Effect of Ca and Sr alkaline earth ions on luminescence properties of BaAl12O19:Eu nanophosphor

Nanosized barium aluminate materials was doped by divalent cations (Ca²⁺, Sr²⁺) and Eu²⁺ having nominal compositions Ba_1−xMxAl₁₂O₁₉:Eu (M=Ca and Sr) (x=0.1-0.5), were synthesized by the combustion method. These phosphors were characterized by XRD, scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) and photoluminescence measurement. The photoluminescence characterization showed the presence of Eu ion in divalent form which gave emission bands peaking at 444 nm for the 320 nm excitation (solid-state lighting excitation), while for 254 nm it gave the same emission wavelength of low intensity (1.5 times) compared to 320 nm excitation. It was also observed that alkaline earth metal (Ca²⁺ and Sr²⁺) dopants increase the intensity of Eu²⁺ ion in BaAl₁₂O₁₉ lattice, thus this phosphor may be useful for solid-state lighting.     

A theoretical study of alkaline-earth cations in crystalline sodium chloride,potassium chloride and potassium bromide

Theoretical methods, based on Mott-Littleton techniques are used to investigate the defect structures of alkali-halide crystals doped with divalent ions. The systems studied are those having Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ as impurity ions in NaCl, KCl and KBr crystals. Our calculations find comparable stability of the (110) nearest-neighbour and (200) next-nearest-neighbour complexes whereas the (211) complex is found to be less stable. Significant trends in the variation of binding energy with dopant ion radius are predicted. We also consider the activation energies for the w₁, w₂, w₃ and w₄ type jumps which occur in the vicinity of the impurity ion. The results are, where possible, compared with experimentally determined defect energies and their implications in diffusion processes of M²⁺ impurities in the alkali halides are discussed.     

Temperature-dependent emission spectra of M2SiO4:Eu (M=Ca, Sr, Ba) phosphors for green and greenish white LEDs

The temperature dependence of emission spectra of alkaline earth ortho-silicates M₂SiO₄ (M=Ca, Sr, Ba) doped with Eu²⁺ ions is investigated. Two emission bands of Sr₂SiO₄:Eu²⁺ show the normal redshift with broadening bandwidth and decreasing emission intensity as an increase in temperature. On the other hand, emission bands of Ca₂SiO₄:Eu²⁺ and Ba₂SiO₄:Eu²⁺ show the anomalous blueshift with increasing temperature. For Ca₂SiO₄:Eu²⁺ and Ba₂SiO₄:Eu²⁺, the temperature dependence of the emission color can be described in terms of back tunneling from the excited state of low-energy emission band to the excited state of high-energy emission band in the configuration coordinate diagram. Our phosphors have a promising potential as phosphors for green or greenish white-light-emitting diode pumped by ultraviolet chip.     

New aspects on the dielectric properties of the alkali halides with divalent impurities

Dielectric losses were measured in the following crystals NaCl+ (Mg²⁺, Co²⁺, Ni²⁺, Sr²⁺, Ca²⁺, Mn²⁺, Zn²⁺, Cd²⁺, Ba²⁺, Pb²⁺), KCl + (Ca²⁺, Sr²⁺, Ba²⁺, ²⁺, Pb²⁺) and KBr + (Sr²⁺, Ba²⁺) in the frequency region 5–500 kHz. It was found that when the divalent cation impurity has an electronic configuration similar to the inert gases (1) no observable deviation from simple Debye theory exists (2) the activation energy Φ increases linearly with the ionic radius of the impurity.The above observations (1) and (2) do not hold when the divalent cation impurity has d-electrons in the outer subshell.     

Photoluminescence characteristics of ZnS nanocrystallites doped with Ti3+and Ti4+

Direct synthesis of ZnS nanocrystallites doped with Ti³⁺ or Ti⁴⁺ by precipitation has led to novel photoluminescence properties. Detailed X-ray diffraction (XRD), fluorescence spectrophotometry, UV–vis spectrophotometry and X-ray photoelectron spectroscopy (XPS) analysis reveal the crystal lattice structure, average size, emission spectra, absorption spectra and composition. The average crystallite size doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2.6±0.2 nm. The nanoparticles can be doped with Ti³⁺ and Ti⁴⁺ during the synthesis without the X-ray diffraction pattern being altered. The strong and stable visible-light emission has been observed from ZnS nanocrystallites doped with Ti³⁺ (its maximum fluorescence intensity is about twice that of undoped ZnS nanoparticles). However, the fluorescence intensity of the ZnS nanocrystallites doped with Ti⁴⁺ is almost the same as that of the undoped ZnS nanoparticles. The emission peak of the undoped sample is at 440–450 nm. The emission spectrum of the doped sample consists of two emission peaks, one at 420–430 nm and the other at 510 nm. Received: 27 April 2001 / Accepted: 16 August 2001 / Published online: 17 October 2001     

Influence of divalent cationic impurities on plasticity of AgCl

The hardening effect of Mg²⁺, Ca²⁺, Sr²⁺, Cd²⁺ and Ba²⁺ impurities in polycrystalline AgCl was investigated. With exception of Mg²⁺ the hardening effect of the impurities decreases with increasing atomic number (atomic weight). The hardening effects of Mg²⁺ and Ca²⁺ are expressively stronger than those of the three remaining impurities. Approximately linear dependence of the stress at the beginning of plastic deformation on the impurity concentration was found. Possible causes of the different hardening effects of the divalent impurities are discussed.     

Surface modification and fluorescent properties of Mn2+-doped and undoped ZnS nanoparticles

Abstract

ZnS nanoparticles and Mn²⁺-doped ZnS nanoparticles were prepared by a reverse micelle reaction system. In addition, ZnS and Mn²⁺-doped ZnS nanoparticles were modified with poly(vinyl alcohol) (PVA) and 1-dodecanethiol (C₁₂H₂₅SH). The average particle size of the ZnS sample is determined around 2.3 nm by using the well-known Scherrer equation, which is in accordance with the results obtained from UV–vis and TEM analysis. Fluorescence intensity of the Mn²⁺-doped ZnS nanoparticles increases with increasing Mn²⁺ content compared with undoped ZnS nanoparticles, and coating PVA can also make fluorescence intensity increase. Different Zn²⁺/S^2- or C₁₂H₂₅SH/Zn²⁺ can affect intensity of PL emission peak and its position, which is discussed in this paper.     

Improved electrochemical properties of Sr2+-doped LiNi0.8Co0.2O2 synthesized via a sol-gel route using tartaric acid as a chelating agent

Single-phase undoped LiNi_0.8Co_0.2O₂ and Sr²⁺-doped LiNi_0.8Co_0.2O₂ were synthesized by a low temperature tartaric acid assisted sol-gel method. Small quantities of Sr²⁺ were used as dopants in order to improve the electrochemical characteristics, especially the capacity and cycling performance of LiNi_0.8Co_0.2O₂. The electrochemical performance of the undoped material was promising with a first discharge capacity of 174 mAh/g and 165 mAh/g after 10 cycles with a 100% cycling efficiency in the tenth cycle. Addition of Sr²⁺ for Li in minimum quantities with the Sr²⁺/Li⁺ dopant mole ratio ranging from 10⁻⁴ to 10⁻⁸ resulted in improved electrochemical properties for dopant mole ratio of 10⁻⁶. The first discharge capacity was 182 mAh/g and the tenth was 174 mAh/g at the 10th discharge. The synthesis of Sr²⁺-doped LiNi_0.8Co_0.2O₂ and its improved electrochemical properties have been discussed for the first time. The improved electrochemical properties of Sr²⁺-doped LiNi_0.8Co_0.2O₂ system are explained based on defect models.     

The influence of local distortions on the static and dynamic properties of copper and silver eightfold-coordinated Jahn-Teller complexes in mixed crystals with a fluorite-type structure

The influence of local distortions on the structure and properties of copper and silver impurity Jahn-Teller complexes in mixed crystals, namely, Ca_xSr_1?x F₂: Me ²⁺ and Sr_1?x Ba_xF₂: Me ²⁺ (0≤x≤1, Me ²⁺=Cu²⁺ or Ag²⁺), is investigated using electron paramagnetic resonance (EPR) spectroscopy at frequencies of 9.3 and 37 GHz in the temperature range 4.2–250 K. Local distortions of the tensile and compressive types are induced by Ca²⁺, Sr²⁺, and Ba²⁺ impurity ions incorporated into the first or second coordination sphere of the cationic environment of the Me ²⁺ impurity ion during crystal growth.     

Shrinking of anionic polyacrylate coils induced by Ca2+, Sr2+ and Ba2+: A combined light scattering and ASAXS study

Anionic polyacrylate chains (NaPA) form precipitates if alkaline earth cations are added in stoichiometric amounts. Accordingly, precipitation thresholds were established for three different alkaline earth cations Ca²⁺, Sr²⁺ and Ba²⁺. Close to the precipitation threshold, the NaPA chains significantly decrease in size. This shrinking process was followed by means of combined static and dynamic light scattering. Intermediates were generated by varying the ratio [MCl₂]/[NaPA] with M denoting the respective alkaline earth cation. All experiments were performed at an inert salt level of 0.01M NaCl. Similar coil-to-sphere transitions could be observed with all three alkaline earth cations Ca²⁺, Sr²⁺ and Ba²⁺. Based on these findings, supplementary conventional and anomalous small-angle X-ray scattering experiments using selected intermediates close to the precipitation threshold of SrPA were performed. The distribution of Sr counterions around the polyacrylate chains in aqueous solution provided the desired scattering contrast. Energy-dependent scattering experiments enabled successful separation of the pure-resonant terms, which solely stem from the counterions. The Sr²⁺ scattering roughly reflects the monomer distribution of the polyacrylate chains. Different ratios of the concentrations of [ SrCl₂]/[NaPA] revealed dramatic changes in the scattering curves. The scattering curve at the lowest ratio indicated an almost coil-like behaviour, while at the higher ratios the scattering curves supported the model of highly contracted polymer chains. Most of X-ray scattering experiments on intermediate states revealed compact structural elements which were significantly smaller than the respective overall size of the NaPA particles.     

Adsorption of formaldehyde on the Fe site of clean and M (Ca, Sr and Ba) doped LaFeO3 (0 1 0) surface

The adsorption of formaldehyde (H₂CO) on the Fe site of clean and M²⁺ (Ca²⁺, Sr²⁺ and Ba²⁺) doped LaFeO₃ (0 1 0) surface have been investigated using the density functional theory (DFT) method. Calculation results show that the oxygen atom of the H₂CO molecule prefers to be adsorbed on the Fe site of the clean LaFeO₃ (0 1 0) surface. The adsorption of H₂CO could change the electronic properties of the LaFeO₃, indicating that the LaFeO₃ could be used as gas sensing material to detect the H₂CO gas. The analysis results of the DOS suggest that the bonding mechanism between the H₂CO molecule and the Fe site is mainly from the interaction between the Fe 3d and H₂CO 2p orbital. Comparing with the binding energy and the net charge-transfer, we find that the M²⁺ (Ca²⁺, Sr²⁺ and Ba²⁺) doping cannot improve the sensitivity of the LaFeO₃ to the H₂CO gas.     

Impurity effect on the Raman spectra of Sr0.6Ba0.4Nb2O6

Impurity effect on the rare earth ion doped Sr_0.6Ba_0.4Nb₂O₆(SBN40) was studied at room temperature. Doping the rare earth ions of Pr³⁺ or Nd³⁺ changed the Raman profile: spectral broadening, central frequency shifts and relative intensity decreasing. Two reasons are considered according to the ferroelectric and optical properties of the rare earth ion doped SBN40: impurity-induced crystal disorder and the crystal structure change. SBN40 is the general disorder crystal and the disorder which is enhanced by doping the rare earth ion is especially strong along the x?y plane of the material compared with that along the polar C-axis.     

Dopant control over the crystal morphology of ceramic materials

Doping is a common way to activate the behavior of ceramics. Its effect is not limited to the bulk: segregation of dopants to the surfaces also yields a way to modify, and ultimately control the crystal morphology. We propose a model that allows us to calculate the surface energy beyond the Langmuir isotherm for doped and defective surfaces from atomic-level simulations. The model also allows us to account for different compositions between the bulk and surface. Computational materials design can thus be applied to optimize simultaneously the crystal behavior at the atomic (surface structure and composition) and mesoscopic (crystal size and shape) length scales. We exemplify the model with orthorhombic CaTiO₃ perovskite doped with Mg²⁺, Fe²⁺, Ni²⁺, Sr²⁺, Ba²⁺ and Cd²⁺ ions, by predicting the effect that different dopants and dopant concentrations have on the crystal morphology. We find that a higher proportion of reactive {0 2 1} and {1 1 1} surfaces are exposed with the presence of divalent Mg²⁺, Fe²⁺ and Ni²⁺ ions than in the undoped material and in perovskite doped with Ba²⁺ and Sr²⁺. Cd²⁺ has only minor effects on crystal morphologies. These findings have important implications for predicting the reactivity of crystals doped with different ions and we show how this can be related to a simple parameter such as the ionic radius. We have tested our newly derived model by comparison with laboratory flux grown single crystals of CaTiO₃, (Ni, Ca)TiO₃ and (Ba, Ca)TiO₃ and find excellent agreement between theory and experiment.     

用正电子湮没研究钙钛矿结构压电陶瓷中的点缺陷

用正电子湮没寿命谱技术对有意掺杂的或被杂质替代的钙钛矿结构压电陶瓷中的点缺陷进行了研究.掺杂了La³⁺的PbTiO₃，掺杂了Sr²⁺，Cd²⁺和La³⁺的Pb(Mg_1/3Nb_2/3)_0.375Ti_0.375Zr_0.25O₃，掺杂了Ca²⁺的BaT 关键词：     

Influence of Ba-doping on structural and luminescence properties of Sr2SiO4:Eu phosphors

Ba²⁺-doped Sr₂SiO₄:Eu²⁺ phosphors were synthesized with the high-temperature solid-state reaction technique. The experimental results, summarized in the successful production of a single-phase powder with fine microstructure of spherical particles with smooth surface, suggest that Ba²⁺-doping favors the stabilization of α′-Sr₂SiO₄. Rietveld refinement of X-ray diffractograms suggests that Ba²⁺ and Eu²⁺ ions occupy the sites of Sr²⁺ in the lattice of α′-Sr₂SiO₄. The produced phosphors show two intense emission bands at green and yellow regions of spectrum, originated from Eu²⁺ ions accommodated at two different sites in the host crystal, whose peaks depend on the concentrations of Ba²⁺ and Eu²⁺. Intense and broad excitation spectra extend from ultraviolet to the blue region.     

Luminescent properties of stabled hexagonal phase Sr1−xBaxAl2O4:Eu (x=0.37-0.70)

Stabled hexagonal phase Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) was prepared by solid-state method. Result revealed that the structure behavior of the SrAl₂O₄:Eu²⁺ calcined at 1350 °C in a reducing atmosphere for 5 h strongly depended on the Ba²⁺ concentration. With increasing Ba²⁺ concentration, a characteristic hexagonal phase can be observed. When 37-70% of the strontium is replaced by barium, the structure of the prepared sample is pure hexagonal. Photoluminescence and excitation spectra of the samples with different x and doped with 2% Eu²⁺ were investigated. Changes in the emission spectra were observed in the two different phases. The green emission at 505 nm from Eu²⁺ was found to be quite strong in the hexagonal phase. The intensity and peak position of the green luminescence from Eu²⁺ changed with increasing content of Ba²⁺. The strongest green emission was obtained from Sr_0.61Ba_0.37Al₂O₄:Eu²⁺. The decay characteristics of Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) showed that the life times also varied with the value of x. Furthermore, the emission colors and decay times varying with x could be ascribed to the variation of crystal lattice.     

Luminescence and chromaticity of alkaline earth aluminate MxSr1−xAl2O4:Eu (M: Ca,Ba)

In order to prepare fluorescent material for UV-LED used as illumination light source, two series of Eu²⁺ doped (1 mol%) alkaline earth aluminate phosphors Ca_xSr_1−xAl₂O₄ and Ba_xSr_1−xAl₂O₄ were prepared. The crystal structure, relative quantum efficiency(Qr), peak wavelength(λ_p), color tuning and chromaticity were investigated by XRD patterns and photoluminescence (PL) on samples prepared by solid solution system (s series) and powder mixing system (m series) respectively. For the s series, the synthesized Ca_xSr_1−xAl₂O₄:Eu²⁺ powders show that the structure transforms from monoclinic to hexagonal at x?0.5, and λ_p increases from 442.3 to 529.7 nm with decreasing x. For the Ba_xSr_1−xAl₂O₄:Eu²⁺ system, the structure transforms from monoclinic to hexagonal at x?0.3, and λ_p decreases from 520.5 to 502.2 nm continuously from x=0 to 1. The shift in λ_p could be explained by the crystal field effect, which is affected by different coulomb attractive forces due to the various fraction of alkaline earth cation in the host lattice. Different phosphor properties prepared by either solid solution or powder mixing methods were characterized by chromaticity measurements for both reflective and transmissive modes.     

Studies on adsorptions of metallic ions in water by zirconium glyphosate (ZrGP): Behaviors and mechanisms

A new adsorbent named zirconium glyphosate [Zr(O₃PCH₂NHCH₂COOH)₂·0.5H₂O, denoted as ZrGP] and its selective adsorptions to Pb²⁺, Cd²⁺, Mg²⁺ and Ca²⁺ ions in water were reported in this paper. Compared to other zirconium adsorbents, such as zirconium phosphate [Zr(HPO₄)₂], ZrGP exhibited highly selective adsorption to Pb²⁺ in solution which contained Pb²⁺, Cd²⁺, Mg²⁺ and Ca²⁺ ions. The loaded ZrGP with metallic ions can be efficaciously regenerated by aqueous solution of HCl (1.0 M) without any noticeable capacity loss, and almost all of it can be reused and recycled. The memory effect on structural regeneration of ZrGP was also found when Mg²⁺ and Ca²⁺ were adsorbed. To be specific, the structure of ZrGP was destroyed due to adsorbing these two ions, but it could be regenerated after the loaded materials were dipped in HCl solution (1.0 M) for several minutes to remove metallic ions.     

Synthesis,characterization and luminescent property of metal-ion-doped terbium complexes of 2,3-Pyrazinedicarboxylate

Using 2,3-pyrazine dicarboxylate (pzdc^2?) as ligand, a series of new terbium complexes Tb₂L₂(HL)(NO₃)10 H₂O, Tb₂Mg₂L₄(HL)(NO₃)14 H₂O, Tb₂Ca₂L₄(HL)(NO₃)14 H₂O, Tb₂Sr₂L₄(HL)(NO₃)14 H₂O, Tb₂Ba₂L₄(HL)(NO₃)14 H₂O, Tb₂Cd₂L₄(HL)(NO₃)14 H₂O, Tb₂Co₂L₄(HL)(NO₃)14 H₂O, Tb₂Ni₂L₄(HL)(NO₃)14 H₂O and Tb₂Zn₂L₄(HL)(NO₃)14 H₂O (L=pzdc^2-) have been synthesized. The complexes were characterized by elemental analysis, ICP-AES, molar conductivity measurement, TG-DSC analysis, IR spectroscopy and UV absorption spectroscopy. The luminescence spectra, luminescence lifetimes and emission quantum efficiencies of the complexes were measured. The results show that doping alkaline earth metal ions have significantly increased the luminescence intensities and quantum efficiencies of the complexes, and the sequence of the quantum efficiencies of the doped complexes is Ba²⁺>Ca²⁺>Mg²⁺>Sr²⁺. The enhancement of luminescence efficiencies may result from the decrease of the concentration quenching effect of Tb³⁺ ions, intramolecular energy transfer from the ligands coordinated with doped ions to Tb³⁺ ions and the lattice distortion of the complexes. The luminescence efficiencies of the Tb³⁺ ions are also enhanced by doping Cd²⁺ and Zn²⁺ ions. However, the complexes doped with Co²⁺ or Ni²⁺ ions exhibit luminescence quenching, which is caused by the energy consumed by these two ions in the form of d-d electron transitions.     

Effect of the ionic radius on jump frequencies of alkaline earth cations in NaCl crystals

By comparing diffusion coefficientsD of bivalent cations Ba²⁺, Ca²⁺, Sr²⁺ in NaCl crystals it was shown that in the temperature range above 550 °CD (Ba²⁺)>D (Sr²⁺)>D (Ca²⁺) is valid. Temperature dependences of jump frequenciesw ₂ of these cations are described byw ₂ (Ba²⁺)=(2·15±0·55) × 10¹² × exp {?(0·817±0.007)/kT};w ₂ (Sr²⁺)=(2·9±1·1) × 10¹² × exp {?(0·84±0.02)/kT} andw ₂ (Ca²⁺)=(5·5±6·5) × 10¹⁰ × exp {?(0·51±0·07)/kT}. It was demonstrated that in NaCl crystals the activation enthalpy and the preexponential factor of the jump frequencyw ₂ increase with increasing ionic radius and mass of the bivalent alkaline earth cation.