Molecular dynamics study of linear and comb-like polyelectrolytes in aqueous solution: effect of Ca2+ ions

All-atom molecular dynamics simulations were employed to provide microscopic mechanism for the salt tolerance of polyelectrolytes dispersants. The conformational variation of polyelectrolytes and interactions between COO^? groups and counterions/water molecules were also studied via radius of gyration and pair correlations functions. Sodium polyacrylate (NaPA) and sodium salts of poly(acrylic acid)–poly(ethylene oxide) (NaPA–PEO) were selected as the representative linear and comb-like polyelectrolyte, respectively. The results show that Ca²⁺ ions interact with COO^? groups much stronger than Na⁺ ions and can bring ion-bridging interaction between intermolecular COO^? groups in the NaPA systems. While in the NaPA–PEO systems, the introduced PEO side chains can prevent backbone chains from ion-bridging interactions and weaken the conformational changes. The present results can help in selecting and designing new-type efficient polyelectrolyte dispersants with good salt tolerance.     

Collapse of sodium polyacrylate chains in calcium salt solutions

The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca²⁺-ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca²⁺ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing the NaPA concentration at a constant Ca²⁺ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca²⁺. Clearly, excessive Na⁺-ions displace the Ca²⁺-ions from the NaPA chains. Received 18 July 2000 and Received in final form 24 August 2000     

The effect of alkaline earth metallic ions on the photoluminescence properties of sol-gel silica glass

Mg²⁺-, Ca²⁺-, Sr²⁺- and Ba²⁺-doped silica glasses have been prepared using sol-gel processing by employing Si(OC₂H₅)₄, MgCl₂6H₂O, CaCl₂2H₂O, SrCl₂6H₂O and BaCl₂2H₂O as precursors, with HCl as a catalyst. The UV–visibleabsorption spectra of the doped samples are almost the same as those of the undoped sample. The absorption bands of alkaline earth metallic ions have not been observed in the doped samples. Strong visible light has been observed from sol- gel silica glasses doped with alkaline earth metallic ions. The relative fluorescence intensity of the Sr²⁺-doped (the impurity mole ratio of Sr²⁺ was 0.268%) and the Ba²⁺-doped (the impurity mole ratio of Ba²⁺ was 0.448%) samples was about 4 times that of the undoped sample. The relative fluorescence intensity of the Mg²⁺-doped (the impurity mole ratio of Mg²⁺ was 0.069%) sample was about 2.5 times that of the pure glass sample. The relative fluorescence intensity of the Ca²⁺- doped (the impurity mole ratio of Ca²⁺ was 0.179%) sample was about 3 times that of the pure glass sample. Alkaline earth metallic ions affect the formation and conversion of luminescent defects in sol-gel silica glass. Thus, the relative fluorescence intensity of the doped samples increases more than that of the undoped sample. Received: 17 April 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

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.     

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.     

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.     

Luminescence characteristics of Eu2+ activated Ca2SiO4 Sr2SiO4 and Ba2SiO4 phosphorsfor white LEDs

<正>This paper investigates the luminescence characteristics of Eu²⁺ activated Ca₂SiO₄,Sr₂SiO₄ and Ba₂SiO₄ phosphors. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices,and show different emission colour variation caused by substituting M²⁺ cations for smaller cations.This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence.These phosphors with maximum excitation of around 370 nm can be applied as a colour-tunable phosphor for light-emitting diodes（LEDs） based on ultraviolet chip/phosphor technology.     

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.     

Critical current density and flux pinning in La2−xPrxCa2x Ba2Cu4+2xOz (x=0.1-0.5) superconductors

Polycrystalline La_2−xPr_xCa_2xBa₂Cu_4+2xO_z (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (T_c^onset values) with increasing dopant concentration. The mixing of rare earth La³⁺ and Pr^3+/4+ ions at rare earth site (La³⁺) along with substitution of divalent Ca²⁺ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.     

Encapsulating MAl2O4:Eu2+, Dy3+ (M = Sr,Ca, Ba) phosphors with triethanolamine to enhance water resistance

Traditional aluminates phosphors with persistent luminescence are chemically unstable to water or moisture. Thus, how to improve the water-resistance of these phosphors is becoming a key issue in their practical applications. In this work, a series of alkaline earth aluminate phosphors including MAl₂O₄:Eu²⁺, Dy³⁺ (M = Sr, Ca, Ba) have been prepared by a co-precipitation synthesis and postannealing approach, using 8-hydroxyquinoline and sodium oxalate as precipitants. The samples before and after encapsulation were well characterized by means of XRPD, FESEM, FT-IR, TGA-DTG and PL techniques as well as water resistance measurements. The precipitants involved can react with Al³⁺ and Sr²⁺ (or Ca²⁺, Ba²⁺) to form complex compounds in aqueous solution, which further convert into porous phosphors by postannealing method under reducing atmosphere. Next, triethanolamine encapsulation at room temperature was conducted onto their surfaces to improve the water resistance. The results reveal that the encapsulation of aluminate phosphors with triethanolamine can effectively enhance the water resistance, and minimally affect on persistent phosphorescence.     

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.     

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.     

Effect of Sr2+-doping on structure and luminescence properties of BaAl2Si2O8:Eu2+ phosphors

Sr²⁺ doped BaAl₂Si₂O₈:Eu²⁺ phosphor was synthesized by chemical co-precipitation method. With the increase of Sr²⁺ concentration, the phase structure of (Ba_0.965 ? xSr_xEu_0.035)Al₂Si₂O₈ changes from hexagonal phase to monoclinic phase owing to large activation energy in SrAl₂Si₂O₈ system. (Ba_0.965 ? xSr_xEu_0.035)Al₂Si₂O₈ phosphor exhibits a broad blue band peaking at 425 nm due to the 4f⁶5d–4f⁷(⁸S_7/2) transition of Eu²⁺ ions. The emission intensity increases, accompanied by the blue shift of emission maximum from 459 to 417 nm with the Sr²⁺ doping concentration increasing. The optimal concentration of Sr²⁺ ion is 40%, and the phosphor shows high color stability in CIE chromaticity diagram. The result indicates that Sr²⁺ doped phosphor not only can enhance the relative intensity but also can adjust the chromaticity coordinate.     

Dielectric properties Ca-substituted barium strontium titanate ferroelectric ceramics

The dielectric and ferroelectric properties of Ba_1−xSr_xTiO₃ (BST) (x=0.10,0.20,0.30,0.40 and 0.60) ceramics and Ba_1−2xSr_xCa_xTiO₃ (BSCT) (x=0.10,0.20,0.30) ceramics have been investigated. The low temperature phase transitions of BST ceramics vanish after Ca²⁺ substitution while the high temperature transition is diffused and relaxed, which becomes more obvious with increasing x. Ca²⁺ substitution obviously decreases the dielectric constant maximum, K_m, of BST ceramics and changes the temperature of dielectric constant maximum, T_m, of BST ceramics. The shift of T_m in BST is attributed mainly to the Sr²⁺ and Ba²⁺ concentration. BST ceramics exhibit almost normal ferroelectric characteristics, while a typical relaxor behavior was observed in BSCT ceramics. The relaxor behavior observations may be understood by a random electric field induced domain state.     

Theoretical design on a new double functional device of 2,2′‐bipyridine‐embedded N‐(9‐pyrenyl methyl)aza‐15‐crown‐5

Theoretical design on a new molecular switch and fluorescent chemosensor double functional device of aza‐crown ether (2,2′‐dipyridine‐embedded N‐(9‐anthraceneyl(pyrenyl)methyl)aza‐15‐crown‐5) was explored. The interactions between ligands and a series of alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) were investigated. The fully optimized geometry structures of the free ligands ( L _1, L ₂) and their metal cation complexes ( L ₁/M²⁺, L ₂/M²⁺) were calculated with the B3LYP/6‐31G(d) method. The natural bond orbital analysis, which is based on optimized geometric structures, was used to explore the interaction of L ₁/M²⁺, L ₂/M²⁺ molecules. The absorption spectra of L _1, L ₂, L ₁/M²⁺, and L ₂/M²⁺, and their excited states were studied by time‐dependent density functional theory. A new type molecular device L ₂(2,2′‐dipyridine‐embedded N‐(9‐pyrenyl methyl)aza‐15‐crown‐5) is designed, which not only has the selectivity for Sr²⁺, and construct allosteric switch, but also has fluorescent sensor performance.     

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.     

Electrical investigation of some fluorides of composition A2MII+MIII+F9 by impedance spectroscopy

The ac conductivity of three fluorides of A₂M^II+ M^{III +} F₉ type, Sr₂NiAlF₉, Sr₂ZnGaF₉ and Ba₂MnCrF₉, between 293 and 773K, has been analyzed within a combined complex impedance and modulus formalism. The antiferroelectric-paraelectric phase transition of Sr₂ZnGaF₉ has been characterized. A hopping mechanism for charge carriers is suggested for the three materials; their electrical performance are relatively weak. Sr₂NiAlF₉ and Sr₂ZnGaF₉ can be considered as ionic conductors of the F- ion. On the contrary, the conductivity in Ba₂MnCrF₉ is rather of an electronic nature.     

Ordre a courte distance et energie interne des composes Sr2TaFeO6 et Sr2TaCrO6 de type perovskite

The disordered samples of the perovskite compounds Sr₂TaFeO₆ and Sr₂TaCrO₆ have been studied using powder X-ray diffuse scattering techniques. The short range order parameters show that on Sr₂TaFeO₆ the Ta⁵⁺ and Fe³⁺ cations are almost completely disordered while high short range order is found between Ta⁵⁺ and Cr³⁺ in Sr₂TaCrO₆. Local displacements of all the ions have been found. In particular the anions move largely toward the positive Ta⁵⁺ ions.The electrostatic energies (order, polarization, displacements) between a “NaCl” order structure-type and the fully disordered perovskite-structure are compared.The existence of the disordered structure can be completely explained by the polarization energy of the anions and Sr²⁺ cations.Qualitatively the displacement energy increases as the radius of the 5+ cations decreases. This allows us to explain the fact that some of these phases are ordered.     

Localised vibrations due to substitutional impurities in calcium fluoride

The localized modes in CaF₂ due to the introduction of substitutional impurities have been investigated using the greens functions method. The weakening of the nearest neighbour interaction in all the alkaline earth fluorides, is estimated from the infrared absorption spectra on localized modes in these crystals. The splitting of the localized modes in CaF₂, due to the presence of additional Sr²⁺ or Ba²⁺ impurities is estimated and satisfactory agreement exists between theory and experiment, within the limits of the simplifications in our calculations.     

Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

To determine the charges of atoms in the lattices of the compounds Tl₂Ba₂Ca _n?1Cu_nO_2n+4 and Bi₂Sr₂Ca _n?1Cu_nO_2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mössbauer spectroscopy on the isotopes ⁶¹Cu(⁶¹Ni) and ⁶⁷Cu(⁶⁷Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on ⁶³Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl₂Ba₂Ca₂Cu₃O₁₀ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀—at oxygen sites in the same plane as the Cu(2) atoms).