• Volume 113, Issue 5-6

      October 2001,   pages  361-702

    • Foreword

      J Gopalakrishnan Ram Seshadri

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    • Basic building units, self-assembly and crystallization in the formation of complex inorganic open architectures

      C N R Rao

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      Careful investigations of open-framework metal phosphates reveal that the formation of these complex architectures is likely to involve a process wherein one-dimensional ladders or chains, and possibly zero-dimensional monomers, transform to higher dimensional structures. The one-dimensional ladder appears to be the primary building unit of these structures. At one stage of the building-up process, spontaneous self-assembly of a low-dimensional structure such as the ladder seems to occur, followed by crystallization of a two-or three-dimensional structure. Accordingly, many of the higher dimensional structures retain the structural features of the 1D structure, indicating the occurrence of self-assembly. These findings mark the beginnings of our understanding of complex supramolecular inorganic materials.

    • Nanotubes and nanowires

      C N R Rao A Govindaraj

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      Synthesis and characterization of nanotubes and nanowires constitute an important part of nanoscience since these materials are essential building units for several devices. We have prepared aligned carbon nanotube bundles and Y-junction nanotubes by the pyrolysis of appropriate organic precursors. The aligned bundles are useful for field emission display while the Y-junction nanotubes are likely to be useful as nanochips since they exhibit diode properties at the junction. By making use of carbon nanotubes, nanowires of metals, metal oxides and GaN have been obtained. Both the oxide and GaN nanowires are single crystalline. Gold nanowires exhibit plasmon bands varying markedly with the aspect ratio. GaN nanowires show excellent photoluminescence characteristics. It has been possible to synthesise nanotubes and nanowires of metal chalcogenides by employing different strategies.

    • Relaxation in binary mixtures: Non-ideality, heterogeneity and re-entrance

      Arnab Mukherjee Goundla Srinivas Sarika Bhattacharyya Biman Bagchi

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      Binary mixtures show many kinds of fascinating dynamical behaviour which has eluded microscopic description till very recently. In this work we show that much of the anomalous behaviour can be explained by building suitable models and carrying out theoretical and simulation studies. Specifically, three well-known problems have been addressed here. (a) Non-ideality in composition dependence of viscosity, (b) re-entrant behaviour of orientational relaxation, and (c) heterogeneity in supercooled binary mixtures. The physical origin of the dynamical behaviour of binary mixtures can be understood in terms of composition fluctuation, a study of which has also been presented in this paper.

    • Electron-electron interactions in the chemical bond: “1/3” Effect in the bond length of hydrogen molecule

      P Ganguly

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      The prominent “1/3” effect observed in the Hall effect plateaus of two-dimensional electron gas (2DEG) systems has been postulated to indicating 1/3 fractional charge quasiparticle excitations arising from electron-electron interactions. Tunneling shot-noise experiments on 2DEF exhibiting fractional quantum Hall effect (FQHE) shows evidence for tunnelling of particles with eand e/3 charges for a constant band mass. A “1/3” effect in the hydrogen molecule is seen in as much as its internuclear distance,dH-H = D + D+, with ¦D+/D¦ = 1/3. This is examined in terms of electron-electron interactions involving electron-and hole quasiparticles, (e-)and(h+), equivalent to those observed in FQHE shot -noise experiments. The(e/m) ratio of the (e) and(h+) quasiparticles is kept at 1: −3. Instead of a 2DEG, these particles are treated as being in flat Bohr orbits. A treatment in the language of charge-flux tube composites for the hydrogen atom as well as the hydrogen molecule is attempted. Such treatment gives important insights into changes in chemical potential and bond energy on crossing a phase boundary during the atom-bond transition as well as on models for FQHE itself.

    • New lithium-ion conducting perovskite oxides related to (Li, La)TiO3

      Litty Sebastian A K Shukla J Gopalakrishnan

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      We describe the synthesis and lithium-ion conductivity of new perovskite-related oxides of the formulas, LiCa1.650.35Ti1.3B1.7O9 (B =Nb,Ta)(I,II), LiSr2Ti2.5W0.5O9 (III) and LiSr1.650.35Ti2.15W0.85O9(IV). OxidesI andII crystallize in orthorhombic (GdFeO3-type) structure, while oxidesIII andIV possess cubic symmetry. All of them exhibit significant lithium-ion conduction at high temperatures, the highest conductivity of ∼ 10−2S/cm at 800°C among the oxides is exhibited by the composition IV. The results are discussed in the light of previous work on lithium-ion conducting perovskite oxides containingd0 cations.

    • Ab initio structure determination via powder X-ray diffraction

      Digamber G Porob T N Guru Row

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      Structure determination by powder X-ray diffraction data has gone through a recent surge since it has become important to get to the structural information of materials which do not yield good quality single crystals. Although the method of structure completion when once the starting model is provided is facile through the Rietveld refinement technique, the structure solutionab initio is still not push-button technology. In this article a survey of the recent development in this area is provided with an illustration of the structure determination of α-NaBi3V2O10.

    • Epitaxial oxide thin films by pulsed laser deposition: Retrospect and prospect

      M S Hegde

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      Pulsed laser deposition (PLD) is a unique method to obtain epitaxial multi-component oxide films. Highly stoichiometric, nearly single crystal-like materials in the form of films can be made by PLD. Oxides which are synthesized at high oxygen pressure can be made into films at low oxygen partial pressure. Epitaxial thin films of highTc cuprates, metallic, ferroelectric, ferromagnetic, dielectric oxides, superconductor-metal-superconductor Josephson junctions and oxide superlattices have been made by PLD. In this article, an overview of preparation, characterization and properties of epitaxial oxide films and their applications are presented. Future prospects of the method for fabricating epitaxial films of transition metal nitrides, chalcogenides, carbides and borides are discussed.

    • Quantum phenomena in magnetic nano clusters

      C Raghu Indranil Rudra Diptiman Sen S Ramasesha

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      One of the fascinating fields of study in magnetism in recent years has been the study of quantum phenomena in nanosystems. While semiconductor structures have provided paradigms of nanosystems from the stand point of electronic phenomena, the synthesis of high nuclearity transition metal complexes have provided examples of nano magnets. The range and diversity of the properties exhibited by these systems rivals its electronic counterparts. Qualitative understanding of these phenomena requires only a knowledge of basic physics, but quantitative study throws up many challenges that are similar to those encountered in the study of correlated electronic systems. In this article, a brief overview of the current trends in this area are highlighted and some of the efforts of our group in developing a quantitative understanding of this field are outlined.

    • Visualizing lone pairs in compounds containing heavier congeners of the carbon and nitrogen group elements

      Ram Seshadri

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      In this mini-review, I discuss some recent work on the stereochemistry and bonding of lone pairs of electrons in divalent compounds of the heavier carbon group elements (SnII , PbII) and in trivalent compounds of the heavier nitrogen group elements (BiIII). Recently developed methods that permit the real-space visualization of bonding patterns on the basis of density functional calculations of electronic structure, reveal details of the nature of s electron lone pairs in compounds of the heavier main group elements — their stereochemistry and their inertness (or lack thereof). An examination of tetragonalP4/nmm SnO, α-PbO and BiOF, and cubic$$Fm\bar 3m$$PbS provides a segue into perovskite phases of technological significance, including ferroelectric PbTiO3 and antiferroelectric/piezoelectric PbZrO3, in both of which the lone pairs on Pb atoms play a pivotal rôle.

    • Infrared and Raman spectroscopic studies of glasses with NASICON-type chemistry

      K J Rao K C Sobha Sundeep Kumar

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      Structures of NASICON glasses of the general formula AB2(PO4)3, where A = Li, Na or K and B = Fe, Ga, Ti, V or Nb, have been investigated using vibrational (IR and Raman) spectroscopies. Phosphate species appear to establish an equilibrium via a disproportionation reaction involving a dynamical bond-switching mechanism where both charge and bonds are conserved. B ions in the system acquire different coordinations to oxygens. Alkali ions cause absorptions due to cage vibrations. All the observed spectroscopic features are consistent with speciation involving disproportionation reactions.

    • Properties of a new magnetic material: Sr2FeMoO6

      D D Sarma Sugata Ray

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      Recently, there have been a large number of investigations of the physical properties of Sr2FeMoO6 and related compounds, in view of their significant negative magnetoresistive property at room temperature and in low applied magnetic fields. We review these investigations, detailing the microscopic mechanism controlling the electronic and magnetic properties of this system.

    • AC impedance behaviour and state-of-charge dependence of Zr0¨5Ti0¨5V0¨6Cr0¨2Ni1¨2 metal-hydride electrodes

      S Rodrigues N Munichandraiah A K Shukla

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      Metal-hydride electrodes made of an AB2 alloy of the composition Zr0¨5Ti0¨5V0¨6Cr0¨2Ni1¨2 are studied for AC impedance behaviour at several of their state-of-charge values. Impedance data at any state-of-chargecomprisetwoRC-time constants and accordingly are analysed by using a nonlinear-least-square-fitting procedure. Resistance of the electrode and frequency maximumf* of the lowfrequency semicircle are found useful for predicting state-of-charge of the metalhydride electrodes.

    • Hydrocarbon chain conformation in an intercalated surfactant monolayer and bilayer

      N V Venkataraman S Vasudevan

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      Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS3 at two different grafting densities. Low grafting densities are obtained on direct intercalation of CTA ions into CdPS3 to give Cd0.93PS3(CTA)0.14. Intercalation occurs with a lattice expansion of 4.8 Å with the interlamellar surfactant ion lying flat forming a monolayer. Intercalation at higher grafting densities was effected by a two-step ion-exchange process to give Cd0.83PS3(CTA)0.34, with a lattice expansion of 26.5 Å. At higher grafting densities the interlamellar surfactant ions adopt a tilted bilayer structure.13C NMR and orientation-dependent IR vibrational spectroscopy on single crystals have been used to probe the conformation and orientation of the methylene ‘tail’ of the intercalated surfactant in the two phases. In the monolayer phase, the confined methylene chain adopts an essentially all-trans conformation with most of the trans chain aligned parallel to the gallery walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and aligned to the confining surface. In the bilayer phase, most bonds in the methylene chain are in trans conformation. It is possible to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene. These high energy conformers disappear on cooling leaving all fifteen methylene units of the intercalated cetyl trimethyl ammonium ion in trans conformational registry at 40 K.

    • Diffusion of hydrocarbons in confined media: Translational and rotational motion

      S Y Bhide A V Anil Kumar S Yashonath

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      Diffusion of monatomic guest species within confined media has been understood to a good degree due to investigations carried out during the past decade and a half. Most guest species that are of industrial relevance are actually polyatomics such as, for example, hydrocarbons in zeolites. We attempt to investigate the influence of non-spherical nature of guest species on diffusion. Recent molecular dynamics (MD) simulations of motion of methane in NaCaA and NaY, benzene in NaY and one-dimensional channels AlPO4−5, VPI−5 and carbon nanotube indicate interesting insights into the influence of the host on rotational degrees of freedom and orientational properties. It is shown that benzene in one-dimensional channels where the levitation parameter is near unity exhibits translational motion opposite to what is expected on the basis of molecular anisotropy. Rotational motion of benzene also possesses rotational diffusivities aroundC6 and C2axes opposite to what is expected on the basis of molecular geometry. Methane shows orientational preference for 2+ 2 or 1 + 3 depending on the magnitude of the levitation parameter.

    • Computer simulation study of water using a fluctuating charge model

      M Krishnan A Verma S Balasubramanian

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      Hydrogen bonding in small water clusters is studied through computer simulation methods using a sophisticated, empirical model of interaction developed by Ricket al (S W Rick, S J Stuart and B J Berne 1994J. Chem. Phys.101 6141) and others. The model allows for the charges on the interacting sites to fluctuate as a function of time, depending on their local environment. The charge flow is driven by the difference in the electronegativity of the atoms within the water molecule, thus effectively mimicking the effects of polarization of the charge density. The potential model is thus transferable across all phases of water. Using this model, we have obtained the minimum energy structures of water clusters up to a size often. The cluster structures agree well with experimental data. In addition, we are able to distinctly identify the hydrogens that form hydrogen bonds based on their charges alone, a feature that is not possible in simulations using fixed charge models. We have also studied the structure of liquid water at ambient conditions using this fluctuating charge model.

    • Effects of hydrogen-bond environment on single particle and pair dynamics in liquid water

      Amalendu Chandra Snehasis Chowdhuri

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      We have performed molecular dynamics simulations of liquid water at 298 and 258 K to investigate the effects of hydrogen-bond environment on various single-particle and pair dynamical properties of water molecules at ambient and supercooled conditions. The water molecules are modelled by the extended simple point charge (SPC/E) model. We first calculate the distribution of hydrogen-bond environment in liquid water at both temperatures and then investigate how the self-diffusion and orientational relaxation of a single water molecule and also the relative diffusion and relaxation of the hydrogen-bond of a water pair depend on the nature of the hydrogen-bond environment of the tagged molecules. We find that the various dynamical quantities depend significantly on the hydrogen-bond environment, especially at the supercooled temperature. The present study provides a molecular-level insight into the dynamics of liquid water under ambient and supercooled conditions.

    • Phase analysis and dielectric properties of oxides obtained in the MgO–(1−x)Nb2O5–(x)Ta2O5 system

      M Thirumal A K Ganguli

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      MgNb2-xTaxO6 (0 ≶x ≶ 2) phases can be obtained as the major phase (75 to 90%) by solid state reactions starting from oxides. These oxides crystallize in the orthorhombic columbite structure tillx = 1.75 and the tetragonal trirutile structure for MgTa2O6 (x = 2.0). For all the compositions there exist secondary phases like Nb2O5 or Ta2O5 in addition to the major AB2O6 phase. Sintered disks (1200°C) show dielectric constants varying between 14.8 and 16.0 for the entire range of composition at a frequency of 500 kHz. The dielectric loss is nearly constant around 0025 to 003 between 0 ≶x ≶ 1 but increases to 017 for the MgTa2O6 phase (x = 2.0). Scanning electron micrographs reveal a gradual decrease in grain size with increase in Ta concentration with a size of 3 micron forthe x=0 composition (sintered at 1200°C) while thex = 2 phase shows a grain size of approximately 0.5 microns. The microwave dielectric constant at ∼14 GHz is found to be 20.9 for thex = 0 composition and 17.7 for thex = 2 composition.

    • Self-assembling bilayers of palladiumthiolates in organic media

      P John Thomas A Lavanya V Sabareesh G U Kulkarni

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      Alkylthiolates of palladium forming a homologous series (butyl to octadecyl) have been prepared and characterized using X-ray diffraction and STM. The thiolates adopt an unusual bilayered lamellar structure, whose thickness is governed by the length of the alkyl chain. These mesophases melt in the temperature range, 60° to 100°C, with the melting point increasing linearly with the thiol chain length. There is evidence to suggest that the alkyl chains are orientationally disordered especially prior to melting.

    • Morpholinium intercalated vanadophosphates

      Sanjeev Sharma A Ramanan J J Vittal

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      The present paper reports the formation of two morpholinium (Morp) incorporated solids precipitated from aqueous vanadate solution acidifed with phosphoric acid: a zero-dimensional, mixed-valent phosphovanadate cluster containing solid, [Morp]6[PVO4 ⊂ VIV3VV11O32(OH)6].2H2O,1 and a two-dimensional layered solid, [Morp]0.23[VIV,VOPO4]1.1H2O,2. While2 precipitates out from the reaction mixture in the presence of a reducing agent hydrazine hydrate,1 crystallises out in its absence.

    • Synthesis and properties of MoSi2 based engineering ceramics

      P Srikari Tantri Anup K Bhattacharya Sheela K Ramasesha

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      Molybdenum disilicide is a high temperature structural ceramic with many attractive properties for engineering applications. Foremost amongst these is its stability in corrosive atmospheres up to about 1600°C. However, there are a few undesirable properties that need to be addressed before it can become a viable material in high temperature applications. Since MoSi2 forms thermodynamically stable composites with both metals and ceramics, many reinforcing materials are incorporated into the matrix to improve the fracture toughness and creep properties. The low temperature oxidation can be controlled by making high density (> 95% of theoretical density) compacts. This article summarizes the important attempts that are made in improving the properties of molybdenum disilicide-based ceramics by the reinforcement with other materials.

    • XRD and UV-Vis diffuse reflectance analysis of CeO2-ZrO2 solid solutions synthesized by combustion method

      G Ranga Rao H Ranjan Sahu

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      A series of ceria-incorporated zirconia (Ce1−xZrxO2,x = 0 to 1) solid solutions were prepared by employing the solution combustion synthesis route. The products were characterized by XRD and UV-Vis-NIR diffuse reflectance spectroscopy. The materials are crystalline in nature and the lattice parameters of the solid solution series follow Vegard’s law. Diffuse reflectance spectra of the solid solutions in the UV region show two intense bands at 250 and 297 nm which are assigned respectively to Ce3+ ← O2−and Ce4+ ← O2− charge transfer transitions. The two vibrational bands in 6960 cm−1 and 5168 cm−1 in the NIR region indicate the presence of surface hydroxyl groups on these materials.

    • Applications of self-assembled monolayers in materials chemistry

      Nirmalya K Chaki M Aslam Jadab Sharma K Vijayamohanan

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      Self-assembly provides a simple route to organise suitable organic molecules on noble metal and selected nanocluster surfaces by using monolayers of long chain organic molecules with various functionalities like -SH,-COOH,-NH2, silanes etc. These surfaces can be effectively used to build-up interesting nano level architectures. Flexibility with respect to the terminal functionalities of the organic molecules allows the control of the hydrophobicity or hydrophilicity of metal surface, while the selection of length scale can be used to tune the distant-dependent electron transfer behaviour. Organo-inorganic materials tailored in this fashion are extremely important in nanotechnology to construct nanoelctronic devices, sensor arrays, supercapacitors, catalysts, rechargeable power sources etc. by virtue of their size and shape-dependent electrical, optical or magnetic properties. The interesting applications of monolayers and monolayer-protected clusters in materials chemistry are discussed using recent examples of size and shape control of the properties of several metallic and semiconducting nanoparticles. The potential benefits of using these nanostructured systems for molecular electronic components are illustrated using Au and Ag nanoclusters with suitable bifunctional SAMs.

    • The many ways of making anionic clays

      Michael Rajamathi Grace S Thomas P Vishnu Kamath

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      Together with hydrotalcite-like layered double hydroxides, bivalent and trivalent metal hydroxides and their hydroxy salts are actually anionic clays consisting of positively charged hydroxide layers with anions intercalated in the interlayer region. The anionic clays exhibit anion sorption, anion diffusion and exchange properties together with surface basicity making them materials of importance for many modern applications. In this article, we discuss many different ways of making anionic clays and compare and contrast the rich diversity of this class of materials with the better-known cationic clays.

    • M4C9+(M = Ti, V): New gas phase clusters

      R Selvan L Gowrishankar T Pradeep

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      New metal-carbon clusters, M4C9+ (M = Ti, V), generated using a combined thermal arc discharge evaporation set-up, have been studied with quadrupole mass spectrometry. Reactivities of these clusters have been investigated by means of association reactions with H2O. Metal-carbon clusters of other compositions have also been studied. We speculate on the mechanism of formation of larger metal-carbon clusters.

    • Subject Index

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