Local structure and electrochemistry of LiNi y Mn y Co1 − 2y O2 electrode materials for Li-ion batteries

A series of LiNi _x Mn _y Co _z O₂ (x = y, z = 1 − 2y) oxides have been synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Layered LiNi _y Mn _y Co_1 − 2y O₂ materials with high homogeneity and crystallinity were synthesized using the wet-chemical method assisted by carboxylic acid as the polymeric agent. The long range and local structural properties are investigated with experiments including X-ray diffraction, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The evolution of the structure is discussed as a function of the cobalt content that confers layer-like behavior on the framework. Electrochemical performance of LiNi _y Mn _y Co_1 − 2y O₂ oxides is tested in cells using nonaqueous 1 M LiPF₆ dissolved in ethylene carbonate–diethyl carbonate. Charge–discharge profiles are investigated as a function of the rate capability and the voltage window. A relation is found between the gravimetric capacity and the cation disorder of the positive electrode as indicated by structural analysis. Fast lithium extraction attributed to the larger interslab space has been observed in the cobalt-rich oxides. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

Magnetic Parameters of Hg1–x Mn x Se1–y S y and Hg1–x Mn x Te1–y S y Crystals

The magnetic susceptibility of Hg_1–x Mn _x Se_1–y S _y and Hg_1–x Mn _x Te_1–y S _y crystals is investigated by the Faraday method at H = 3 kOe in the temperature interval T = 77–300 K. It is established that the specific features of are due to Mn–S–Mn–S, Mn–Se–Mn–Se, and Mn–Te–Mn–Te clusters and mixed Mn–Se–Mn–S and Mn–Te–Mn–S clusters of different sizes in which the indirect exchange antiferromagnetic interaction between Mn atoms is realized through chalcogen atoms. Based on the dependences 1/_Mn = f(T), the magnetic parameters are determined and their dependences on the crystal composition (x and y) are established.     

Incomplete magnetic ordering in Fe2(1−y)Mg1+y Ti y O4 spinels with intermediate compositiony

Static magnetization measurements on the ferrimagnetic spinels Fe_2(1?y)Mg_1+y Ti _y O₄ withy=0.3, 0.4, 0.5, and 0.6 show that these compounds have no well-defined orderdisorder transition temperature and that their ferrimagnetism may not be described in terms of the Néel theory. From the Mössbauer spectra we conclude that a temperature dependent number of the ferric ions does not participate in the ferrimagnetism of those compounds with compositiony≧0.4. The explanation of the observed magnetic and Mössbauer properties is based on the assumption that each ferric ion must have at least two magnetic linkages of the type Fe _A ³⁺ ?O^2??Fe _B ³⁺ in order to couple its magnetic moment to the neighbouring ones over the entire temperature interval between 0 K and the respective Néel temperature.     

Martensitic transformation in La1−x Yb x Ag1−y In y

The structural phase transformation of La_1–x Yb _x Ag_1–y In _y has been studied on single crystals by low temperature Laue-technique. The martensitic transformation in this pseudobinary intermetallic alloy has to be characterized as a weak orthorhtombic distortion of a single I centered unit cell (c/a1.04;a/b1.006) and a collective slipping or twinning of these cells that gives a fixed orientation between the remaining cubic room temperature structure and the martensitic phase. Above room temperature exists an order-disorder transformation from the CsCl-B2 structure to an at room temperature metastable W-A2 structure. There is no dramatic change in the physical properties of this alloy by substituting La by Yb, so we may approximate our results to LaAg_1–y In _y .This paper is dedicated to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Growth and Characterization of CdSe1 − y S y Nanofilms Obtained by Chemical Bath Deposition

In this work, the growth and characterization of cadmium selenide sulphur (CdSe_1???y S _y ) deposited by chemical bath deposition (CBD) technique at the reservoir temperature of 20?±?2 °C are presented, varying the thiourea volume added to the growth solution in the range of 0–30 ml. The films chemical stoichiometry was determined by energy-dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) analysis and Raman scattering reveal that CdSe_1???y S _y -deposited films showed hexagonal wurtzite crystalline phase. The average size of the crystalline grain in relation to the sulphur volume varies in the range of 1.48–9.2 nm that was determined by using the Debye-Scherrer equation for the direction (100), which is confirmed by analyzing the grain average diameter by high-resolution transmission electron microscopy (HRTEM). Raman scattering shows that the lattice dynamics is characteristic of bimodal behaviour and the multipeak adjust of the first optical longitudinal mode for the CdSeS denotes, in all cases, the Raman shift of the characteristic peak in the range of 177–181 cm^?1 of the CdSe crystals associated with the sulphur incorporation. CdSe_1???y S _y band gap energy can be varied from 1.86 to 2.11 eV by varying the thiourea volume added in the growth solution measured by transmittance at room temperature.     

Modulated quasi-two-dimensional antiferromagnetic structures in La1 − y Nd y MnO3 + δ manganites

The structural, electronic, and magnetic phase transitions induced by the isovalent substitution of the rare-earth Nd³⁺ ion for the La³⁺ ion with a larger radius have been investigated in the system of self-doped manganites La_{1 ? y} Nd _y MnO_{3 + δ} (0 ≤ y ≤ 1; δ ～ 0.1). For the average radius of the ions in A-sites of the lattice 〈r _A 〉 〈 1.19 Å (y 〉 0.5), the phenomena revealed in the manganites are as follows: the ordering of Mn e _g orbitals, the transition from the pseudocubic O* phase to the orthorhombic O’ phase, the opening of the dielectric Jahn-Teller gap, the frustration of the collinear ferromagnetic (FM) state, and the transition from the lowtemperature canted FM to canted antiferromagnetic (AFM) state of Mn spins. It is assumed that, in samples with neodymium concentrations y = 0.9 and 1.0 (〈r _A 〉 ≈ 1.16 Å) at temperatures T < 12 K, there coexist A- and E-type modulated AFM states similar to the sinusoidal and helical structures of Mn spins, which were previously studied in RMnO₃ multiferroics. The magnetic T-H phase diagrams of these samples are characteristic of quasi-two-dimensional antiferromagnets with a very low (zero) magnetic anisotropy in the ab planes. Under these conditions, the phase transition from the A-type AFM phase to the spin-flop state occurs in a relatively weak magnetic field. The AFM ordering of the Nd magnetic moments with a critical phase transition temperature T _Nd ≌ 6 K is induced in magnetic fields with a strength H ≥ 3.5 kOe. For the NdMnO_{3 + δ} manganite in a magnetic field H = 10.7 kOe, the curves M(T) are characterized by additional very narrow peaks near temperatures T ₁ ≌ 4.5 K and T ₂ ≌ 5 K. The additional features revealed for the first time in the magnetization near T = 0 are assumed to be caused by the quantization of the spectrum of free holes in the ab planes by a strong magnetic field.     

Lattice distortion-induced phase transformations in La1 − y Pr y MnO3 + δ manganites

The structural and magnetic phase transformations that occur in the system of self-doped La_{1 ? y} Pr _y MnO_{3 + δ} (δ ≈ 0.1, 0 ≤ y ≤ 1) manganites in the temperature range 4.2–300 K are studied by X-ray diffraction and measuring the temperature and field dependences of dc magnetization. The low-temperature magnetic phase transformations induced by the substitution of Pr for La correlate well with the structural phase transformations at T = 300 K, which indicates a strong coupling of the electronic and magnetic subsystems of La_{1 ? y} Pr _y MnO_{3 + δ} manganites with the crystal lattice. The anomalies of the magnetic and structural properties detected in this work in the form of peaks and inflection points in the concentration dependences of the magnetization and lattice parameters of the pseudocubic phase of La_{1 ? y} Pr _y MnO_{3 + δ} (0.1 ≤ y ≤ 0.7) in the temperature range 4.2–300 K are explained in terms of the existing concepts of the effect of Fermi surface nesting on the renormalization of the density of states and the hole dispersion near E _F in the presence of a strong coupling of holes with low-frequency optical phonons, which results in their transformation into quasiparticles. The narrow peak in the magnetization curve M(y) of La_{1 ? y} Pr _y MnO_{3 + δ} that is detected near y = 0.3 at T = 4.2 K is assumed to correspond to the peak of coherence of quasiparticles with a low energy of coupling with the crystal lattice near E _F, which was found earlier in the photoelectron emission spectra of manganites. The disappearance of the narrow magnetization peak with increasing Pr concentration is explained by the transition of charge carriers from the mode of “light” holes weakly coupled to one of the soft phonons to the mode of “heavy” holes strongly coupled to several phonons. The transition between phases with strongly different effective quasiparticle masses proceeds jumpwise; that is, it has features of the metal-insulator Mott transition and is accompanied by the transition from 3D to 2D quasiparticle motion in planes ab.     

Optical properties of GaN x As y P1?x?y semiconductor quaternary solid solutions

The photoluminescence method was used in the temperature range of 15?C300 K to study the optical properties of quantum-dimensional heterostructures based on GaN _x As _y P_(1?x?y) solid solutions, synthesized on the surface of a GaP (100) substrate. The calculations for the band-gap width of the GaN _x As _y P_(1?x?y) solid solution with the use of the band anticrossing model are shown to agree well with experimental data.     

Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Elastic properties of (Pb y Sn1−y )2P2S6 solid solutions

Elastic properties of (Pb _y Sn_1?y )₂P₂S₆ solid solutions were studied using Brillouin scattering technique. Different scattering geometries were used for sound velocities determination that make it possible to find all components of the stiffness tensor. The concentration dependencies of volume compressibility, the Grüneisen parameter and Debye temperature were investigated. The results obtained were used to analyze chemical bonding with substitution of tin by lead at room temperature in the crystals under consideration.     

Photoelectric properties of Pb1−x−y Sn x Ge y Te:In epitaxial films

Pb_1–x–y Sn _x Ge _y Te:In epitaxial films are examined in a wide temperature interval and at various background fluxes. These films have high sensitivity to infrared radiation in the spectral range <20m. The lifetime depends exponentially on temperature and varies from several seconds at T=10 K to 10^–2 s at T=20 K. The two-electron model of Jahn-Teller centers is proposed to explain the results. Multielement photoresistors based on these films are fabricated and D*=1.7×10¹³ cm Hz^1/2 W^–1 at T=25 K is achieved. Noise of the photoresistors is independent of background flux when it varies from 10¹² cm^–2 s^–1 to 10¹⁸ cm^–2 s^–1. As compared with Si:Ga and Ge:Hg photoresistors, the responsitivity is several orders larger at the operating temperature 25–30 K.     

Raman spectra and restructurizations in the system of solid solutions NaTa y Nb1−y O3

By recording the Raman spectra, concentrational restructurizations in the system of solid solutions NaTa_yNb_1−yO₃ were investigated. The regions of relatively stable existence of solid solutions (y<0.2, y>0.8) and the region of a rather high disordering (0.2<y<0.9) with maximum deformation of the elementary cell at y=0.5 have been revealed. It is found that at y=0.5 a concentrational phase transition is observed, and at y ∼ 0.25 and 0.8 concentrational restructurizations occur. It is shown that the octahedral anions of BO₆ are centrosymmetric when y<0.2 and y>0.9, and the system of solid solutions NaTa_yNb_1−yO₃ is antiferroelectric when y<0.2. In the range of concentrations 0.2<y< 0.9, the structure of NaTa_yNb_1−yO₃ is characterized by the presence of polar clusters. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 192–198, March–April, 2000.     

Effect of doping with iron on the charge ordering in La0.33Ca0.67Mn1−y Fe y O3 (y = 0, 0.05) manganites

The formation and specific features of the superstructure in La_0.33Ca_0.67Mn_1?y Fe _y O₃ (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90–300 K, and the high-resolution electron microscope images recorded at temperatures of 91–92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature T _CO determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell «3a × b × c» (where a ≈ b ≈ √2a _c , c ≈ 2a _c , and a _c ～ 3.9 Å is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature T _CO by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by a _c √2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature T _CO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn³⁺ Jahn-Teller ions as a result of their replacement by Fe³⁺ non-Jahn-Teller ions.     

ODMR Study of Zn1−x Mn x Se/Zn1−y Be y Se and (Cd1−x ,Mn)Te/Cd1−y Mg y Te Diluted Magnetic Semiconductor Quantum Wells

The effects of microwave pumping with a frequency of 60 GHz on the magneto-optical properties of diluted magnetic semiconductors (DMSs) are studied in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Resonant heating of the Mn²⁺ ions in the electron spin resonance conditions leads to an increase in the Mn-spin temperature, which exceeds the bath temperature by up to 5.2 K, as detected by the shift of exciton emission line and decrease of its integral intensity. Nonresonant heating mediated by free carriers is also observed through variation of the polarization degree of emission. Direct measurements of spin–lattice relaxation times for both materials using time-resolved optically detected magnetic resonance (ODMR) technique have been performed. The mechanisms of ODMR in nanostructures of DMSs are discussed.     

关于函数y=aebx拟合的问题

介绍了函数ｙ＝ａｅ＾ｂｘ的３种拟合方法，并举例说明３种方法的应用。     

Preparation and characterization of Li2.9Sc1.9−y Y y Zr0.1(PO4)3 (where y = 0, 0.1) solid electrolyte ceramics

The solid electrolyte Li_2.9Sc_{1.9? y} Y _y Zr_0.1(PO₄)₃ (where y = 0, 0.1) compounds belong to monoclinic symmetry (space group P2₁/n) at room temperature. The Zr 3d, Sc 2p, P 2p, Y 3d, O 1s, and Li 1s core level X-ray photoelectron spectra (XPS) were fitted. The Li ions in ceramics without Y occupy two different positions and in the ceramics with Y they occupy one position in the lattice. The deconvolutions of the Zr 3d, P 2p, Sc 2p, and Y 3d core level XPS are associated with different valence states on the surfaces of the investigated ceramics. Anomalies of enthalpy, change of activation energy of ionic conductivity, anomalies of dielectric permittivity in the temperature range 420–520 K of investigated compounds were found. The phenomena are related to diffuse structure phase transition in the compounds. At temperatures 600 and 900 K, the compounds belong to orthorhombic symmetry (space group Pbcn).     

Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

Al, F-doped new perovskite lithium ion conductors (x=0.11) have been prepared by solid state reaction. It is found that a pure perovskite-structured phase with space group of P4mm(99) exits in the composition range of 0<y≤0.10. The sample with y=0.02 possesses the highest ionic conductivity of 1.06×10⁻³ S/cm at room temperature, and its decomposing voltage is 2.3 V. The factors affecting the conductivity of this system are discussed.     

57Fe Mössbauer study of the electrochemical reaction of Li with FeP y (y = 1,2)

A complete ⁵⁷Fe Mössbauer study was undertaken to elucidate the electrochemical mechanism occuring during the cycling of the FeP _x (x?= 1,2)/Li batteries. Upon discharge, for FeP, the conversion reaction FeP + 3Li → Fe + Li₃P was confirmed, whereas for FeP₂, FeP and a disordered magnetic unknown phase A are observed. On charge, the starting material is partially reconstructed only in the case of FeP. Once the first cycle is achieved, the same redox process occurs in both the FeP/Li and FeP₂/Li cells, between LiFeP, FeP, Fe° (or phase A) and Li₃P.