It is argued that a porous insertion electrode can be modeled, for purposes of ac impedance analysis, by a modified Randles equivalent circuit containing a generalized constant-phase-angle impedance in series with the double-layer capacitance. The impedance spectrum of the disintercalation system Li,_xCoO~ showed two time-dependent semicircles, indicating the need for a further modification of the equivalent circuit by a physical process that could be represented by a resistor/capacitor combination. An adsorption process and a Li § surface layer are each shown to refine to identically low R factors and to be represented by equivalent circuits that transform one into the other, each having circuit values with reasonable values for the physical processes represented. Time-dependent experiments could eliminate the adsorption models, and electron microscopy gave direct evidence for the surface-layer model; further indirect evidence came from constant-voltage experiments and the preparation of electrodes with greater electrode/electrolyte interface area. Although propylene carbonate appears to be kinetically stable in contact with Li,_~CoO2 at potentials of up to 4.5V vs. lithium, mixtures of propylene carbonate with oxide particles having x = 0.34 produce thick polymer films.Many successful battery systems, primary as well as secondary, are based on electrodes that utilize insertion compounds. An insertion compound is a mixed electronic/ionic conductor into which atoms may be topotactically inserted over a wide range of solid solution. The mobile species generally enters the host as a cation, donating an electron(s) to the conduction band of the host matrix; it may also enter as an anion, taking an electron(s) from the conduction band of the host matrix. The most successful insertion-compound electrodes now in use are based on mobile H ~ or Li * ions in the host matrix, and the search for alternate insertion compounds represents an active area of research.Although the reversible insertion/removal of mobile Li ions has been extensively studied in layered structures such as LixTiS2 (0 -< x -< 1) (1) and Li~V60,3 (0 -< x < 7) (2), fundamental electrochemical investigations have been mostly limited to studies of the ionic diffusion within the insertion compounds, as this has been assumed to be the rate-limiting process. However, other processes, such as charge transfer and adsorption at the surface, are important and could prove rate limiting, especially if new materials provide a high alkali-ion mobility within the host. In this paper, we report an elecrochemical investigation of the system L i, ~CoO~ (0 < x < 1), a system first explored by Mizushima et al. (3), in which the interfacial processes prove to be increasingly important as x increases.The kinetics of insertion-electrode reactions have been most commonly studied by potentiostatic or galvanostatic pulse measurements. However, ac measurements are also applicable; in fact, they have been utilized extensively in classical electrochemistry (4). Whereas, in principl...

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New red phosphor materials of general formula Gd3-xMO7:xEu(3+) (M = Nb, Sb, and Ta) were prepared using a high temperature solid state reaction route. Detailed structural studies using XRD, FT-IR and Raman spectroscopic techniques showed that niobate and tantalate samples crystallized in the weberite type structure whereas the antimonate sample in the fluorite structure. Photoluminescence properties of the three compositions are correlated with their crystal structures. It was observed that more ordering occurs in the lattice when an M site is doped from Sb to Nb to Ta. Although niobate and tantalate samples possess similar structures more distortions were noticed in the tantalate sample increasing the radiative transition probabilities. Due to the more ordered structure of the Gd3TaO7 host lattice resulting in a more uniform distribution of Eu(3+) ions, the tantalate system showed better luminescence properties. The variation in the luminescence intensity with various Eu(3+) concentrations in the Gd3TaO7 host lattice was also studied to calculate the optimum doping concentration.

Stannate-based pyrochlore-type red phosphors CaGd(1-x)SnNbO7:xEu(3+), Ca(1-y)Sr(y)Gd(1-x)SnNbO7:xEu(3+), and Ca(0.8-x)Sr0.2GdSnNbO(7+δ): xEu(3+) were prepared via conventional solid-state method. Influence of cation substitution and activator site control on the photoluminescence properties of these phosphors are elucidated using powder X-ray diffraction, Rietveld analysis, Raman spectrum analysis, and photoluminescence excitation and emission spectra. The Eu(3+) luminescence in quaternary pyrochlore lattice exemplifies as a very good structural probe for the detection of short-range disorder in the lattice, which otherwise is not detected by normal powder X-ray diffraction technique. The Eu(3+) emission due to magnetic dipole transition ((5)D0-(7)F1 MD) is modified with the increase in europium concentration in the quaternary pyrochlore red phosphors. (5)D0-(7)F1 MD transition splitting is not observable for low Eu(3+) doping because of the short-range disorder in the pyrochlore lattice. Appearance of narrow peaks in Raman spectra confirms that short-range disorder in the crystal lattice disappears with progressive europium doping. By using Sr as a network modifier ion in place of Ca we were able to increase the f-f transition intensities and europium quenching concentration. The influence of effective positive charge of the central Eu(3+) ions when it replaces a metal ion having lower oxidation state such as Ca(2+) was also investigated. The relative intensities of A1g (∼500 cm(-1)) and F2g (∼330 cm(-1)) Raman vibrational modes get inverted when Eu(3+) ions replaces Ca(2+) ions instead of Gd(3+) as trivalent europium ions can attract the electron cloud of oxygen ions strongly in comparison with divalent calcium ions. The influence of positive charge effect of Eu(3+) in Ca0.7Sr0.2GdSnNbO7+δ:0.1Eu(3+) phosphor is greatly strengthened the charge transfer band and (7)F0-(5)L6 transition intensities than that of the Ca0.8Sr0.2Gd0.9SnNbO7:0.1Eu(3+) phosphor. Our results suggest that the photoluminescence properties can be enhanced by simple compositional adjustments in the quaternary pyrochlore-type red phosphors.