Anharmonicity and fragility in lithium borate glasses

Since the physical properties of lithium borate glasses xLi₂O-(1-x)B₂O₃ (0<x< 0.28) vary over a wide range with the composition, this binary system is particularly suitable for studying the relationship between vibrational anharmonicity and fragility. The density, the linear expansion coefficient, the longitudinal and transverse ultrasonic velocities and their respective temperature coefficients of the velocities are measured, from which the vibrational anharmonicity in lithium borate glasses is evaluated with the help of the Grüneisen parameter at the Debye cut-off frequency and the Anderson-Grüneisen parameter: these two parameters plotted vs. composition have the same characteristics with minima at x≈0.08. The fragility is evaluated from the temperature width of the glass transition; the fragility also shows a minimum at x≈0.08. The presence of minima at x≈0.08 is ascribable to the fact that the crosslinking density between six-membered rings in the glass reaches a maximum at this composition. We show that the anharmonic parameters strongly correlate with the fragility metrics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Intense white light luminescent Dy3+ doped lithium borate glasses for W-LED: A correlation between physical,thermal, structural and optical properties

In this article the physical, thermal structural and optical properties of Dy³⁺ doped lithium borate glasses have been studied for white LED application. The emission spectra shows two intense emission bands at around 483 nm and 574 nm corresponds to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions along with one feeble band at 663 nm corresponds to ⁴F_9/2 → ⁶H_11/2 transition. The average lifetime <τ> of Dy³⁺ were found to be about 2.95 and 4.94 ns for blue and yellow emission bands respectively. CIE chromaticity diagram shows glass LBD-4 containing 0.5 mol% Dy₂O₃ with colour co-ordinates x = 0.33 and y = 0.37 have highest emission intensity. These glasses having emission in the white region and thus can be used for bright white LED's and modern white LED bulbs.     

Thermal expansion studies on some gamma-irradiated binary borate glasses

The thermal expansion, density and molar volume of some binary borate glasses were measured before and after exposure to a gamma-ray dose of 10³ kGy. The expansion curves for all glasses, which were measured from room temperature to above the softening temperature, displayed similar characteristics. Increase of the lead oxide content decreased the thermal coefficient of expansion, but the effects of different alkali metal cations were shown to depend on their ionic radii. The various proposed mechanisms of thermal expansion are dealt with. The experimental results could be explained by considering the bond strengths, the polarizing powers of the different cations and the damage produced by radiation. The possible compaction of the structure due to irradiation is also discussed.     

Structural and optical properties of barium titanate modified bismuth borate glasses

Glass samples with composition (70B₂O₃–29Bi₂O₃–1Dy₂O₃) modified with Barium titanate (BT), where BT is added in different successive weight percents, have been synthesized by conventional melt quenching technique. X-ray diffraction studies were performed in order to confirm the amorphous nature of the samples. The density of the samples has been found to decrease with an increase in the BT content, whereas an opposite trend has been observed in the molar volume. The analysis of FTIR and Raman spectra of the samples depicts that the glass network is built up of mainly BiO₆, BiO₃, BO₃ and BO₄ units. Its detailed analysis also revealed that the glass structure depends upon the amount of BT in the glass matrix and hence it acts as a modifier in the glass network. Introduction of BT into the glass matrix leads to the conversion of BO₃ trigonal units into BO₄ tetrahedral units, which results in a decrease in the degree of disorder in the glass network and makes the glass system more stable. The values of Urbach energy obtained for the prepared samples also confirmed the decrease in disorder in the glass network. The optical absorption measurements carried out for well-polished samples show a decrease in optical band gap energy with an increase in BT content whereas the molar refractivity shows the reverse trend. The Hydrogenic excitonic model applied to the studied glasses suggested that the present glass system favors direct transitions. The metallization criterion of the presently studied samples suggests that the prepared glasses may be potential candidates for nonlinear optical applications.     

Optical characterization of Eu3+ and Tb3+ ions doped cadmium lithium alumino fluoro boro tellurite glasses

This article reports on the development and spectral results of Eu(3+) and Tb(3+) ions doped cadmium lithium alumino fluoro boro tellurite (CLiAFBT) glasses in the following composition. 40TeO2-30B2O3-10CdO-10Li2O-10AlF3 (Hostglass) (40-x)TeO2-30B2O3-10CdO-10Li2O-10AlF3-xEu2O3 (40-x)TeO2-30B2O3-10CdO-10Li2O-10AlF3-xTb4O7 where x=0.25, 0.50, 0.75, 1.0, 1.25 mol%. Glass amorphous nature and thermal properties have been studied using the XRD and DSC profiles. From the emission spectra of Eu(3+):glasses, five emission transitions have been observed at 578 nm, 592 nm, 612 nm, 653 nm, 701 nm and are assigned to the transitions (5)D(0)→(7)F(0), (7)F(1,)(7)F(2), (7)F(3) and (7)F(4), respectively, with λ(exci)=392 nm ((7)F(0)→(5)L(6)). In case of Tb(3+):glasses, four emission transitions ((5)D(4)→(7)F(6,)(7)F(5), (7)F(4) and (7)F(3)) are observed at 488 nm, 543 nm, 584 nm and 614 nm, respectively, with λ(exci)=376 nm. Decay curves and energy level diagrams have been plotted to evaluate the life times and to analyze the emission mechanism.     

Non-debye nature in thermal relaxation and thermal properties of lithium borate glasses studied by modulated DSC

Complex heat capacity, C _p ^*=C _p ^'–iC _p ^', of lithium borate glasses xLi₂O·(1–x)B₂O₃ (molar fraction x=0.00–0.30) has been investigated by Modulated DSC. We have analyzed the shape of C _p ^* by the Cole-Cole plot, performed fitting by the Havriliak-Negami equation, and then determined the parameters related to the non-Debye nature of thermal relaxation. Moreover, the concentration dependence of the thermal properties has been investigated. Glass transition temperatures become higher with the increase of molar fraction of Li₂O and shows the board peak around x=0.26. Temperature ranges of glass transitions become narrower with the increase of Li₂O concentration.     

Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi2O3-TiO2-TeO2 system

Within the Bi₂O₃-XO₂-TeO₂ (X=Ti, Zr) systems, a large glass-forming domain was found for X=Ti, but no glass formation was evidenced for X=Zr. Densities, glass transition (T_g), crystallization (T_c) temperatures and Raman spectra of the relevant glasses were studied as functions of the composition. The Raman spectra of the glasses were interpreted in terms of the structural transformations produced by the modifiers. It was established that the addition of Bi₂O₃ and TiO₂ content to TeO₂ glass influences the T_g temperature in a similar manner: this value progressively increases with the increase of the modifier concentration. However, the structural evolutions are different: (a) the addition of TiO₂ to TeO₂ glass keeps the polymerized framework structure in transforming a number of Te-O-Te bridges into the Te-O-Ti ones without producing any tellurite anions (i.e., the [TeO₃]²⁻ groups); (b) on the contrary, the addition of Bi₂O₃ destroys the glass framework by giving rise to the island-type [Te_nO_m]^2(m−2n)− complex tellurites anions, thus causing a depolymerization of the glass.     

Spectroscopic,mechanical and magnetic characterization of some bismuth borate glasses containing gadolinium ions

The ultrasonic parameters, the optical parameters along with the IR spectroscopy and magnetic susceptibility studies have been employed to explore the role of Gd₂O₃ in the structure of the glasses xGd₂O₃–60B₂O₃–10MoO₃–(30-x)Bi₂O₃, with 0 ≤ x ≤ 7 mol %. IR analysis indicates that Gd₂O₃ is preferentially incorporated into the borate network-forming BO₄ units. It is assumed that Bi₂O₃ and MoO₃ enter the structure as modifiers in the form of BiO₆ and MoO₆ only. The compositional dependence of the mechanical and the optical parameters are interpreted in terms of the transformation of the structural units BO₃ into BO₄, the increase in the number of bridging oxygen atoms, and the substitution of high bond strength Gd–O, in the place of low bond strength Bi–O bond. The results of the magnetic susceptibility reveal the paramagnetic behavior as described by the Curie-Weiss law and indicating the presence of weak antiferromagnetic exchange interactions between Gd³⁺ ions. The magnetic entropy change of the glasses was determined according to the temperature and magnetic field dependence of magnetization.     

The non-isothermal devitrification of lithium germanate glasses

The non-isothermal devitrification of lithium germanate glasses, examined by DTA and XRD, is reported and discussed. The glass compositions are expressed by the general formula:xLi₂O(1?x)GeO₂ withx=0.050, 0.125, 0.167, 0.200 and 0.250. All the glasses studied, unlike GeO₂ glass, exhibit internal crystal nucleation without the addition of any nucleating agent. The devitrification processes occur in one or more steps. Phases which crystallized at each step are identified and crystallization mechanisms proposed. These crystallization mechanisms are related to structures of the crystallizing phases. Activation energy values as well as those for glass transition temperatures, do not vary linearly with increase in Li₂O content but pass through a maximum atx=0.200.     

Processing and characterization of new oxy-sulfo-telluride glasses in the Ge-Sb-Te-S-O system

New oxy-sulfo-telluride glasses have been prepared in the Ge-Sb-Te-S-O system employing a two-step melting process which involves the processing of a chalcogenide glass (ChG) and subsequent melting with TeO₂ or Sb₂O₃. The progressive incorporation of O at the expense of S was found to increase the density and the glass transition temperature and to decrease the molar volume of the investigated oxy-sulfo-telluride glasses. We also observed a shift of the vis-NIR cut-off wavelength to longer wavelength probably due to changes in Sb coordination within the glass matrix and overall matrix polarizability. Using Raman spectroscopy, correlations have been shown between the formation of Ge- and Sb-based oxysulfide structural units and the S/O ratio. Lastly, two glasses with similar composition (Ge₂₀Sb₆S₆₄Te₃O₇) processed by melting the Ge₂₃Sb₇S₇₀ glass with TeO₂ or the Ge₂₃Sb₂S₇₂Te₄ glass with Sb₂O₃ were found to have slightly different physical, thermal, optical and structural properties. These changes are thought to result mainly from the higher moisture content and sensitivity of the TeO₂ starting materials as compared to that of the Sb₂O₃.     

Dynamical processes in sol-gel derived rare-earth doped glasses

We have investigated the dynamical processes of rare-earth photoluminescence in silica glasses produced by the sol-gel method. Generally, static spectroscopic properties are used as a tool to probe the local structure near dopants as well as for the study of glass formation. Rare earth ions are preferred structural probes, especially the Eu³⁺ ion for its sensitivity to local structural modifications and its spectroscopic simplicity. By an analysis of the decaying emission that follows a pulsed laser excitation, we have studied several dynamical processes. Importance of a distribution of different environments for the dopant is clearly seen even for the wet gels. The evolution of the lifetimes allow to study the densification process.     

单离子导体聚合硼酸锂盐的研究进展

目前商业化锂离子电池常用的锂盐LiPF6,对水极其敏感,热稳定性差,尤其是在高温条件下的应用存在着一定的安全隐患.种类多且环境友好的新型有机硼酸锂盐越来越受到人们的重视.本文综述了近年来几种锂盐的合成方法,电化学性能,各自存在的优点和不足以及本课题组在聚合硼酸锂盐方向取得的系列研究进展,并对锂盐和聚合物电解质的发展方向进行了展望.     

Fluorescence properties of Eu3+ ions doped borate and fluoroborate glasses containing lithium, zinc and lead

The influence of glass composition on the fluorescence properties of Eu3+ ions doped borate and fluoroborate glasses modified with Li+, Zn2+ and Pb2+ cations have been investigated. The magnitude of splittings of 7F1 levels are analyzed using crystal-field (CF) analysis. The relative intensities of 5D0 --> 7F2 to 5D0 --> 7F1 transitions, crystal-field strength parameters and decay times of the 5D0 level have been determined and are found to be lower for Pb based glasses than those of Zn/Li based glasses. The lifetimes of 5D0 level are found to increase when borate glasses are modified with pure fluorides than with oxides and oxyfluorides. The fluorescence decay of 5D0 level fits perfect single exponential in the Eu3+:glass systems studied which indicates the absence of energy transfer between Eu3+ ions in these glasses.     

Thermal properties and stability of lithium titanophosphate glasses

DTA was used to study thermal properties and thermal stability of (50-x)Li₂O-xTiO₂-50P₂O₅ (x=0–10 mol%) and 45Li₂Ot-yTiO₂-(55-y)P₂O₅ (y=5–20 mol%) glasses. The addition of TiO₂ to lithium phosphate glasses results in a non-linear increase of glass transition temperature. All prepared glasses crystallize under heating within the temperature range of 400–540°C. The lowest tendency towards crystallization have the glasses with x=7.5 and y=10 mol% TiO₂. X-ray diffraction analysis showed that major compounds formed by annealing of the glasses were LiPO₃, Li₄ P₂O₇, TiP₂O₇ and NASICON-type LiTi₂(PO₄)₃. DTA results also indicated that the maximum of nucleation rate for 45Li₂O-5TiO₂-50P₂O₅ glass is close to the glass transition temperature.     

Optical characterization of Eu3+ and Tb3+ ions doped zinc lead borate glasses

This paper reports on the spectral analysis of Eu3+ or Tb3+ ions (0.5 mol%) doped heavy metal oxide (HMO) based zinc lead borate glasses from the measurement of their absorption, emission spectra and also different physical properties. From the XRD, DSC profiles, the glass nature and glass thermal properties have been studied. The measured emission spectrum of Eu3+ glass has revealed five transitions (5D0-->7F0, 7F1, 7F2, 7F3 and 7F4) at 578, 591, 613, 654 and 702 nm, respectively, with lambdaexci=392 nm (7F0-->5L6). In the case of Tb3+:ZLB glass, four emission transitions such as (5D4-->7F6, 7F5, 7F4 and 7F3) that are located at 489, 542, 585 and 622 nm, respectively, have been measured with lambdaexci=374 nm. For all these emission bands decay curves have been plotted to evaluate their lifetimes and the emission processes that arise in the glasses have been explained in terms of energy level schemes.     

Synthesis and characterization of a new layered lead borate

A new lead borate, Pb[B₈O₁₁(OH)₄], has been hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction, FTIR, powder X-ray diffraction and thermogravimetric analysis. The compound crystallizes in the monoclinic system, space group P2₁/n (No. 14), , , , β=90.327(2)°, , and Z=4. The structure consists of layers of 9-membered borate rings enclosing Pb²⁺ cations. Adjacent borate layers are interconnected via ionic Pb-O bonds and hydrogen bonding to form a 3D supramolecular network.     

Studies on lithium acetate doped chitosan conducting polymer system

The structure of chitosan contains the amine group that can act as electron donors. Complexation between chitosan and the salt can be proven by infrared and X-ray photoelectron spectroscopy methods. The NH₂, NH₃⁺ and OC-NHR vibrations which can be observed at 1590, 1560 and 1650 cm⁻¹ shift to lower wave numbers when the complexes are formed. The after deconvolution Li 1s core level spectrum of the chitosan-salt complexes can contain several gaussian components one of which has a binding energy peak at 55.2 eV which signifies Li-N interaction. The component that peaks at ∼403 eV in the N 1s core level spectrum complements the proof of N-Li interaction. The highest conductivity achieved for a plasticized chitosan-salt complex is of the order 10⁻⁶ S/cm using lithium acetate as the doping salt. Transference number studies prove that this material is ionic conductor and from transient ionic current studies that mobility of the ions is of the order of 10⁻⁴ cm²/V s.     

Synthesis and characterization of rare-earth doped SrBi2Nb2O9 phase in lithium borate based nanocrystallized glasses

Glass composites comprising of un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites are fabricated in the glass system 16.66SrO-16.66[(1−x)Bi₂O₃-xSm₂O₃]-16.66Nb₂O₅-50Li₂B₄O₇ (0?x?0.5, in mol%) via the melt quenching technique. The glassy nature of the as-quenched samples is established by differential thermal analyses. Transmission electron microscopic studies reveal the presence of about 15 nm sized spherical crystallites of the fluorite-like SrBi_1.9Sm_0.1Nb₂O₉ phase in the samples heat treated at 530 °C. The formation of layered perovskite-type un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites with an orthorhombic structure through the intermediate fluorite phase is confirmed by X-ray powder diffraction and micro-Raman spectroscopic studies. The influence of samarium doping on the lattice parameters, lattice distortions, and the Raman peak positions of SrBi₂Nb₂O₉ perovskite phase is clarified. The dielectric constants of the perovskite SrBi₂Nb₂O₉ and SrBi_1.9Sm_0.1Nb₂O₉ nanocrystals are relatively larger than those of the corresponding fluorite-like phase and the precursor glass.     

Conductivity and modulus formulation in lithium modified bismuth zinc borate glasses

The conductivity and modulus formulation in lithium modified bismuth zinc borate glasses with compositions xLi₂O–(50-x) Bi₂O₃–10ZnO–40B₂O₃ has been studied in the frequency range 0.1 Hz–1.5 × 10⁵ Hz in the temperature range 573 K–693 K. The temperature and frequency dependent conductivity is found to obey Jonscher's universal power law for all the studied compositions, the dc conductivity (σ_dc), crossover frequency (ω_H), and frequency exponent (s) have been estimated from the fitting of the experimental data of ac conductivity with Jonscher's universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating centre (H_f) and enthalpy of migration (H_m) have been estimated. It has been observed that number of charge carriers and ac conductivity in the lithium modified bismuth zinc borate glasses increases with increase in Li₂O content. Further, the conduction mechanism in the glass sample with x = 0 may be due to overlapping large polaron tunneling, whereas, conduction mechanism in other studied glass samples more or less follows diffusion controlled relaxation model. The ac conductivity is scaled using σ_dc and ω_H as the scaling parameter and is found that these are suitable scaling parameter for conductivity scaling. Non-Debye type relaxation is found prevalent in the studied glass system. Scaling of ac conductivity as well as electric modulus confirms the presence of different type of conduction mechanism in the glass samples with x = 0 and 5 from other studied samples. The activation energy of relaxation (E_R) and dc conductivity (E_dc) are almost equal, suggesting that polarons/ions have to overcome same barrier while relaxing and conducting.     

Spectroscopic studies of charge-ordering system in organic conductors

Charge localization generates a non-uniform charge distribution in some organic conductors. Phase transitions accompanying such a localization of charge are studied by using infrared and Raman spectroscopy. We first introduce θ-(BEDT-TTF)₂MM′(SCN)₄ (M=Rb, Cs, Tl; M′=Zn, Co) as typical examples of a charge-ordering system, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. We apply the same spectroscopic technique to α′-(BEDT-TTF)₂IBr₂, θ-(BDT-TTF)₂Cu(NCS)₂, and (TTM-TTP)I₃, which show the phase transitions from low-resistivity to high-resistivity state, where TTM-TTP is 2,5-bis[4,5-bis(methylsulfonyl)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene.