Now showing 1 - 10 of 28
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    Host-sensitized emission of LiInW2O8 wolframites: From red-Eu3 to white-Dy3 phosphors
    (01-09-2011)
    Asiri Naidu, S.
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    Boudin, S.
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    Raveau, B.
    The LiInW2O8:Eu3, LiInW2O 8:Dy3 and LiInW2O8:Eu 3/Dy3 phosphors were synthesized by solid-state reaction and their photoluminescence properties were studied. Under UV excitation, the LiInW2O8:Eu3 phosphor exhibits an intense red emission whereas the LiInW2O8:Dy3 and LiInW2O8:Dy3/Eu3 phosphors show a white emission. The WO6 octahedra play a major role in the luminescence of the host lattice, characterized by a blue emission under UV excitation. The emission of activator ion results from an efficient energy transfer from the LiInW2O8 host lattice to the Eu 3 and Dy3 ions. The LiIn0.97Dy30.03W2O8 and LiIn0.965 Dy 30.03Eu30.005W2O 8 samples, optimized for white emission, are interesting candidates for solid-state lighting applications. © 2011 Elsevier Inc.
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    Electrochemical performance of LiMSnO 4 (M=Fe, In) phases with ramsdellite structure as anodes for lithium batteries
    (01-11-2004)
    Satya Kishore, M. V.V.M.
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    Raveau, B.
    LiMSnO 4 (M=Fe, In) compounds were synthesized by high temperature solid-state reaction method and the electrochemical studies were carried out vs. lithium metal. Lithium is reversibly intercalated and deintercalated in LiFeSnO 4 with a constant capacity of ∼90 mAh/g. In situ X-ray diffraction data show that ramsdellite structure is stable for lithium intercalation and deintercalation in LiFeSnO 4. Galvanostatic discharge/charge of LiFeSnO 4 in the voltage window 0.05-2.0 V shows a reversible capacity of ∼100 mAh/g. The observed capacity in LiFeSnO 4 is due to the two processes involving alloying/dealloying of Li 4.4Sn and formation/decomposition of Li 2O. In contrast, the new isotypic oxide LiInSnO 4 does not exhibit any lithium intercalation due to the absence of mixed valence for indium. Its reversible capacity is strongly dependent on the voltage window. LiInSnO 4 exhibits severe capacity fading on cycling in the voltage window 0.05-2.0 V, but shows a stable capacity of ∼90 mAh/g in the voltage range 0.75-2.0 V. In situ XRD patterns of Li xFeSnO 4 (1≤x≤2) at various Li contents during initial lithium intercalation and deintercalation. © 2004 Elsevier Inc. All rights reserved.
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    Studies on Magnetic Properties of MnTi1-xNbxO3System
    (15-02-1998)
    Ramakrishnan, P. A.
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    Majhi, J.
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    Subba Rao, G. V.
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    Maignan, A.
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    Raveau, B.
    Synthesis and characterization of electrical and magnetic properties of ilmenite phases of the type MnTi1-xNbxO3have been carried out. Single phase materials could be obtained for 0.0≤x≤0.25. The electrical conductivity increases with increasing Nb content. Magnetic susceptibility studies show that the phases exhibit 2D antiferromagnetic behavior. The magnetic susceptibility data has been analyzed using Fisher's specific heat to determine the long range ordering temperature. © 1998 Academic Press.
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    New layered hydrogenophosphate, protonic conductor: Mn(H2PO 4)2
    (07-07-2008)
    Baies, R.
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    Pralong, V.
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    Caignaert, V.
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    Saradhi, M. P.
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    Raveau, B.
    A new hydrogenophosphate Mn(H2PO4)2 has been synthesized from an aqueous solution. Its ab initio structure resolution shows that the original layered structure of this phase consists of PO 2(OH)2 tetrahedra and MnO5OH octahedra, sharing corners to form [MnP2O8H4]∞ layers, whose cohesion is ensured through hydrogen bonds. The excitation and emission spectra of this phase are characteristic of Mn2+ species. This phosphate is shown to be a good protonic conductor with a conductivity of 10-4.4 S/cm at 90°C (363 K). © 2008 American Chemical Society.
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    Scheelite based red phosphors for white LEDs
    (29-02-2012)
    Naidu, S. Asiri
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    Raveau, B.
    Synthesis, crystal structure and optical properties of AgY 0.95Eu 0.05(WO 4) 2-x(MoO 4) x and AgY 1-xEu x(MoO 4) 2 phases with scheelite related structures are described. The 5D 0- 7F 2 electric dipole transition at 615 nm is predominant in the photoluminescence (PL) emission spectra, under 465 nm excitation. The critical concentration of Eu 3+ is x 0.5 in AgY 1-x(MoO 4) 2, beyond which concentration quenching is observed. In AgY 0.95Eu 0.05(WO 4) 2-x(MoO 4) x phases, the 5D 0- 7F 2 emission intensity of Eu 3+ is increased with the increase of Mo content. © 2011 The Electrochemical Society.
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    Magnetization relaxation and aging in spin-glass (La,Y)1-xCaxMnO3 (x = 0.25, 0.3 and 0.5) perovskite
    (07-04-1998)
    Maignan, A.
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    Sundaresan, A.
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    Raveau, B.
    Magnetization relaxation and AC-susceptibility measurements have been carried out on a disordered (La,Y)1-xCaxMnO3 (x = 0.25, 0.3 and 0.5) system where 〈r(La,Y)〉 = 〈rPr〉 = 1.18 Å. Resistivity and low-field susceptibility measurements suggest that the sample with x = 0.25 develops a dominant ferromagnetic component below 130 K. With increase of x, the ferromagnetic component decreases at the benefit of a charge-ordered antiferromagnetic one. At low temperatures, for x = 0.3, the magnetic frustration due to those two kinds of competing interactions leads to spin-glass-like behavior. This behavior is more pronounced for x = 0.5 as the strength of these competing interactions becomes equal. Magnetization relaxation measurements show that aging phenomenon exists in all the samples. The parameters obtained by fitting the M(t) data with a fractional exponential function are correlated with the transport and other magnetic properties. © 1998 Elsevier Science B.V. All rights reserved.
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    Crystallite size constraints on lithium insertion into brookite TiO2
    (23-06-2008)
    Anji Reddy, M.
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    Pralong, V.
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    Raveau, B.
    We demonstrate lithium insertion into brookite TiO2 in the nanophase regime. The extent of lithium insertion is significantly influenced by the crystallite size. A maximum of 0.95 Li/ TiO2 can be inserted into 10 nm size crystallites and the extent of lithium insertion is low (0.23 Li) in 33 nm crystallites. The reversibility decreases with an increase in crystallite size. The contrasting behavior of brookite and rutile TiO2 suggests that the structural features of brookite TiO2 may play an important role in determining Li insertion behavior. © 2008 The Electrochemical Society.
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    Lithium intercalation into nanocrystalline brookite TiO2
    (08-01-2007)
    Reddy, M. Anji
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    Kishore, M. Satya
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    Pralong, V.
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    Raveau, B.
    Lithium intercalation in phase-pure nanocrystalline brookite TiO2 is demonstrated for the first time. Galvanostatic studies show that 0.9 Li per formula unit can be intercalated into this phase in initial discharge. Ex situ X-ray diffraction studies on the electrodes at different levels of lithium intercalation show that the structure is stable toward lithium intercalation and deintercation. In the initial charge, an irreversible capacity loss is observed. However, on further cycling, the phase shows excellent cycling behavior. A reversible capacity of 170 mAhg-1 is observed even after 40 cycles. © 2006 The Electrochemical Society.
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    Synthesis and electrochemical properties of a new vanadyl phosphate: Li4VO(PO4)2
    (01-10-2006)
    Kishore, M. Satya
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    Pralong, V.
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    Caignaert, V.
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    Raveau, B.
    The exchange of lithium for proton in VO(H2PO4)2 has been studied. Beside the continuous exchange from VO(H2PO4)2 to Li2H2VO(PO4)2, a new cathode material Li4VO(PO4)2 has been synthesized, whose structure is closely related to that of VO(H2PO4)2. The electrochemical evaluation of Li4VO(PO4)2 vs. Li shows that it undergoes reversible lithium deintercalation/intercalation at high voltage, ∼4.0 V with a reversible capacity of ∼70 mAh/g. © 2006 Elsevier B.V. All rights reserved.
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    Electrochemical Li insertion studies on WNb12O33-A shear ReO3 type structure
    (01-05-2010)
    Saritha, D.
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    Pralong, V.
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    Raveau, B.
    Electrochemical lithium insertion studies on WNb12O33 synthesized by solid state reaction (SSR) are carried out in the voltage range 1.0-3.2 V. During first discharge 15.6 Li are inserted with a specific capacity of 221 mAh/g. WNb12O33 is also synthesized by sol-gel (SG) technique with a view to enhance the rate capability and cycling properties. The SSR and SG samples are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling. Electrochemical cycling performance of SG samples is superior to that of the SSR sample at high 'C' rates. The sample synthesized by SG method exhibits high specific capacity of 142 mAh/g after 20 cycles at 20C rate. © 2010 Elsevier Inc. All rights reserved.