gd₂SiO₅∶Eu³⁺ is an efficient visible quantum cutter.     

Ligand hyperfine interaction in Gd<Superscript>3+</Superscript> tetragonal centers in CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript> and the structure of the impurity nearest surroundings

ENDOR experimental spectra of Gd³⁺ tetragonal impurity centers in CaF₂ and SrF₂ crystals were used to determine the superhyperfine interaction (SHFI) constants of the impurity with ¹⁹F nuclear spins of its first coordination sphere and the compensator ion. The distances in the Cd³⁺F₉ complex were estimated within the model of isotropic SHFI constants suggested in [1]. An analysis of the data on the SHFI and spin-Hamiltonian constants [2] in terms of the superposition model indicates significant changes in the contributions (due to the Gd³⁺ mixed states) to these parameters for the tetragonal centers in comparison with the corresponding contributions for the cubic and trigonal centers in the same crystals.     

Photoinduced clustering of Eu and the influence of the magnetic field on it in NaCl: Eu crystals

It has been demonstrated that, in NaCl crystals supersaturated with an Eu impurity (0.1 at %), ultraviolet light with a wavelength of 200–370 nm initiates spontaneous aggregation of Eu²⁺−V _k ⁻ impurity-vacancy dipoles into EuCl₂ flat precipitates lying in the (013) crystallographic plane. A constant magnetic field of 15 T accelerates the photoinduced formation of EuCl₂ nanoclusters at a temperature of 300 K.     

Line emission of SrBe2Si2O7:Eu2+ and BaBe2Si2O7:Eu2+

The compounds SrBe₂Si₂O₇ and BaBe₂Si₂O₇ both have the barylite structure. With 254 nm excitation, the Eu²⁺-activated compounds give UV emission peaking at 360 nm (Sr) and at 375 nm (Ba). Maximum quantum efficiencies of 40% (Sr) and 65% (Ba) were measured. The emission consists of a 5d-4f band emission as well as 4f-4f line emission, in contrast to many other Eu²⁺-activated oxides which generally show only 5d-4f band emission. At 77°K, both compounds show only the 4f-4f line emission peaking at 360 nm. At higher temperatures, 5d-4f band emission shows up at the cost of the line emission. A thermal equilibrium is assumed between the lowest excited 5d and 4f levels. The energy difference between these levels, calculated from the variation in the line-band intensity ratio with temperature, was computed to be 0.15 eV (Sr) and 0.09 eV (Ba). The occurrence of the line emission in the barylites is correlated with the weakness of the crystal field at the Eu²⁺ ions and with the high quenching temperature of the 5d-4f band emission.     

A tunable green alkaline-earth silicon-oxynitride solid solution (Ca1? x Sr x )Si2O2N2:Eu2+ and its application in LED

A series of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ (x=0.0–0.97, y=0.03) phosphors were synthesized by high-temperature solid-state reaction. The XRD patterns confirm the formation of a solid solution of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺. An intense tunable green light is observed with the increasing ratio of Sr/Ca. With an increase in x, the excitation and emission spectra show a redshift and blueshift, respectively, due to large centroid shift and small Stokes shift. The temperature dependent luminescence is also investigated in the temperature range of 77–450 K. The Huang–Rhys factor and the thermal-quenching temperature are determined. Intense green LEDs were successfully fabricated based on the (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ phosphor and near-ultraviolet (∼395 nm) GaN/blue (460 nm) InGaN chips. All the results indicate that the solid solution (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ is a promising phosphor applicable to near-UV and blue LEDs for solid-state lighting.