Pressure effect on mechanical stability and ground state optoelectronic properties of Li2S2 produced by Lithium−Sulfur batteries discharge: GGA-PBE,GLLB-SC and mBJ investigation

ABSTRACT

According to several previous works, Li₂S₂ occurs during discharge of lithium–sulfur (Li–S) batteries. Several information on the physical properties of this compound remain unknown including those of its ground state. In this work, a study of the pressure effect on the elastic and optoelectronic properties of Li₂S₂ in its tetragonal structure of P4₂/mnm space group was carried out. According to the previous works and according to the structural results, which have been obtained by GGA-PBE and optPBE-vdW funtionals, we have found that P4₂/mnm-Li₂S₂ energy is the lowest compared to other studied structures (P6₃/mmc, P-62m and Immm), this can indicate that P4₂/mnm space group structure may represent its ground state structure. The study of the pressure effect on the elastic properties has shown that P4₂/mnm-Li₂S₂ maintains its mechanical stability for a pressure range between 0 and 2.5?GPa. The study of the directional dependence of Young's modulus has shown that it is anisotropic and therefore indicates the elastic anisotropy of the studied compound. According to the electronic results, the fundamental band-gap of P4₂/mnm-Li₂S₂ is of an indirect nature, its values found by mBJ and GLLB-SC are close to each other. The pressure effect on the possible transitions between the valence band top and the conduction band bottom and on the variations of the refractive index and the absorption coefficient according to both optical directions has been studied by determining the dielectric function. The found optical results have shown that P4₂/mnm-Li₂S₂ has negative uniaxial birefringence.     

Determination of optical nonlinearities in Cu(mpo)2 by Z-scan technique

The optical nonlinearities of Cu(mpo)₂ [formula = C₁₀H₈N₂CuO₂S₂] complex have firstly been investigated by using the Z-scan technique with a nanosecond-pulsed Nd:YAG laser at its second harmonic (532 nm) radiation. The nonlinear optical absorption and refraction coefficients of Cu(mpo)₂ have been measured with the different on-axial peak irradiances I ₀ at the waist ranging from 0.48 to 5.66 GW/cm². The nonlinear transmittance characteristics exhibit the near resonant two-photon absorption (TPA) at 532 nm and are explained by a population redistribution model. The nonlinear absorption originates from the near resonant TPA while the mechanism of the nonlinear refraction is the near resonant TPA transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant TPA.     

Application of a characterized difference-frequency laser source to carbon monoxide trace detection

A tunable continuous wave(cw) mid-infrared(MIR) laser based on difference-frequency generation(DFG) in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported.Two visible and near-infrared diode lasers were used as pump and signal sources.The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm.The phase-matching(PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths.The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U,et al.(Willer U,Blanke T and Schade W 2001 Appl.Opt.40 5439).Then the calculated PM temperatures are compared with the experimental values.The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide(12C16O) absorption line in a laboratory-fabricated absorption cell.The enhanced sensitivity of about 0.6×10 4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.     

Evolution of ferroelectric SrBi2Nb2O9 phase embedded in tellurite glass

Glasses with the composition, [(100-x)TeO₂- x(SrO–Bi₂O₃–Nb₂O₅)] with x = 20, 30 and 40 (in mol %) were prepared. The X-ray diffraction (XRD) pattern and differential thermal analysis (DTA) for the as-prepared samples confirmed the amorphous and glassy characteristics, respectively. The SrBi₂Nb₂O₉ phase in tellurite glass for HT773 sample at x = 40 mol % is formed and confirmed by the Rietveld refinement. DTA curves for all glass samples exhibit two endothermic dips while the two broad exothermic peaks at lower x reduced to one at higher x. Infrared (IR) results revealed that the glassy matrix are composed of TeO₃, TeO₃₊₁, TeO_4, BiO₆ and NbO₆ structural units. The changes in the density (ρ), molar volume (V_m), oxygen molar volume (V₀) and oxygen packing fraction (OPD) have correlated with structural changes in the glass network. The optical studies show an absorption bands below the absorption edge in the glass samples.     

Point defects related to 260 K thermostimulated luminescence in α-Al2O3

Abstract

Thermo- and photostimulated processes are studied in reduced hydrogen containing α-Al₂O₃ excited by UV light. It is found that UV excitation in F absorption band at 90 K results in a ionization of the F-centers and capture of released electrons at defects thus producing an anisotropy absorption band at 4.2 eV and the dominant thermoluminescence (TL) peak at 260 K. The 260 K TSL peak is accompanied by complete bleaching of the 4.2 eV absorption band and vice versa—by light stimulation in the region of the 4.2 eV band the 260 K TSL peak disappear and released electrons recombine with F⁺-centers. Both the effect of the preliminary high-temperature thermal treatment of samples on formation of 4.2 eV-centers and the observed dichroism characteristics allows to conclude that corresponding complex defect contains hydrogen and can involve vacancy pair.     

微弧等离子喷涂碳纳米管/纳米Al2O3-TiO2复合涂层的吸波性能研究

将碳纳米管与纳米Al₂O₃-TiO₂陶瓷粉末超声共混制备了碳纳米管/纳米Al₂O₃-TiO₂复合粉末,测试了复合粉末在2—18GHz波段的电磁参数.研究表明：随着碳纳米管质量分数的增加,碳纳米管/纳米Al₂O₃-TiO₂复合粉末的复介电常数和损耗角不断增大.当碳纳米管质量分数和厚度增加时,复合粉末对电磁波的反射率峰值先增加后减小,而谐振频率不断向低频移动.采用微弧等离子喷涂制备了7wt%碳纳米管/纳米Al₂O₃-TiO₂复合吸波涂层,当厚度为1.5mm时,涂层最小反射率为-24.0dB,当厚度为2.0mm时,涂层小于-10dB的频带宽为3.60GHz,当温度为500℃高温时,1.0mm厚的涂层最小高温反射率为-12.2dB,小于-10dB频带宽为2.0GHz.复合涂层的实际厚度D与理论厚度d呈线关系：d=0.898D+0.515. 关键词： 等离子喷涂 碳纳米管 2O₃-TiO₂')" href="#">纳米Al₂O₃-TiO₂ 吸波性能     

MgF2单晶的THz光谱研究

利用THz时域光谱技术对MgF₂晶体(样品1)和MgF₂：Co晶体(样品2)在0.5—2.5 THz的吸收特性进行了研究.在0.5—2.5 THz波段，样品1吸收系数α(ν)随频率ν增加而增大，最大值为24 cm^-1.样品2的吸收系数比样品1大得多，Co掺杂使晶格吸收带边向低频移动，而且样品2在1.9 THz有吸收峰，吸收系数达到70 cm^-1，由此求出F^--Co²⁺离子键伸缩振动的键力常数K为3.40×10^-2　N/cm.这一结果表明，THz光谱分析有可能成为研究晶体化学键的一种重要手段.利用光学常数之间的关系计算了两个样品在0.5—2.5 THz的介电函数的实部ε₁(ν)，得到样品1的ε₁(ν)值在4.67至4.73之间，样品2的ε₁(ν)值在4.62至5.01之间. 关键词： THz辐射 光谱 2晶体')" href="#">MgF₂晶体     

Influence of Y2Cu2O5 nanoparticles doping on superconducting properties of YBa2Cu3O7-δ

ABSTRACT

The blue phase of YBa₂Cu₃O_{7- δ} (YBCO) family, Y₂Cu₂O₅ (Y202) nanoparticles were prepared and doped into (YBCO) superconductor and the effect of doping on critical current density and critical temperature was investigated. Y202 nanoparticles with particle sizes of 47, 107 and 206?nm were prepared by a sol–gel combustion method and added into the YBCO superconductor by 0.5–2?wt.%. XRD and scanning electron microscope measurements were used to characterize the samples. The measurement of critical current density at 77?K revealed that the doped superconductors had larger critical current density compared to the undoped superconductors. For a fixed dopant concentration, by increasing the size of nanoparticles, the J_c was increased. For the samples including 0.5?wt.% of nanoadditives, J_c was higher. The highest critical current density of 137?A/cm² was measured for the superconductors containing 0.5?wt.% of 206?nm Y202 nanoparticles. Also, by increasing the nanoparticles concentration, the T_c was reduced.     

New compounds with incommensurate phases: Cs2CdI4 and Cs2ZnI4

Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs₂CdI₄ and Cs₂ZnI₄. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P2₁/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs₂ZnI₄ crystal in a broad pre-transition region (T - T _i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).     

Dielectric and spectroscopic investigations of lithium aluminium zirconium silicate glasses mixed with TiO2

Li₂O–Al₂O₃–ZrO₂–SiO₂ glasses mixed with different concentrations of TiO₂ (ranging from 0 to 5.0?mol%) were synthesised and their dielectric properties (dielectric constant, loss tan?δ, a.c. conductivity σ) investigated over wide ranges of frequency and temperature. Studies of optical absorption, ESR, infrared (IR) and photoluminescence properties have also been undertaken. A decrease in dielectric parameters with increasing concentrations of TiO₂ has been observed and this is attributed to an increasing proportion of titanium ions occupying network-forming positions rather than going into interstitial positions. A.C. conductivity in the high-temperature region appears to be connected both to electronic transfer and ionic movements, but conduction attributed to such processes seems to be hampered by the entry of titanium ions into the network-forming positions. Analysis of the results of the IR spectral studies have indicated that there is a decreasing degree of disorder in the glass network with increasing TiO₂ content. The optical absorption and ESR spectral studies have revealed that titanium ions exist in both Ti³⁺ and Ti⁴⁺ states in the glasses. Luminescence spectra exhibited an emission band in the visible region and the luminescence efficiency increased with TiO₂ content. The excitation of substitutionally positioned octahedral Ti⁴⁺ ions is identified as being responsible for the observed luminescence emission.     

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract

High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD₃ND₃)₂MnCl₄. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) Å; b = 7.409 (8) Å c = 17.126 (12) Å and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (?10%) of the inter-layer distance. Space groups P2/c or P2₁/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.     

Tm:Y2SiO5晶体的生长和光谱性质研究

采用中频感应提拉法生长了高质量的Tm:Y₂SiO₅(Tm:YSO)晶体,测定了晶体的晶格常数和分凝系数.运用劳厄照相法确定了单斜晶系Tm:YSO晶体的三个偏振轴〈010〉,D₁和D₂,在室温下测量了三个偏振轴方向的吸收光谱、荧光光谱和荧光寿命,计算了晶体吸收峰的吸收线宽和吸收截面.研究发现,相对于其他两个偏振轴方向,D₁方向在790nm处出现较强的吸收峰, 关键词： 2SiO₅')" href="#">Tm:Y₂SiO₅ 单斜晶系 吸收光谱 荧光光谱     

Thermoluminescence and opitical absorption of BaF2 crystals

Abstract

Nominally pure and Dy-doped BaF₂ crystals were investigated concerning their optical absorption (OA) and thermoluminescence (TL) properties. Peaks at 120—150 and 200°C were observed for a heating rate of 1.7°C/s. The TL response for γ-rays and the TL emission spectra were obtained for these peaks. Except for the purest crystal, all BaF₂ crystals produced OA bands before irradiation typical of Ce³⁺ ions. After irradiation, Dy doped crystals showed bands due to Dy²⁺ ions. A nominally pure sample gave bands related to Ce²⁺ ions and photochromic centers of Ce³⁺ ions. and photochromic centres of Ce³⁺ ions. The correlation between some OA bands and TL peaks is discussed.     

Microscopic distortion and order parameter in langbeinite K2Cd2(SO4)3

The structural data on the orthorhombic phase of K₂Cd₂(SO₄)₃ at four different temperatures of Abraham et al. (1978) [J. Chem. Phys., 68, 1926] is reanalyzed in terms of frozen symmetry modes. The eigenvector of the primary distortion present in the orthorhombic phase is derived. The temperature variation of the amplitude of the order parameter is determined and compared with those of the amplitude of the frozen secondary distortion and the amplitude of the orthorhombic strain. In contrast with some previous literature, it is shown that, within experimental accuracy, all these magnitudes exhibit the correlation expected from a conventional Landau model.     

Structures and luminescence properties of Yb3＋ in the double perovskites Ba2YB＇O6 （B＇=Ta5＋, Nb5＋） phosphors

The Yb³⁺ doped Ba₂YB'O₆ (B'=Ta⁵⁺, Nb⁵⁺) were prepared by high temperature solid-state reaction method, their structures were determined by x-ray diffraction and refined by Rietveld method. The diffuse reflection absorption, excitation and emission spectra of Yb³⁺:Ba₂YB'O₆ (B'=,Ta⁵⁺, Nb⁵⁺) were measured at room temperature. Under the excitation of ultraviolet light, these phosphors exhibit broad charge transfer band emissions of TaO₆ or NbO₆ centre with large Stokes shift. The Yb³⁺ doped into these hosts are situated at Y³⁺ sites of cubic symmetry (O_h). The experimental energy levels of Yb³⁺ in Ba₂YTaO₆ and Ba₂YNbO₆ were determined by photoluminescence and diffuse reflection absorption spectra. Their wavefunctions and theoretical energy levels were obtained by diagonalising the Hamiltonian matrix. The experimental energy levels were fitted by Levenberg--Marquardt iteration algorithm to determine crystal field parameters. Then, the magnetic-pole transition line strengths of Yb³⁺:Ba₂YB'O₆(B'=Ta⁵⁺, Nb⁵⁺) from (²F_5/2)Γ₈^- to the low-energy states were calculated.     

Synthesis and photocatalytic oxidation of different organic dyes by using Mn2O3/TiO2 solid solution and visible light

Mn₂O₃/TiO₂ solid solution was prepared from two different oxides, manganese oxide (from KMnO₄ and ethanol) and TiO₂, these samples were characterized by BET, XRD, EDAX, SEM, FT-IR, ESR, XPS and UV–vis absorption spectroscopy. Photocatalytic activities of Mn₂O₃/TiO₂ powder was investigated by photooxidation of different dyes like Rhodamine B, thymol blue, methyl orange and Bromocresol green under visible light (300-W Xe lamp; λ > 420 nm). The results show that the alloy of TiO₂ with 1 mol% of Mn₂O₃ (MNT1) exhibit photocatalytic activity 3–5 times higher than that of P25 TiO₂ for oxidation of various dyes (RB, TB, MO and BG). The average particle size and crystallite size of MNT1 were found to be 100 nm and 12 nm measured from SEM and XRD, respectively. The EPR spectra of the Mn₂O₃/TiO₂ samples is a sharp five-line Mn(III) component centered on g_eff = 1.99.     

Study of the excitation properties of the Si3O2 cluster under an external electric field

The structure of the Si₃O_x (x=2, 3) cluster is investigated; we find that the geometry of Si₃O₂ is similar to that of Si₃O₃ except for the oxygen-deficient defect structure (Si-Si band) which exists only in the Si₃O₂ cluster. It is known that oxygen-deficient defects are used to explain visible luminescence (especially blue, purple and ultraviolet light) from silicon-based materials, which are directly bound up with the excited states of the molecules. Therefore the excitation properties of the two clusters are also studied. Our results show that the absorption spectrum of Si₃O₂ is concentrated in the visible light region. In contrast, the absorption spectrum of Si₃O₃ is mainly located in the ultraviolet light region. The calculations are perfectly consistent with experimental data and also support the theory of oxygen-deficient defects.     

γ-irradiation effect on the spectroscopic and electrical properties of K2SO4—Na2SO4 mixed system

Abstract

Measurements of electron paramagnetic resonance, infrared and electrical properties were carried out for the K₂SO₄—Na₂SO₄ mixed system before and after γ-irradiation. EPR measurements revealed the presence of a quartet of lines characterized by an isotropic g-value of 2.0034. These lines are mainly attributed to the formation of a SO^? ₃ center which results from the interaction of γ-rays with the sulfate ion. A decrease in the absorption intensity of the Infrared radiation was observed after γ-irradiation due to radiation damage in the sulfate group. The electrical conductivity, σ, was measured for the K₂SO₄—Na₂SO₄ system before and after γ-irradiation in the temperature range from 30 up to 430°C. A considerable decrease in the conductivity value accompanied by an increase in the activation energy was observed after γ-irradiation. The energy of formation of Frenkel defects was estimated to be 2.94eV. The current-voltage characteristics were measured at different temperatures in order to estimate the type of conduction in the samples. Isothermal annealing kinetics was investigated at different temperatures before and after γ-irradiation. The electrical conductivity decreases with increasing time of annealing and the annealing process is dominated by a unique rate process.     

High-pressure optical studies on R-line fluorescence lifetime in Al2O3:V2+

The effect of high hydrostatic pressure (up to 10.3?GPa) at room temperature on fluorescence lifetime τ for R line (²E→⁴A₂ transition) in ruby Al₂O₃:V²⁺ was studied. The performed studies show the linear increase of τ with increasing pressure. At 10.3?GPa, τ is about 1.36 times higher than at ambient pressure. The obtained trend was explained by a model which considered the effect of pressure on τ through an induced change of line position, inter-ionic distance, compressibility, and molecular polarizability. A good agreement between the calculated and experimental values for τ was obtained.     

Effect of heavy atom disorder on the intensities of powder XRD lines of YBa2Cu3O7-y

Abstract

It is shown that heavy atom disorder resulting from Y ? Ba, Ba ? Cu and Y ? Cu interchanges due to local stoichiometric constraints can cause significant changes in the intensities of X-ray powder diffraction lines of YBa₂Cu₃O_7-y . A comparison of the theoretically predicted intensities with published patterns for specimens prepared by the conventional dry route reveals that Ba ? Cu interchanges involving copper atoms in the CuO₂ planes can occur quite frequently and may be mistaken for (00l) texture.