R. Gundakaram

Effect of Co doping in LiMn2O4

Abstract Lithium transition-metal oxides Li(Mn2−xCox)O4 (0≤x≤0.5) are synthesized by solid state reaction. X-ray and electrochemical data show that the replacement of Mn3+ (d4) ions by Co3+ (d6) in the octahedral framework of the spinel eliminates the local disorder present in the lattice around the [Mn3+O6] octahedra. The capacity loss observed in the undoped Li/LiMn2O4 cell is about 25% after 20 cycles, whereas that for the x=0.1 and 0.2 doped spinel materials is about 0.48 and 1%, respectively. The good capacity retention of Li(Mn2−xCox)O4 (0.1≤x≤0.5) electrode is attributed to the stabilization of the spinel structure by Co doping for Mn ion sites. The chemical substitution of Co3+ for Mn3+ in LiMn2O4 improves the efficiency in maintaining electrochemical capacity over a large number of cycles without sacrificing initial reversible capacity at room temperature.

Cr doping in the La1:2Sr1:8Mn2O7 system

The effect of doping Cr in the Mn site of the La1.2Sr1.8Mn2O7 system has been studied. Addition of Cr modifies the transport and magnetic properties of the parent phase. With increasing Cr, the insulator-metal transition observed in the parent phase is suppressed and insulating behaviour is induced. In the range of doping studied (25%), the compositions show ferromagnetic behaviour with the Curie temperature decreasing with increasing Cr. The unit cell volume also shows a decrease. However, the magnetoresistance ratio is not significantly affected. We compare these results with an earlier study of doping Cr in the three-dimensional LaMnO3 structure, where it was seen that the ferromagnetic and magnetoresistance characteristics are sensitive to Cr doping. The results of the present study suggest that the layered manganates are more accommodative to doping compared to the three-dimensional perovskites.

Crystal and electronic structures of inverse spinel-type LiNiVO4

Nearly monophasic LiNiVO4 has been successfully prepared by the sol-gel method. The crystal structure has been determined by powder X-ray diffraction with Rietveld refinement. The material is found to be of the inverse spinel-type structure (ie. V in tetrahedral site and (Ni,Li) in octahedral site) with lattice parameter a of 8.2233(1) A. Moreover, mutual chemical substitution between V and Ni has been found. X-ray absorption near edge structure (XANES) of the LiNiVO4 has been examined. Vanadium has a tetrahedral symmetry as found by XANES of V K-edge absorption. Based on the energy shifts of the absorption peaks in the XANES spectra correlated with the effective charge, the valences of V and Ni in LiMn2O4 are determined to be ; 51 and 21 respectively.

Evidence for electron-doped (n-type) superconductivity in the infinite-layer (Sr0.9La0.1)CuO2 compound by X-ray absorption near-edge spectroscopy

Abstract X-ray-absorption near-edge structure (XANES) measurements of the O–K and Cu–L edges of the series (Sr1−xLax)CuO2 with x=0 and 0.1 have been performed. The appearance of the Cu2p3d104s state in (Sr0.9La0.1)CuO2 is an indication of formal Cu+ formation which provides a direct evidence for n-type superconductivity in this composition.

Effect of Pb doping in high-Tc Bi2Sr2CaCu2Oy superconductors studied by X-ray absorption near-edge structure spectroscopy

Abstract An investigation of the variation of hole concentration within the CuO 2 planes using X-ray absorption near-edge structure spectroscopy with synchrotron radiation in the (Bi 2− x Pb x )Sr 2 CaCu 2 O y (0⩽ x ⩽0.3) series of high- T c superconductors synthesized by solid-state reaction in air is presented. Pb is found to have the valence of +2 in the samples of this series. Moreover, the hole concentration increases with increasing Pb content. The results lead us to conclude that the substitution of Pb 2+ in the Bi 3+ sites of the (Bi,Pb)-2212 system causes the compounds to be in the over-doped state. This effect results in the compounds having a high critical current density.

Studies into the phase transformation of Bi-2223 precursor powders using X-ray diffraction and SQUID susceptibility measurements

An optimized processing route for formation of the Bi-2223 phase, from a highly reactive precursor powder, is of obvious importance in achieving a high J/sub c/ in powder-in-tube (PIT) tapes, Here, we report the dependence of the SQUID susceptibility signal on the phase content and heat-treatment of the Bi-2223 precursor powders. Magnetic susceptibilities as measured by SQUID reveal differences in the powder quality not seen by x-ray powder diffraction. In particular, Bi-2223 detection is enhanced using SQUID. The rate of phase transformation to the 2223 phase in various precursor powders exhibiting significantly different SQUID signals will be discussed.

Tuning Pb content in High-Tc Bi2Sr2CaCu2Oy superconductors studied by X-ray absorption near-edge structure spectroscopy

The variation of hole concentration within the CuO2 planes via tuning Pb content in (Bi2−xPbx)Sr2CaCu2Oy (0.2≤x≤0.6) has been investigated by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. The profile of XANES spectra indicated that Pb can be incorporated into Bi-2212 phase under nitrogen atmosphere around x=0.6. However, after annealing the nitrogen-sintered samples in oxygen, Pb solubility is found to be decreased only up to x=0.2 in (Bi2−xPbx)Sr2CaCu2Oy, showing that compounds with high Pb content are more stable under oxygen-free atmosphere.

Effect of secondary phases in the precursor powders on the transformation to the (Bi,Pb)-2223 phase

The (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 2 system [also know as (Bi,Pb)-2223] has shown promise for use in superconducting tapes and wires due to its high transition temperature and high critical current density. The tapes and wires are usually fabricated by the powder-in-tube method, after which they are subjected to thermal and mechanical treatments. Depending on the nature of the heat treatment, phase transformations occur in the powder and it is of paramount importance to understand the response of the powders to different processing conditions. In this study, we have synthesized a precursor powder of the nominal composition Bi 1.8 Pb 0.33 Sr 1.87 Ca 2 Cu 3 O y by spray pyrolysis. The powder was then calcined under controlled conditions between 760 and 800°C in an atmosphere between 0 and 21% O 2 for 2 to 24 hours with controlled heating and cooling, such that different amounts of Pb are incorporated into the majority phase. The resultant powders were then converted to the (Bi,Pb)-2223 phase with suitable heat treatments. Using powder Xray diffraction and magnetization measurements, the volume fractions of the secondary phases in the precursors were estimated and the effect of the phases on the conversion to the (Bi,Pb)-2223 phase was studied. We show that the ratio of the intensity of the (020) and (115) reflections of the (Bi,Pb)-2212 phase can be used to tune the precursor powders for optimal conversion. While a lower fraction of the secondary phases such as CaO and CuO is desirable in the precursor powders, a higher volume fraction of Ca 2 PbO 4 seems to help in the rapid conversion to the (Bi,Pb)-2223 phase.