Colossal magnetoresistance in layered manganite Nd2−2x Sr1+2x Mn2O7 (0 ≤ x ≤ 0.5)

The layered manganite Nd_2−2x Sr_1+2x Mn₂O₇, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd_2−2x Sr_1+2x Mn₂O₇ do not form the single-phase layered compound with A₃B₂O₇ structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd_1−z Sr _z MnO₃, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.     

Effect of Ca doping on the superconducting properties of GdBaSrCu3O7−δ bulk samples

Gd_1−xCa_xBaSrCu₃O_7−δ samples (0  x  0.1) were prepared via solid-state reaction. Four-point probes method was used for resistance versus temperature measurements. Results show decrease in T_c by increasing x content. This variation is assumed to be irrelevant to the different phases or impurity effects since X-ray patterns show all samples are tetragonal single-phase. Ca doping decreases the oxygen content and lattice parameters of the samples. It is suggested that Ca prevents the dislocation of oxygen, and then disrupts the hole concentration of the system and antiferromagnetic correlation at CuO₂ planes. Subsequently, destroys the superconductivity of the samples.     

Thermoluminescence of Chromium Doped Cd2Nb2O7 Crystals

The exposure of the Cd₂Nb₂O₇:Cr crystal to monochromatic light with wavelength shorter than 600 nm at 12 K decreases the Cr^3? photoluminescence emission and allows us to observe thermoluminescence within 60–200 K during subsequent heating of the crystal at a rate of 0.077 K/s. The glow curve of thermoluminescence revealed a rather complex structure with two pronounced intensive glow peaks at 91 and 180 K and four rather weak peaks at 99, 117, 132, and 155K. An analysis of the glow peaks by “an initial rise” gave evidence of many various charge carrier traps with activation energies from 150 to 470 meV. Since a comparison of the thermoluminescence and photoluminescence emission spectra showed that these spectra are identical, it was concluded that the thermoluminescence glow peaks are associated with thermal release of charge carriers from shallow traps followed by Cr^4?→Cr^3?(²E) and/or Cr^2?→Cr^3?(²E) charge capture and ²E→⁴A₂ radiative decay of Cr^3?(²E) ions.     

Effect of fluorine in Li1.27Cr0.2Mn0.53O2 cathode for secondary lithium batteries

Fluorine substituted Li_1.27Cr_0.2Mn_0.53O₂ electrode, prepared by sol–gel method, was investigated in the present work. Thermal analysis was done on this cathode material and found to be thermally stable with a loss of weight near 300 °C. Influence of fluorine substitution on the structural and electrochemical properties of the Li_1.27Cr_0.2Mn_0.53O₂ electrode was studied by X-ray diffraction (XRD) and field emission scanning electron microscope. XRD pattern of the fluorine-doped Li_1.27Cr_0.2Mn_0.53O₂ cathode material quenched at 900 °C indicates a phase pure material. The charge–discharge profile of the prepared cathode material showed that the fluorine substitution for oxygen in the cathode material resulted in improved capacity retention. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006.     

Effect of doping on the luminescence properties of Li2B4O7

In this work, we have investigated the influence of doping agents on the luminescence properties of multiply doped Li₂B₄O₇ and the temperature lag between TSL materials and the heating element. The results of thermoluminescence studies show that the Ag doping leads to the appearance of a new glow curve peak at 165 °C and the increasing sensitivity of Li₂B₄O₇:Cu,Ag,P is correlated with copper and phosphate concentrations. Under the excitation at 245 nm the emission spectra show maxima at 365 and 450 nm in the ceramic, crystal and glass. The low energy shift in the latter system might be related to the local structural distortion in the glass around Cu⁺ ions.     

层状钙钛矿结构锰氧化物Ca3Mn2O 7的压致结构相变

采用金刚石压砧高压装置（DAC）,对层状钙钛矿结构锰氧化物Ca3Mn2O7的粉末样品进行了高压能散X射线衍射实验。实验结果表明,在0－35GPa压力范围内,Ca3Mn2O7晶体结构发生了两次相变。在1．3GPa左右,由原来的四方相转变为正交相,在9．5GPa左右,又由正交相向新的四方相转变。     

Orthorhombic to tetragonal phase transition in high TC superconductor YBa2Cu3O7-y and NdBa2Cu3O7-z under high pressure

Abstract

X-ray powder diffraction measurements for YBa₂Cu₃O_7-y and NdBa₂Cu₃O_7-y were made at the intense synchrotron radiation source under high pressure up to 5 GPa. These samples were wrapped tightly in platinum foil to avoid deoxidizing atmosphere. The orthorhombic to tetragonal transition temperature increases with pressure in both samples. These results are discussed on the basis of the disordering of the oxygen atoms on the chain sites.     

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.     

Density functional analysis of the magnetic structures of Sr2MGe2O7 (M=Mn,Co)

To examine the difference between the magnetic structures of Sr₂MGe₂O₇ (M=Mn, Co), we evaluated their spin exchange interactions by performing energy-mapping analysis based on density functional theory calculations. The calculated intra- and inter-layer spin exchanges correctly predict the G-type and C-type antiferromagnetic structures of Sr₂MnGe₂O₇ and Sr₂CoGe₂O₇, respectively, and the Curie–Weiss temperatures estimated from these spin exchanges are also in good agreement with the experiment. The ∥c and ⊥c orientations of the spins in the ordered magnetic structures of Sr₂MnGe₂O₇ and Sr₂CoGe₂O₇, respectively, were also examined by considering spin–orbit coupling.     

Normal co-ordinate analysis of the zero wave-vector vibrations of DyBa2Cu3O7

Using Wilson’s F-G matrix method a normal co-ordinate analysis of the spectral frequencies and form of the zero-wave vector vibrations of the high temperature superconductor, orthorhombic DyBa₂Cu₃O₇ has been performed. The vibrational frequencies and the potential energy distribution of the 21 infrared-active and 15 Raman-active modes are presented. The potential constants employed here are presented and evaluated vibrational frequencies are compared with the available experimental values.     

Effect of chloride doping concentration on enhancement of protonic conductivity of mesoporous Al2O3

Mesoporous Al₂O₃ was prepared by a sol-gel method with doping different amount of LaCl₃. The proton conductivity of mesoporous Al₂O₃ increased with increasing the doping concentration up to the optimum doping, 0.1 Cl/Al mole ratio. The surface acidities of different samples were investigated by NH₃-TPD method. The change trends of surface acidity are consistent with that of proton conductivity. It indicates that the proton conductivity of mesoporous Al₂O₃ can be increased by enhancing its surface acidity via the chloride doping.     

The effect of doping by IV-family elements on the electronic structure and electrical characteristics of Sb2O5

The electronic structures of doped Sb₂O₅ by IV-family elements (Si, Ge and Sn) were examined using the density function theory (DFT). Density of states (DOSs) results showed that the substituted IV-family elements act as acceptors in Sb₂O₅. Partial DOSs indicates that by substituting Ge(Ge _Sb ) or Sn(Sn _Sb ), there may be a larger contribution to the total DOSs near E _F than by substituting Si, which suggests that doping Ge or Sn in Sb₂O₅ produces better ptype doping compared to doping Si. Formation energy results show that IV-family elements are more likely to exist in the substituted position rather than in the interstitial position in Sb₂O₅, decreasing any self-compensation effect and making it easier for IV-family elements to realize ptype doping in Sb₂O₅. Ionization energy results show that Ge _Sb or SnSb, two among the three impurities considered, act as shallow acceptors in Sb₂O₅, thus producing a higher concentration of holes.      

An “EPR” study of YBa2Cu3O7 and related high-temperature superconductors

Besides ag ≈ 2 signal which disappears on cooling to the superconductingT _c , YBa₂Cu₃O₇ and related oxides show a near-zero-field signal in the superconducting state with certain unusual features attributable to a “superconducting glassy state”. Contribution No. 443 from Solid State and Structural Chemistry Unit.     

TiO2 coating of LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

TiO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ materials were prepared by the hydrolyzation of Ti(OBu)₄. The impact of TiO₂ coating on the structure and electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ was investigated using X-ray diffraction, scanning electron microscope, and charge–discharge tests. The results indicated that TiO₂ coating did not affect the lattice of LiNi_1/3Co_1/3Mn_1/3O₂, but exhibited obvious effects on its discharge capacity and cycling stability. As coated TiO₂ increased from 0.0 to 2.0 mol%, the initial capacity of samples decreased slightly, but the cycling stability over 2.5∼4.3 V increased remarkably. The capacity retention reached 99.5% at the 50th cycle at a coating amount of 2.0 mol%.     

Identification of the high-temperature superconducting phase in the Y-Ba-Cu-O system as the perovskite YBa2Cu3O7±δ

The oxide responsible for high-temperature superconductivity (onset ∼100 K, zero resistance above liquid N₂ temperature) is found to be YBa₂Cu₃O₇±δ. Contribution No. 432 from the Solid State and Structural Chemistry Unit     

Effect of nominal copper concentration in YBa2Cu3±x O7−δ on the superconducting transition

Variation of the copper concentration in the nominal composition YBa₂Cu_3±x O_7−δ markedly influences the superconducting transition. The transition occurs at the highest temperature with lower width, whenx = +0·25. Communication No. 82 from Materials Research Laboratory.     

Synthesis of spherical LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

Spherical LiNi_1/3Co_1/3Mn_1/3O₂ was successfully prepared by controlled crystallization. The preparation started with the spherical coprecipitate of Ni_1/3Co_1/3Mn_1/3CO₃ from NiSO₄, CoSO₄, MnSO₄, NH₄HCO₃, and NH₃·H₂O, followed by pyrolysis of Ni_1/3Co_1/3Mn_1/3CO₃ at 600°C for 3 h. The X-ray diffraction analysis showed that the homogeneous cubic (Ni_1/3Co_1/3Mn_1/3)₃O₄ was obtained after the pyrolysis. Spherical LiNi_1/3Co_1/3Mn_1/3O₂ was obtained by sintering of the mixture of as-obtained (Ni_1/3Co_1/3Mn_1/3)₃O₄ and LiOH·H₂O at 900°C for 6 h in air. As-prepared spherical LiNi_1/3Co_1/3Mn_1/3O₂ presented initial discharge capacity of 162.9 mA h g⁻¹ and capacity retention of 98% at 50th cycle.     

Structure of grain boundaries in YBa2Cu3O7 thin-film step-edge junctions

The atomic structure of 90° [100] (or [010]) tilt grain boundaries in YBa₂Cu₃O₇ thin-film step-edge junctions and, for comparison, in the interface between a-axis and c-axis oriented YBa₂Cu₃O₇ grains is investigated by means of high-resolution transmission electron microscopy. For (100)(001)-type boundaries two different structures are found. In the first a (001) CuO₂ plane of one grain faces a (100) Y–Ba–O plane of the other grain, in the second a (001) BaO plane faces a (100) Cu–O plane. In the former structure an incomplete unit cell of YBa₂Cu₃O₇ terminates at the boundary and a smaller strain in the adjacent CuO₂ planes is detected in comparison with the latter. It is found that a combination of a partial dislocation with a 124 stacking fault is a way to accommodate the lattice mismatch between c and 3a of YBa₂Cu₃O₇ in the boundary. For a symmetric (103)(103)-type boundary a displacement of the Cu-atoms of the CuO₂ planes is found near the boundary plane. From this a redistribution of the oxygen atoms around the Y-atoms located right in the boundary plane is inferred. The possible effect of the boundary structure on the superconducting properties of YBa₂Cu₃O₇ films is discussed.     

Anomalous behaviour of sound velocity in the highT c superconductor YBa2Cu3O7

The longitudinal acoustic wave velocity in YBa₂Cu₃O₇ has been measured in the range 300 to 85 K using the ultrasonic pulse superposition technique. The sound velocity shows a steep drop in its value at 260 K and decreases continuously till the superconducting transition temperature. BelowT _c the sound velocity increases steeply. A hysteresis is seen when the experiments are performed during a cooling-warming cycle. The results are compared with recent measurements by Ewertet al on the samples with the same chemical composition.     

Kinetics of oxygen desorption in Y Ba2Cu3O7-δ

In this paper the kinetics of oxygen desorption from Y Ba₂Cu₃O_7-δ has been investigated and measurements carried out in the temperature range 350–750 C. It is shown that the oxygen desorption is diffusion-controlled (and not surface-controlled), and characteristic of diffusion from small grains in a loosely coupled porous matrix. At every temperature a number of time constants with well-defined ratios appear and have been identified and measured. These time constants can be related to the average bulk diffusion constant which shows an Arrhenius behaviour with deviations around 750°C which are ascribed to the well-known orthorhombic to tetragonal transition. In this paper we establish the basic underlying physical mechanism of oxygen desorption in Y Ba₂Cu₃O_7-δ.