Electronic structure of high-temperature superconductors II. La-Ba-Cu-O systems

Electronic structure of La_2–x Ba _x CuO₄ superconductors has been studied. Model systems La₁₀CuO ₂₂ ^–12 , La₁₀cuO ₁₄ ⁺⁴ and La_gBaCuO ₁₄ ⁺³ with various Ba positions have been investigated using the quasirelativistic INDO molecular orbital method. The oxidation state of copper atoms corresponds to Cu(II). The influence of Ba/La substitution on copper atoms is much lower than on their oxygen ligands sphere.     

Structure, electrical properties and temperature characteristics of?Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–Bi0.5Li0.5TiO3 lead-free piezoelectric ceramics

(1−x−y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃– yBi_0.5Li_0.5TiO₃ lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique, and their structure, electrical properties, and temperature characteristics have been studied systematically. The ceramics can be well-sintered at 1050–1150 °C. The increase in K⁺ concentration decreases the grain-growth rate and promotes the formation of grains with a cubic shape, while the addition of Li⁺ decreases greatly the sintering temperature and assists in the densification of BNT-based ceramics. The results of XRD diffraction show that K⁺ and Li⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. As x increases from 0.05 to 0.50, the ceramics transform gradually from rhombohedral phase to tetragonal phase and consequently a morphotropic phase boundary (MPB) is formed at 0.15≤x≤0.25. The concentration y of Li⁺ has no obvious influence on the crystal structure of the ceramics. Compared with pure Bi_0.5Na_0.5TiO₃, the partial substitution of K⁺ and Li⁺ for Na⁺ lowers greatly the coercive field E _c and increases the remanent polarization P _r of the ceramics. Because of the MPB, lower E _c and large P _r, the piezoelectricity of the ceramics is improved significantly. For the ceramics with the compositions near the MPB (x=0.15–0.25 and y=0.05–0.10), the piezoelectric properties become optimum: piezoelectric coefficient d ₃₃=147–231 pC/N and planar electromechanical coupling factor k _P=20.2–41.0%. In addition, the ceramics exhibit relaxor characteristic, which probably results from the cation disordering in the 12-fold coordination sites. The depolarization temperature T _d shows a strong dependence on the concentration x of K⁺ and reaches the lowest values at the MPB. The temperature dependences of the ferroelectric and dielectric properties at high temperatures may imply that the ceramics may contain both the polar and non-polar regions at temperatures above T _d.     

Special points on the phase diagrams of langbeinites. The case of (x,T)-phase diagram of mixed K2xRb2(1−x)Cd2(SO4)3 crystals

We have studied special points that appear on the (x, T)-phase diagram of mixed K_2xRb_2(1?x)Cd₂(SO₄)₃ crystals. This phase diagram has been constructed issuing from the changes in domain structure occurring in the course of phase transitions. We have found that two triple points on the (x, T)-phase diagram of the above solid solutions are very close to each other in the phase space and can change their places or transform into a quadruple point. These special points are associated with disappearance of ferroelectric phases with the symmetries P1 and P2₁ taking place when Rb₂⁺ are substituted with K₂⁺ ions.     

Optical absorption behavior of Tl<Superscript>+</Superscript> DOPED Rb(Br<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>I<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) system

The optical absorption behavior of Tl⁺ doped Rb(Br_1–x I _x ) mixed crystals (with x = 0.00, 0.05, and 0.10) grown under vacuum by slow cooling from the melt has been studied. Absorption spectra of the mixed crystals recorded at room temperature showed that the characteristic A-absorption band of Tl⁺ ions in the Rb(Br_1–x I _x ) system (0.1 mol. %) with x = 0.00 (i.e., RbBr:Tl⁺) broadened with the iodine content towards the low energy side. Changes in the absorption spectra of the mixed crystals are due to creation of some complex centers involving Tl⁺, Br^–, and I^- ions with energy levels inside the band gap while forming the mixed crystal. The absorption spectra of gamma-irradiated mixed crystals showed the F-band, which shifted towards the low energy side due to the existence of iodine ions in the mixed crystals.     

Ionic conduction and effect of immobile cation substitution in binary system (AgI)4/5–(PbI2)1/5

Samples of stoichiometry (AgI)₄(PbI₂)_1?x (CdI₂) _x , (0 ≤ x ≤ 0.4), have been prepared and studied by electrical conductivity, X-ray powder diffraction and DSC techniques. The ionic conductivity of samples was found to increase with temperature, and an abrupt increase at phase transition temperature was observed. The Cd⁺²-doped samples exhibited lower phase transition temperature compared to that of the pure samples. The ionic conductivity decreases with an increase in Cd⁺² content in pre-transition, while enhances in conductivity result in Cd⁺² content samples of x ≤ 0.2 in the post-transition region. Different resources of investigation confirmed the solubility limit of Cd⁺² in the high-temperature phase to be x = 0.2. The change in the ionic conductivity of Cd⁺²-doped samples is explained by the increase in the defect concentration and the free volume available in the lattice. The drop in phase transition temperature of Cd²⁺-doped systems is attributed to the lattice distortion and the increase in the defect–defect interaction.     

Effect of laser irradiation on the electrical properties of dysprosium doped LiCo-Ferrite

The effect of laser irradiation on the electrical properties of Li_0.5+z Co _z Dy _x Fe_2.5?2z?x O₄ ferrite (0.0 ≤ x ≤ 0.2, z = 0.1) has been studied in the temperature range 300 K ≤ T ≤ 750 K at frequencies of 10 kHz?5 MHz, using a LIMO-IR laser diode, at a wavelength of 808 nm. It was found that laser irradiation increases the polarization, the resistivity and the paramagnetic region. As the result of electronic rearrangement and lattice defects, small polorons and clusters were created. The doping of LiCo-Ferrite by Dy³⁺ increases both the AC and DC resistance of the investigated material. The variation of the AC and DC resistance with the Dy-content (x) obeys the following correlations R _ac/100 = 50x ²+4x+0.005 and R _dc/1000 = 31x ²+0.099x+0.09, respectively. A peculiar behaviour was obtained for the sample with Dy-content x = 0.075, as the resistance notably decreases. The applicable result is that laser irradiation increases the resistance of LiCo-ferrite by about 17% while its doping by dysprosium at x = 0.15 increases the resistance by about 23%. Its value is nearly stable for the temperature range from 340 to 480 K.     

Structural evolution of the MoO3(010) surface during lithium intercalation

Atomic force microscopy (AFM) has been used to characterize the structural evolution of the MoO₃(010) surface during the initial stage of Li⁺ intercalation. Lithiation was observed in situ using model cells comprised of a single crystal MoO₃ cathode, a dilute propylene carbonate (PC)-based electrolyte, and an Li metal anode. The insertion of Li⁺ into MoO₃ results in the topotactic nucleation and growth of acicular Li_xMoO₃ (x0.25) precipitates at the (010) surface. Because the interlayer spacing of Li_xMoO₃ (d=7.88 Å) is greater than that of MoO₃ (d=b/2=6.93 Å), the Li_xMoO₃ precipitates expand out of the (010) surface as they grow into the MoO₃ crystal along 010. The local strain associated with this expansion causes the Li_xMoO₃ precipitates to crack parallel to the MoO₃001 and 010 axes once their height exceeds approximately 20 nm. With continued Li⁺ intercalation, cracking becomes more prominent, and the (010) surface begins to fragment and disintegrate. Anisotropies in Li⁺ ion uptake and the influence of surface morphology on Li_xMoO₃ precipitation and crack propagation are described.     

交叉关联噪声对光学双稳系统平均首通时间的影响

研究了乘性色噪声和加性白噪声驱动的光学双稳系统中噪声对系统两个方向平均首通时间T₊（x_s1→x_s2）和T_-（x_s2→x_s1）的影响（平均首通时间是指从一稳态出发越过势垒到另一稳态所用时间的平均值）.通过Novikov定理和Fox近似方法得到相应的Fokker-Planck方程,利用最速下降法得到T₊（x_s1→x_s2）和T_-（x_s2→x_s1）的表达式.研究发现：乘性噪声强度Q和加性噪声强度D对T₊（x_s1→x_s2）的影响相同,对T_-（x_s2→x_s1）的影响不同;T₊（x_s1→x_s2）随乘性色噪声自关联时间τ的增大而增大,但随噪声间的交叉关联强度λ的增大而减小;T_-（x_s2→x_s1）随τ的增大而减小,随λ的增大而增大. 关键词： 平均首通时间 光学双稳系统 乘性色噪声 加性白噪声     

Ti1－xCrxO2±δ体系的相关系、晶体结构和磁性能研究

采用溶胶凝胶法制备了Ti_1-xCr_xO_2±δ体系系列样品.利用扫描电子显微镜(SEM)，X射线光电子能谱(XPS)，粉末X射线衍射分析(XRD)方法研究了Ti_1-xCr_xO_2±δ系列样品的颗粒尺寸、形貌、组分化学态、相关系和固溶区范围；并利用超导量子干涉磁强计对样品的磁性能进行了研究.采用Rietveld结构精修的方法研究了Cr的不同掺杂量对TiO₂晶体结构的影响，研究表明，1000℃烧结的样品的固溶区范围是x=0—0.03，为金红石单相；随着Cr掺杂量的增加，金红石相晶胞参数规律性地减小；当x>0.03，为金红石相和CrO₂相两相共存.综合XRD和磁性测量结果，500℃烧结的样品的固溶区范围是x=0—0.02，为锐钛矿单相；随着Cr掺杂量的增加，锐钛矿相晶胞参数规律性地减小；当x≥0.04，为锐钛矿相和绿铬矿相(Cr₂O₃)两相共存.XPS实验结果表明，500℃和1000℃退火的样品中Cr都是以Cr⁺³和Cr⁺⁶两种化学态存在，1000℃烧结的样品中可能有更多的Cr³⁺转化为Cr⁶⁺.根据M-H和M-T曲线的测试结果发现，本文500℃烧结的Ti_1-xCr_xO_2±δ体系样品当x=0—0.02时，为室温铁磁性.当x≥0.04时，由铁磁相和顺磁相所组成，在低温下有较强的铁磁性；室温下主要是顺磁相，铁磁相只占据很小的体积分数. 关键词： 1-xCr_xO_2±δ体系')" href="#">Ti_1-xCr_xO_2±δ体系 相关系 固溶区 磁性能     

Rietveld refinements and vibrational spectroscopic studies of Na1−xKxPb4(PO4)3 lacunar apatites (0≤x≤1)

Synthesis of apatites, Na_1−xK_xPb₄(PO₄)₃ 0≤x≤1, with anion vacancy was carried out using solid state reactions. The solid solution of apatite-type structure crystallizes in the hexagonal system, space group P6₃/m (No 176). Rietveld refinements showed that 75% of Pb²⁺ cations are located in the (6h) sites; the ninefold coordination sites (4f) are equally occupied by the other 25% lead cations and the K⁺ and Na⁺ monovalent ions.The structure can be described as built up from [PO₄]³⁻ tetrahedra and Pb²⁺ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. These tunnels are connected by cations of mixed sites (4f) half occupied by Pb²⁺ and half by Na⁺/K⁺ mixed cations. The assignment of the observed frequencies in the Raman and infrared spectra is discussed on the basis of a unit cell group analysis and by comparison with other apatites. The Raman modes of all the compositions show some linear shifts of the frequencies as a function of the composition toward lower values due the substitutions of Na⁺ by K⁺ with a larger radius.     

CNDO studies of Y-Ba-Cu-O superconductors II. Simple models of YBa2Cu3Ox

The electronic structure of YBa₂Cu₃O_x (x=6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu₃O₁₀]^–15, [Ba₈Cu₃O₁₀]⁺¹, [Y₈Cu₃O₁₀]⁺⁹ of the non-superconducting (x=6) and [Cu₃O₁₂]^–17, [Ba₈Cu₃O₁₂]^–1, [Y₈Cu₃O₁₂]⁺⁷ of the superconducting (x=7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region.     

Phase interactions in TiN<Subscript>x</Subscript>(TiB<Subscript>x</Subscript>)-<Emphasis Type="Italic">n</Emphasis>-Si-<Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript>-Si contacts and their thermal degradation due to rapid thermal annealing

The temperature stability of TiN_x(TiB_x)-n-Si-n ⁺-Si, Au-TiN_x(TiB_x)-n-Si-n ⁺-Si, and Au-Ti(Mo)-TiN_x(TiB_x)-n-Si-n ⁺-Si Schottky-barrier contacts subjected to rapid thermal annealing in hydrogen at temperatures T=400, 600, and 800°C is studied. It is shown that structural and morphological transformations and the related degradation of electrophysical characteristics in interstitial alloys (titanium nitrides and borides) start at 600°C. Reasons for the degradation of the barrier properties of titanium borides and nitrides are discussed.     

Theoretical studies of vibrational spectra of [N(CH3)4]2ZnCl4−yBry compounds with y = 0, 2 and 4

The infrared and Raman spectra of [N(CH₃)₄]₂ZnCl_4?yBr_y, where y = 0, 2 and 4, have been analyzed with ab initio calculations of the vibrational characteristics of constitutive polyhedra, tetramethylammonium [N(CH₃)₄]⁺ and [ZnCl_4?xBr_x]^2? (x = 0, 1, 2, 3 and 4) tetrahedra. The optimized geometries, calculated vibrational frequencies, infrared intensities and Raman activities are calculated using Hartree–Fock and density functional theory B3LYP methods with 3-21G, 6-31G(d) and 6-311G⁺(d,p) basis sets. Calculation of the root mean square difference δ_rms between the observed and calculated frequencies allows to give scaling factors and to deduce that the best agreements are obtained by B3LYP/6-311G⁺(d,p) for [N(CH₃)₄]⁺ and B3LYP/3-21G for [ZnCl_4?xBr_x]^2?. The present study establishes a strongly reliable assignment of the vibrational modes of [ZnCl_4?xBr_x]^2? tetrahedra based on comparison between experimental and ab initio calculations, both of the frequencies and the intensities of the Raman signals.     

Electrical properties of MWCNT-composite (Se80Te20)100−xAgx (0 ≤ x ≤ 4) chalcogenide glasses

This article describes the preparation of multi-walled carbon nanotube (MWCNT) chalcogenide glass composite by the melt-quenching technique. MWCNT composite (Se₈₀Te₂₀)_100?xAg_x (0 ≤ x ≤ 4) bulk samples are characterized by the XRD, SEM and EDX. The electrical measurements were carried out in the temperature range of the 308-388 K. Cole–Cole plot has been used to determine the electrical conductivity at room temperature. It has been observed that MWCNT chalcogenide composite have higher value of electrical conductivity than pure glass. The results have been discussed on the basis of increased ionic conductivity (Ag⁺ ions) in MWCNT doped (Se₈₀Te₂₀)_100?xAg_x (0 ≤ x ≤ 4) bulk samples.     

Lithium ions conduction in Li2xMn1-xPS3 films

Li_2xMn_1-xPS₃ films have been synthesized by exfoliating MnPS₃ through the successive intercalations of K⁺ and Li⁺ ions. Their dielectric response has been measured from 80 to 350 K in the frequency range of (10²–10⁶) Hz. The obtained data have been analyzed in terms of both complex permittivity ε* and the ac conductivity σ_ac. The frequency dependence of σ_ac has been interpreted in terms of the Jonscher’s law, whose exponent n decreases by increasing temperature. The n values lie between 0.479 and 0.501 and are typical of materials in which the ac conductivity is due to multiple hops of carriers. By analyzing the σ_dc temperature dependence, the observed dielectric response has been attributed to the intercalated lithium ions, and the Li_2xMn_1-xPS₃ films have been classified as hopping charge carrier systems.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2−x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O²⁻→Cu²⁺+O⁻, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2−x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

Properties of Perturbative Solutions of Unilateral Matrix Equations

A left-unilateral matrix equation is an algebraic equation of the form a ₀+a ₁ x+a ₂ x ²+·+a _n x ⁿ =0 where the coefficients a _r and the unknown x are square matrices of the same order and all coefficients are on the left (similarly for a right-unilateral equation). Recently certain perturbative solutions of unilateral equations and their properties have been discussed. We present a unified approach based on the generalized Bezout theorem for matrix polynomials. Two equations discussed in the literature, their perturbative solutions and the relation between them are described. More abstractly, the coefficients and the unknown can be taken as elements of an associative, but possibly noncommutative, algebra.     

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x Na x MnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na⁺ ion substitution for Ca²⁺ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La_0.7Ca_0.3−x Na _x MnO₃ manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ ². The temperature dependence of magnetization of all samples obeys the Bloch T ^3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na⁺ ions causes a decrease in total exchange integral Aof the samples.      

Thermodynamic study of lithium insertion in V6O13 and Li1+xV3O8

The enthalpies of Li⁺ insertion into two V oxides of interest as cathodes for secondary Li batteries, i.e. V₆O₁₃ and Li_1+xV₃O₈ have been directly measured by solution calorimetry. By comparing the integral molar enthalpies ΔH⁰/x with the corresponding ΔG⁰/x values, ΔS⁰/x higher than expected have been found for Li_1+xV₃O₈. This has been correlated to a reorganization of its distorted structure induced by the initial Li⁺ insertion. Two other cathode materials, having the same variation of the potential with Li⁺ content, i.e. MoO₃ and (Mo_0.3V_0.7)₂O₅, show even higher ΔS⁰/x values. The entropic values stemming from structural reorganizations add to the configurational entropies of inserted ions in determining the initial profile of E as a function of Li⁺.     

Impurity distribution and electrical characteristics of boron-doped Si1−x Ge x /Si p +/N heterojunction diodes produced using pulsed UV-laser-induced epitaxy and Gas-immersion laser doping

A two-step pulsed UV-laser process which independently controls the metallurgical and electrical junction depth of a Si_1–x Ge _x /Si heterojunction diode has been implemented. Pulsed Laser-Induced Epitaxy (PLIE) combined with Gas-immersion Laser Doping (GILD) are used to fabricate boron-doped heteroepitaxial p ⁺/N Si_1–x Ge _x /Si layers and diodes. Borontrifluoride is used as the gaseous dopant source in the GILD process step. Boron incorporation and activation are investigated as a function of laser energy fluence and the number of laser pulses using SIMS and Halleffect measurements. The dose of incorporated dopant is on the order of 10¹³ cm^–2 per pulse. The B profiles obtained are flat except for a peak at the interface resulting from segregation effects. The B and Ge distributions are compared with shifts in the turn-on voltage of p ⁺/N Si_1–x /Si heterojunction diodes produced by the process. The GILD/PLIE process is spatially selective with the resulting diodes fabricated being quasiplanar. Hole mobilities in the heavily doped Si_1–x Ge _x films are found to be slightly lower than in comparable Si films.Presently at the Oregon Graduate Institute, Beaverton, OR 97006, USA