Role of monovalent alkali ions in the Yb3+ centers of CaF2 laser crystals

Yb³⁺ and M⁺ monovalent alkali ions (M⁺ = Li⁺, Na⁺, K⁺)-co-doped CaF₂ cubic laser crystals were grown by the micro-pulling-down method (μ-PD) under CF₄ atmosphere. Structural and spectroscopic characterizations of Yb³⁺ in substitution of Ca²⁺ (absorption, emission and decay curves) were carried out to study the effect of M⁺ ions as charge compensators.     

Spectroscopic properties, energy transfer and structural analysis of Sr2CeO4:M+ and Sr2CeO4:Eu3+, M+ (M+ = Li+, Na+, K+)

The room-temperature luminescent emission characteristics of Sr₂CeO₄:M⁺ and Sr₂CeO₄:Eu³⁺,M⁺ (M⁺ = Li⁺, Na⁺, K⁺) have been investigated under UV excitation. By introducing appropriate alkali metal cations dopants (Li⁺, Na⁺, K⁺) into the crystalline lattice, not only emission color of the blue-white-emitting Sr₂CeO₄ doped with low Eu³⁺ content can be tuned to green, but also the red emission intensity of Sr₂CeO₄ doped with high Eu³⁺ concentration is strengthened significantly. The relevant mechanisms have been elucidated in detail.     

Charge storage performance of lithiated iron phosphate/activated carbon composite as symmetrical electrode for electrochemical capacitor

In this study, a symmetric electrochemical capacitor was fabricated by adopting a lithium iron phosphate (LiFePO₄)-activated carbon (AC) composite as the core electrode material in 1.0 M Na₂SO₃ and 1.0 M Li₂SO₄ aqueous electrolyte solutions. The composite electrodes were prepared via a facile mechanical mixing process. The structural properties of the nanocomposite electrodes were characterised by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis. The electrochemical performances of the prepared composite electrode were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy (EIS). The experimental results reveal that a maximum specific capacitance of 112.41 F/g was obtained a 40 wt% LiFePO₄ loading on an AC electrode compared with that of a pure AC electrode (76.24 F/g) in 1 M Na₂SO₃. The improvement in the capacitive performance of the 40 wt% LiFePO₄–AC composite electrode is believed to be attributed to the contribution of the synergistic effect of the electric double layer capacitance (EDLC) of the AC electrode and pseudocapacitance via the intercalation/extraction of H⁺, OH⁻, Na⁺ and SO₃²⁻ and Li⁺ ions in LiFePO₄ lattices. In contrast, it appears that the incorporation of LiFePO₄ into AC electrodes does not increase the charge storage capability when Li₂SO₄ is used as the electrolyte. This behaviour can be explained by the fact that the electrolyte system containing SO₄²⁻ only exhibits EDLC in the Fe-based electrodes. Additionally, Li⁺ ions that have lower conductivity and mobility may lead to poorer charge storage capability compared to Na⁺ ions. Overall, the results reveal that the AC composite electrodes with 40 wt% LiFePO₄ loading on a Na₂SO₃ neutral electrolyte exhibit high cycling stability and reversibility and thus display great potential for electrochemical capacitor applications.     

Structural,ferroelectric, optical properties of A-site-modified Bi0.5(Na0.78K0.22)0.5Ti0.97Zr0.03O3 lead-free piezoceramics

We reported the role of A-site modification on the structural, ferroelectric, optical and electrical field-induced strain properties of Bi_0.5(Na_0.78K_0.22)_0.5Ti_0.97Zr_0.03O₃ lead-free piezoceramics. The Li⁺ ions with concentration from 0 to 5 mol% were used to substitute at A-site. There was no phase transition when Li⁺ ions was added up to 5 mol%. The electric field-induced strain (S_max/E_max) values increased from 600 to 643 pm/V for 2 mol% Li⁺-added which results from distortion both rhombohedral and tetragonal phase structures. The band gap reduced from 2.88 to 2.68 eV and the saturation polarization decreased from 46.2 to 26.1 μC/cm² when Li⁺ ions concentration increased from 0 to 5 mol% respectively. We expect that this work could be helpful for further understanding the role of A-site dopants in comparison with B-site modification in lead-free Bi_0.5(Na,K)_0.5TiO₃-based ceramics.     

Synthesis and electrochemical characterization of M2Mn3O8 (M = Ca,Cu) compounds and derivatives

M₂Mn₃O₈ (M = Ca²⁺, Cu²⁺) compounds were synthesized and characterized in lithium cells. The M²⁺ cations, which reside in the van der Waals gaps between adjacent sheets of Mn₃O₈⁴⁻, may be replaced chemically (by ion-exchange) or electrochemically with Li. More than 7 Li⁺/Cu₂Mn₃O₈ may be inserted electrochemically, with concomitant reduction of Cu²⁺ to Cu metal, but less Li can be inserted into Ca₂Mn₃O₈. In the case of Cu²⁺, this process is partially reversible when the cell is charged above 3.5 V vs. Li, but intercalation of Cu⁺ rather than Cu²⁺ and Li⁺/Cu⁺ exchange occurs during the subsequent discharge. If the cell potential is kept below 3.4 V, the Li in excess of 4 Li⁺/Cu₂Mn₃O₈ can be cycled reversibly. The unusual mobility of + 2 cations in a layered structure has important implications both for the design of cathodes for Li batteries and for new systems that could be based on M²⁺ intercalation compounds.     

Luminescent properties of R doped Sr2SiO4:Eu(R=Li, Na and K) red-emitting phosphors for white LEDs

Sr₂SiO₄:Eu³⁺ and Sr₂SiO₄:Eu³⁺ doped with R⁺(R⁺=Li⁺, Na⁺ and K⁺) phosphors were prepared by conventional solid-state reaction and investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. XRD patterns and SEM reveal that the optimal firing condition for Sr₂SiO₄:Eu³⁺ was 1300 °C for 4 h. The excitation and emission spectra indicate that the phosphor can be effectively excited by ultraviolet (395 nm) and blue (466 nm) light and emits intense red light peaked at around 614 nm corresponding to the ⁵D₀→⁷F₂ transitions of Eu³⁺. In the research work, the effect of R⁺ contents on luminescence property and the Eu³⁺ concentration quenching process have also been investigated. The Eu³⁺ concentration quenching mechanism was verified to be a multipole-multipole interaction and the critical energy-transfer distance was calculated to be around 14.6 Å. The dopant R⁺(R⁺=Li⁺, Na⁺ and K⁺) as charge compensator in Sr₂SiO₄:Eu³⁺ can further enhance luminescence intensity, and the emission intensity of Sr₂SiO₄:Eu³⁺ doping Li⁺ is higher than that of Na⁺ or K⁺.     

Transport processes in heavy metal fluoride glasses

Na self-diffusion, Li self-diffusion, Na⁺–Li⁺ ion exchange, electrical conductivity, and mechanical relaxation have been studied below T_g on glasses of the system ZrF₄–BaF₂–LaF₃–AF (A=Na, Li), with A=10, 20, 30 mol%. Compared to the transport mechanism in alkali-containing silicate glasses, the mechanisms in these non-oxide glasses are anomalous. Thus the self-diffusion coefficient of Na decreases with increasing NaF content, whereas that of Li increases with increasing LiF content. Both the electrical conductivity and the Na⁺–Li⁺ ion exchange reach a minimum at ≈ 20 mol% LiF, and the mechanical relaxation shows one peak for the 20 and 30 mol% LiF-glasses and two peaks for the glass with 10 mol% LiF, evidencing both a contribution of F⁻ and Li⁺ ions to the transport. Moreover, the presence of the three partially interacting mobile species F⁻, Na⁺, Li⁺ obviously leads to an anionic–cationic mixed ion effect. Applying the Nernst–Einstein equation to the Li⁺ transport in LiF-containing glasses shows that its mechanism is dissimilar to that in oxide glasses. Calculated short jump distances possibly can be interpreted as an Li⁺ movement via energetically suitable sites near F⁻ ions. Likewise the Nernst–Planck model, successfully applied to the ionic transport in mixed alkali silicate glasses, obviously does also not hold for the present heavy metal fluoride glasses.     

Effects of alkali metal ions on upconversion photoluminescence intensity of Er3+-doped Y2O3 nanocrystals

We investigate upconversion emissions in alkali metal ions (Li⁺, Na⁺, and K⁺) and Er³⁺-codoped Y₂O₃ nanocrystals. By introducing Li⁺, upconversion intensity is significantly enhanced, while Na⁺ and K⁺ hardly have this influence. FT-IR data give evidence that the main mechanisms of the enhanced upconversion emission cannot be attributed to the decreased surface defects. EXAFS data and variations of enhanced upconversion intensities in different samples indicate that Li⁺ can occupy the interstitial sites in lattice and thus arouse large site asymmetry. In addition, locations in the samples and effects on the upconversion emission of Na⁺ and K⁺ are discussed.     

Luminescent properties of Eu3+ and Sm3+ activated M2SiO4 (M=Ba,Sr and Ca) red-emitting phosphors for WLEDs

Eu³⁺ and Sm³⁺ activated M₂SiO₄ (M=Ba, Sr and Ca) red-emitting phosphors were synthesized by a solid state reaction. The results of XRD and SEM measurements show that the samples are single phase and have irregular shape. The excitation and emission spectra indicate that these phosphors were effectively excited by ultraviolet (395 nm) and blue (466 nm) light and exhibited red performance. The charge compensator R⁺ (R⁺=Li⁺, Na⁺ and K⁺) injecting into the host efficiently enhanced the luminescence intensity of the M₂SiO₄: Eu³⁺ and M₂SiO₄: Sm³⁺ phosphors. The emission intensity of M₂SiO₄: Eu³⁺ and Sm³⁺ doping Li⁺ were higher than that of Na⁺ or K⁺.     

Rhodamine B-based “turn-on” fluorescent and colorimetric chemosensors for highly sensitive and selective detection of mercury (II) ions

Two novel macromolecules based on 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine covalently bound to one and two units of rhodamine-B moieties, 1 and 2, were prepared and utilized as fluoroionophores and chromophores for the optical detection of Hg²⁺ ions. The sensors were readily prepared by a conventional two-step synthesis. Especially, sensor 1 exhibits high sensitivity and selective OFF–ON fluorescence enhancement and chromogenic change upon binding to Hg²⁺, which served as a “naked-eye” indicator by a noticeable color change of the solution (from colorless to pink–red color). 1 is shown to discriminate various competing metal ions, particularly Ag⁺ and Cu²⁺, as well as Cd²⁺, Na⁺, Li⁺, K⁺, Ba²⁺, Co²⁺, Ni²⁺, Mg²⁺, Mn²⁺ and Al³⁺, with a detection limit of 10 ppb.     

Preparation and characteristics of Li4Ti5O12 with various dopants as anode electrode for hybrid supercapacitor

Spinel-Li₄Ti₅O₁₂ is successfully synthesized by a solid phase synthesis. The Li₄Ti₅O₁₂ powders with various dopants (Al³⁺, Cr³⁺, Mg²⁺) synthesized at 800 °C are in accordance with the Li₄Ti₅O₁₂ cubic spinel phase structure. The dopants are inserted into the lattice structure of Li₄Ti₅O₁₂ without causing any changes in structural characteristics. In order to study the effect on various dopants, the hybrid supercapacitor is prepared by using un-doped Li₄Ti₅O₁₂ and doped Li₄Ti₅O₁₂ in this work. The electrochemical performance of the hybrid supercapacitor is characterized by impedance spectroscopy and cycle performance. The results show Cr³⁺ and Mg²⁺ dopants enhance the conductivity of Li₄Ti₅O₁₂. Also, Al³⁺ substitution improves the reversible capacity and cycling stability of Li₄Ti₅O₁₂. It is found that effect of dopant on the electrochemical performance of Li₄Ti₅O₁₂ as electrode material for hybrid supercapacitor where the EDLC and the Li ion secondary battery coexist in one cell system.     

Ultrasound-assisted Li+/Na+ co-intercalated exfoliation of graphite into few-layer graphene

In this work, we developed a novel approach for few-layer graphene by employing Li⁺/Na⁺ co-intercalated exfoliation assisted by ultrasound method. The experiments were conducted under the ultrasonic power of 300 W and the frequency of 40 kHz without the participation of any organic solvent. The effect of Li⁺/Na⁺ proportion on the exfoliation of graphite was intensively investigated. The structure and morphology of the as-exfoliated graphene nanosheets (UGN) was determined by a series of characterizations. The results showed that the thicknesses of the as-exfoliated graphene nanosheets were about 2.38–2.56 nm (about 7–8 layers) at the optimal Li⁺/Na⁺ ratio. The potential application of the as-exfoliated graphene nanosheets as additive in grease was evaluated by four-ball friction tester. The results demonstrated that the antifriction and antiwear performances of the grease with 0.06 wt% graphene were significantly improved by 21.35% and 30.32% relative to pure grease, respectively. The friction mechanism was proposed by detecting the worn surfaces.     

二价Ba2+、Pb2+和四价Ti4+在硼硅酸盐玻璃系统中对Li+/Na+离子交换的影响

本文讨论了在硼硅酸盐玻璃系统中引入二价Ba²⁺、Pb²⁺和四价Ti⁴⁺后,由于Pb²⁺和Ti⁴⁺配位结构的变化,引起Pb²⁺、Ti⁴⁺场强及其与O^2-离子间距的变化,对Li⁺/Na⁺离子交换产生影响,从而改变折射率差值(ΔN)和折射率分布.     

Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb³⁺/Er³⁺ co-doped Gd₆MoO₁₂ and Yb³⁺/Er³⁺/Li⁺ tri-doped Gd₆MoO₁₂ phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb³⁺ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb³⁺ concentration quenching effects, the critical distance between Yb³⁺ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er³⁺(⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺(⁴I_15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li⁺ ions mixing, the upconversion emission enhanced largely, and the optimum Li⁺ concentration was obtained while fixed the Yb³⁺ and Er³⁺ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er³⁺ after Li⁺ ions doping into the system. This result indicates that Li⁺ is a promising candidate for improving luminescence in some case.     

Raman study on electrochemical lithium insertion into multiwalled carbon nanotubes

We have carried out in situ Raman studies during the electrochemical insertion of lithium ions (Li⁺) into pristine and thermally treated multiwalled carbon nanotubes (MWNTs). We found an improved structural integrity as well as the removal of defects in the thermally treated tubes. The different Li⁺ insertion behaviors above 0.5 V in as‐grown and thermally treated tubes could be explained by the presence of defects on the outer surface of the tubes. No change of Raman spectra from 2.8 to 0.8 V is characterized by the coverage of Li⁺ on the outer surface of tubes, whereas the upshift of G band and the absence of a separated G band below 0.75 V indicate the formation of diluted graphite intercalation below the stage‐2 phase (LiC₁₂). Copyright © 2008 John Wiley & Sons, Ltd.     

Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

Summary With the aid of the Sanderson model for nonmolecular structure a qualitative criterion is derived for forecasting off-centre configurations of monovalent impurity ions (Li⁺, F⁻, Cl⁻, Na⁺, Ag⁺, Cu⁺) in alkali halide crystals. The same criterion is checked for Mn²⁺ impurity ions in some oxides. We also present a critical review of the criteria introduced in the past, as well as of the experimental results up to now known in the literature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

掺杂石墨烯吸附特性的第一性原理研究

利用第一性原理方法研究了一氧化碳分子在本征和硼、氮、铝、磷掺杂的有限尺寸石墨烯上的吸附机理.结果表明,石墨烯作为一氧化碳传感器时的性能依赖于掺杂元素.本征、硼和氮掺杂石墨烯吸附一氧化碳时的吸附能较低,为物理吸附.铝、磷掺杂石墨烯的吸附能显著提高,比本征、硼和氮掺杂时高出约一个数量级,且铝和磷原子从石墨烯中突出,使其发生局部弯曲.铝掺杂石墨烯增强了石墨烯与一氧化碳分子之间的相互作用,可以提高石墨烯的气敏性和吸附能力,是一氧化碳传感器的最佳候选材料之一.     

Hofmeister series and ionic effects of alkali metal ions on DNA conformation transition in normal and less polarised water solvent

Normal and less polarised water models are used as the solvent to investigate Hofmeister effects and alkali metal ionic effects on dodecamer d(CGCGAATTCGCG) B-DNA with atomic dynamics simulations. As normal water solvent is replaced by less polarised water, the Hofmeister series of alkali metal ions is changed from Li⁺ > Na⁺ ? K⁺ ? Cs⁺ ? Rb⁺ to Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺. In less polarised water, DNA experiences the B→A conformational transition for the lighter alkali metal counterions (Li⁺, Na⁺ and K⁺). However, it keeps B form for the heavier ions (Rb⁺ and Cs⁺). We find that the underlying cause of the conformation transition for these alkali metal ions except K⁺ is the competition between water molecules and counterions coupling to the free oxygen atoms of the phosphate groups. For K⁺ ions, the ‘economics’ of phosphate hydration and ‘spine of hydration’ are both concerned with the DNA helixes changing.     

Effect of Li-substitution on piezoelectric and ferroelectric properties of (Bi0.92Na0.92−xLix)0.5Ba0.06Sr0.02TiO3 lead-free ceramics

New lead-free ceramics (Bi_0.92Na_0.92−xLi_x)_0.5Ba_0.06Sr_0.02TiO₃ have been fabricated by a conventional ceramic technique and their electrical properties have been studied. X-ray diffraction studies reveal that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. The partial substitution of Li⁺ for Na⁺ increases the remanent polarization P_r of the ceramics. Because of the large P_r and low coercive field E_c, the ceramics with x = 0.075–0.125 exhibit excellent piezoelectric properties: d₃₃ = 189–235 pC/N, k_p = 33.6–36.3% and k_t = 51.6–54.3%. The ceramics exhibit relaxor behaviors after the substitution of Li⁺ for Na⁺. Our results also suggest that polar and non-polar phases may coexist in the ceramics at temperatures above the depolarization temperature T_d.     

First applicability of ZnGa2O4 : Ge, Li, Mn phosphor for a plasma display panel

The green emission intensity of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ excited by the vacuum ultraviolet line of 147 nm reaches 70% of commercial green Zn₂SiO₄:Mn²⁺. The vacuum ultraviolet excitation spectra consist of four peaks. In a plasma display test bed filled with Ar and Ne plasma discharged by a radio-frequency generator of 13.6 MHz, ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ and commercial Zn₂SiO₄:Mn²⁺ phosphor screens show a linear increase in luminance with increasing self bias voltages. Increasing gas pressures cause the luminance to increase. Also, on increasing the self bias voltages and the gas pressures, the current densities of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphor screens are increased; this is the same behavior as that of the commercial phosphor.