• Volume 118, Issue 1

      January 2006,   pages  7-154

    • Recent development in metal oxides and related materials - Foreword

      S Natarajan T N Guru Row

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    • Improved synthesis of carbon nanotubes with junctions and of single-walled carbon nanotubes

      F L Deepak A Govindaraj C N R Rao

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      Pyrolysis of thiophene over nickel nanoparticles dispersed on silica is shown to yield Y-junction carbon nanotubes with smaller diameters than those obtained by the pyrolysis of organometallic-thiophene mixtures. In the presence of water vapour, the pyrolysis of organometallic-hydrocarbon mixtures yields single-walled nanotubes, as well as relatively narrow-diameter carbon nanotubes with Y-junctions. Pyrolysis of organometallic-hydrocarbon mixtures, in the absence of water vapour, only gives nanotubes with T- and Y-junctions.

    • Influence of fuel ratios on auto combustion synthesis of barium ferrite nano particles

      D Bahadur S Rajakumar Ankit Kumar

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      Single-domain barium ferrite nano particles have been synthesized with narrow particle-size distribution using an auto combustion technique. In this process, citric acid was used as a fuel. Ratios of cation to fuel were maintained variously at 1:1, 1:2 and 1:3. The pH was 7 in all cases. Of all three cases, a cation to citric acid ratio of 1:2 gives better yield in the formation of crystalline and single domain particles with a narrow range of size distribution. Most particles are in the range of 80 to 100 nm. Maximum magnetization and coercivity values are also greater for 1:2 ratios. These values measured at room temperature are found to be 55 emu/gram and 5000 Oe respectively. XPS and ESR studies support the results.

    • Fluorescence resonance energy transfer (FRET) in chemistry and biology: Non-Förster distance dependence of the FRET rate

      Sangeeta Saini Harjinder Singh Biman Bagchi

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      Fluorescence resonance energy transfer (FRET) is a popular tool to study equilibrium and dynamical properties of polymers and biopolymers in condensed phases and is now widely used in conjunction with single molecule spectroscopy. In the data analysis, one usually employs the Förster expression which predicts (l/R6) distance dependence of the energy transfer rate. However, critical analysis shows that this expression can be of rather limited validity in many cases. We demonstrate this by explicitly considering a donor-acceptor system, polyfluorene (PF6)-tetraphenylporphyrin (TPP), where the size of both donor and acceptor is comparable to the distance separating them. In such cases, one may expect much weaker distance (as l/R2 or even weaker) dependence. We have also considered the case of energy transfer from a dye to a nanoparticle. Here we find l/R4 distance dependence at large separations, completely different from Förster. We also discuss recent application of FRET to study polymer conformational dynamics.

    • A new form of MgTa2O6 obtained by the molten salt method

      Ashok K Ganguli Shikha Nangia Meganathan Thirumal Pratibha L Gai

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      Using molten salt route (with NaCl/KCl as the salt) we have been able to synthesize a new form of magnesium tantalate at 850°C. Powder X-ray diffraction data could be indexed on an orthorhombic unit cell with lattice parameters, ‘a’ = 15.36(1) Å, ‘b’ = 13.38(1) Å and ‘c’ = 12.10(1) Å. High resolution transmission electron microscopy and electron diffraction studies confirm the results obtained by X-ray studies. Energy dispersive X-ray spectroscopy helps ascertain the composition of MgTa2O6. The title compound shows a dielectric constant of ∼24 with a low dielectric loss of 0.006 at 100 kHz at room temperature. Dielectric constant is nearly unchanged with rise in temperature while the loss shows a very marginal increase (0.007 at 300°C).

    • Crystal structure and ionic conductivity of a new bismuth tungstate, Bi3W2O10.5

      B Muktha T N Guru Row

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      The compound Bi3W2O10.5 was synthesized by the solid-state technique from Bi2O3 and WO3 in stoichiometric quantities. Single crystals were grown by the melt-cooling technique and the crystal structure was solved in the tetragonalI4/m space group witha = 3.839 (1) Å,c = 16.382 (5) Å,V = 241.4 (1) Å3,Z = 4 and was refined to anR index of 0.0672. The structure represents a modification of the Aurivillius phase and consists of [Bi2O2]2+ units separated by WO8 polyhedra. a.c. impedance studies indicate oxide ion conductivity of 2.91 10−5 Scm−1 at 600°C.

    • Design and fabrication of an automated temperature programmed reaction system to evaluate 3-way catalysts Ce1−x−y,(La/Y)xPtyO2−δ

      Arup Gayen Tinku Baidya G S Ramesh R Srihari M S Hegde

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      A completely automated temperature-programmed reaction (TPR) system for carrying out gas-solid catalytic reactions under atmospheric flow conditions is fabricated to study CO and hydrocarbon oxidation, and NO reduction. The system consists of an all-stainless steel UHV system, quadrupole mass spectrometer SX200 (VG Scientific), a tubular furnace and micro-reactor, a temperature controller, a versatile gas handling system, and a data acquisition and analysis system. The performance of the system has been tested under standard experimental conditions for CO oxidation over well-characterized Ce1−−x−y Ptx(La/Y)yO2-δ catalysts. Testing of 3-way catalysis with CO, NO and C2H2 to convert to CO2, N2 and H2O is done with this catalyst which shows complete removal of pollutants below 325°C. Fixed oxide-ion defects in Pt substituted Ce1−y(La/Y)yO2−y/2 show higher catalytic activity than Pt ion-substituted CeO2.

    • Synthesis, structure and magnetic properties of the polyoxovanadate cluster [Zn2(NH2(CH2)2NH2)5][{Zn(NH2(CH2)2NH2)2}2{V18O42(H2O)}].xH2O (x ∼ 12), possessing a layered structure].xH2O (x ∼ 12), possessing a layered structure

      Srinivasan Natarajan K S Narayan Swapan K Pati

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      A hydrothermal reaction of a mixture of ZnCl2, V2O5, ethylenediamine and water gave rise to a layered poly oxovanadate material [Zn2(NH2(CH2)2NH2)5][{Zn(NH2(CH2)2NH2)2}5{V18O42(H2O)}].xH2O (x ∼ 12) (I) consisting of [V18O42]12− clusters. These clusters, with all the vanadium ions in the +4 state, are connected together through Zn(NH2(CH2)2NH2)2 linkers forming a two-dimensional structure. The layers are also separated by distorted trigonal bipyramidal [Zn2(NH2(CH2)2NH2)5] complexes. The structure, thus, presents a dual role for the Zn-ethyl-enediamine complex. The magnetic susceptibility studies indicate that the interactions between the V centres inI are predominantly antiferromagnetic in nature and the compound shows highly frustrated behaviour. The magnetic properties are compared to the theoretical calculations based on the Heisenberg model, in addition to correlating to the structure. Crystal data for the complexes are presented.

    • A density matrix renormalization group study of low-lying excitations of polythiophene within a Pariser-Parr-Pople model

      Mosumi Das S Ramasesha

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      Symmetrized density-matrix-renormalization-group calculations have been carried out, within Pariser-Parr-Pople Hamiltonian, to explore the nature of the ground and low-lying excited states of long polythiophene oligomers. We have exploitedC2 symmetry and spin parity of the system to obtain excited states of experimental interest, and studied the lowest dipole allowed excited state and lowest dipole forbidden two photon state, for different oligomer sizes. In the long system limit, the dipole allowed excited state always lies below the lowest dipole forbidden two-photon state which implies, by Kasha rule, that polythiophene fluoresces strongly. The lowest triplet state lies below two-photon state as usual in conjugated polymers. We have doped the system with a hole and an electron and obtained the charge excitation gap and the binding energy of the 11Bu exciton. We have calculated the charge density of the ground, one-photon and two-photon states for the longer system size of 10 thiophene rings to characterize these states. We have studied bond order in these states to get an idea about the equilibrium excited state geometry of the system. We have also studied the charge density distribution of the singly and doubly doped polarons for longer system size, and observe that polythiophenes do not support bipolarons.

    • Formation of high nuclearity mixed-valent polyoxovanadates in the presence of copper amine complexes

      Jency Thomas Sanjeev Sharma S E Lofland K V Ramanujachary A Ramanan

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      Two new Müller-type clusters, a one-dimensional solid [Cu(en)]24[Cl ⊂V15O36]−12H2O1, and a three-dimensional solid [Cu(pn)]24[Cl ⊂V18O42]·12H2O2, have been synthesised by employing identical hydrothermal conditions except varying the nature of organic diamine.1 crystallised in a chiral space groupP212121 witha = 12.757(1),b = 18.927(2) andc = 28.590(3) Å, andZ =4.2 crystallised in a tetragonal system with space groupP4/nnc,a = 15.113(1) andc = 18.542(3) Å, andZ = 2. Mixed-valent vanadium ions in structures1 and2 have been established both by magnetisation and bond-length bond-valence measurements. Chemistry of formation of high nuclearity polyoxovanadate clusters is discussed.

    • A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

      Dinesh Topwal U Manju Sugata Ray S Raj D D Sarma S R Krishnakumar M Bertolo S La Rosa G Cautero

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      Disordered Sr2FeMoO6 shows a drastic reduction in saturation magnetization compared to highly ordered samples, moreover magnetization as a function of the temperature for different disordered samples shows qualitatively different behaviours. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples, is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+xMo1-xO6 with Fe-rich (x > 0) and Mo-rich (x < 0) regions.

    • A low-cost lead-acid battery with high specific-energy

      S K Martha B Hariprakash S A Gaffoor D C Trivedi A K Shukla

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      Lightweight grids for lead-acid battery grids have been prepared from acrylonitrile butadiene styrene (ABS) copolymer followed by coating with lead. Subsequently, the grids have been electrochemically coated with a conductive and corrosion-resistant layer of polyaniline. These grids are about 75% lighter than those employed in conventional lead-acid batteries. Commercial-grade 6V/3.5Ah (C20-rate) lead-acid batteries have been assembled and characterized employing positive and negative plates constituting these grids. The specific energy of such a lead-acid battery is about 50 Wh/kg. The batteries can withstand fast charge-discharge duty cycles.

    • Structural, magnetic and magnetotransport properties of La0.7−xCexBa0.3MnO3 (x = 0.0–0.4)

      R Vengadesh Kumara Mangalam A Sundaresan

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      Structural, magnetic and magnetotransport properties of La0.7−xCexBa0.3MnO3 (x = 0–0.4) have been investigated although some unreacted secondary phases of CeO2 were present. The rhombohedral structure (R-3c) forx = 0 transforms to orthorhombic with the space groupImma forx = 0.3. All samples showed ferromagnetic transition above 300 K and a negative magnetoresistance. For x>0.1, magnetization data measured at 1 T showed a decrease at low temperatures (T < 50 K) due to antiferro-magnetic coupling between Ce-local moments and Mn-moments. Forx = 0.4, the resistivity showed a maximum around 200 K which corresponds to ordering temperature of cerium. Since these results are similar to that observed in the Sr-containing La0.5−xCexSr0.5MnO3 (x = 0–0.4) system, we suggest that the cerium ions are in the trivalent state and the anomalous behaviour has been attributed to a Kondo-like effect.

    • Inclusion of poly-aromatic hydrocarbon (PAH) molecules in a functionalized layered double hydroxide

      L Mohanambe S Vasudevan

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      The internal surface of an Mg-Al layered double hydroxide has been functionalized by anchoring carboxy-methyl derivatized β-cyclodextrin cavities to the gallery walls. Neutral polyaromatic hydrocarbon (PAH) molecules have been included within the functionalized solid by driving the hydrophobic aromatic molecules from a polar solvent into the less polar interior of the anchored cyclodextrin cavities by a partitioning process. The optical (absorption and emission) properties of the PAH molecules included within the functionalized Mg-Al layered double hydroxide solid are similar to that of dilute solutions of the PAH in non-polar solvents. The unique feature of these hybrid materials is that they are thermally stable over a wide temperature range with their emission properties practically unaltered.

    • Syntheses, structure and intercalation properties of low-dimensional phenylarsonates, A(HO3AsC6H5)(H2O3AsC6H5) (A = Tl, Na, K and Rb)

      Bonthu Krishna Kishore Reddy K Prabhakara Rao K Vidyasagar

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      Four new low-dimensional phenylarsonates, A(HO3AsC6H5)(H2O3AsC6H5) (A = Tl(1), Na(2), K(3) and Rb(4)), have been synthesized and characterized by X-ray diffraction, spectroscopic and thermal studies. They crystallize in triclinic unit cells and have approximately planar arrangement of A+ ions, coordinated to oxygen atoms of phenylarsonates, on both sides. Structure of thallium phenylarsonate as determined by single crystal X-ray diffraction, is one-dimensional, whereas those of alkalimetal analogues are two-dimensional. Successful intercalation reactions of compounds1 and2 with primaryn-alkyl amines have been demonstrated.

    • Line broadening in the PXRD patterns of layered hydroxides: The relative effects of crystallite size and structural disorder

      Grace S Thomas P Vishnu Kamath

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      Layered hydroxides crystallize in a hexagonal structure and incorporate a number of different types of structural disorders as an exigency of anisotropic bonding. Structural disorder contributes to the non-uniform broadening of lines in the powder X-ray diffraction pattern. Common among the disorders are stacking faults, which broaden theh0ℓ/0kℓ reflections. Interstratification selectively broadens the 00l reflections and turbostratic disorder broadens the 0kℓ reflections. The line broadening caused by structural disorder has to be discounted before estimates of particle size are made by applying the Scherrer formula.

    • Ionic conduction in the solid state

      P Padma Kumar S Yashonath

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      Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these solids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simulations on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed.

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