• Volume 5, Issue 3-4

      August 1983,   pages  1-380

    • Dedication

      C N R Rao

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    • Raman spectroscopy of intermediate valent (IV) systems: Electron-phonon coupling and charge fluctuation

      A Jayaraman

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      Raman scattering measurements on intermediate valent rare earth chalcogenides crystallizing in the NaCl structure are reviewed. The experimentally obtained Raman intensities are compared with the theoretically calculated intensities, using a lattice dynamical model in which the electron lattice interaction has been expressed in terms of local intraionic charge deformabilities, with monopolar (breathing Γ1+), dipolar (Γ15), and quadrupolar (Γ12+) symmetry. Phonon anomalies in SmS, Sm0·75 Y0·25S and YS are described and discussed. Finally, evidence for a new excitation involving 4f charge fluctuations is presented, which is observed as a dispersionless excitation in the acoustic-optic phonon gap in (IV) materials.

    • Mercury chalcogenides under pressure

      T G Ramesh V Shubha P S Gopalakrishnan

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      The mercury chalcogenides, HgTe and Hg1−x Cdx Te, have received considerable attention in recent years because of their use in infrared photon detectors. This article is concerned with some recent experiments on the Γ68 band crossover in these systems induced by pressure. It is shown that although the inverted band model is valid for HgTe, the conclusions of the earlier workers on the pressure induced Γ68 crossover are erroneous. Experimental results show that it is difficult to observe the band crossover due to an intervening structural phase transition from cubic zinc blend to the cinnabar structure. However pressure experiments on the semiconductor alloy system Hg0·9Cd0·1 Te clearly indicate this crossover. New results on the zinc blend-cinnabar phase transformation are also presented. The striking behaviour of thermoelectric power in the high pressure cinnabar phase is correlated with the available experimental data on the structurally similar elemental semiconductors like selenium and tellurium.

    • High pressure studies of liquid crystalline transitions: Recent results

      R Shashidar

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      For sometime now we have been conducting high pressure studies on various types of liquid crystalline phase transitions. Two topics of current interest in liquid crystalline systems are (i) the phenomenon of reentrance, and (ii) multicritical points. In this paper we present some of the recent results of our pressure studies dealing with these two aspects.

    • The kinetics of pressure-induced polymorphic transformations

      A K Singh

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      The various methods of obtaining the kinetics data in pressure-induced transformations have been briefly reviewed. Some of the recent results obtained for the pressure-induced alpha-omega transformation in titanium, and fcc-bcc transformation in ytterbium have been discussed. The general features of the kinetics data have been compared with those for other transformations available in literature.

    • Hydrothermal synthesis of chromium dioxide powders and their characterisation

      V Abdul Jaleel T S Kannan

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      Chromium dioxide (CrO2) powders have been synthesised by decomposing CrO3 and Cr2O5 powders under hydrothermal conditions in the temperature range of 300–500°C and pressure range of 250–1200 bars. Oxides of antimony and iron have been used as modifiers to induce acicular morphology. A novel method of using alkali metal salts such as chlorides and carbonates as mineralisers produces CrO2 with superior magnetic characteristics. The particle size distributions have been correlated with the magnetic properties of the materials. The products obtained have properties rendering them useful for magnetic recording applications.

    • The verwey transition

      C M Srivastava

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      The origin of Verwey transition in magnetite is investigated. It is shown that the transport properties in the high temperature phase of magnetite can be accounted by a phonon-induced correlated electron transfer mechanism. Assuming that the condensation of this phonon mode leads to Verwey transition, the isotope and pressure effects have been explained. The electron energy band diagram consistent with the electrical transport behaviour in the low temperature phase has been constructed.

    • Anomalous photovoltaic effect and disorder in ZnS crystals

      M T Sebastian P Krishna

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      Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by uv light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed.

      This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2H to 3C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations.

    • Electrochemical ways of tapping solar energy: an appraisal

      A K Shukla R Manoharan K V Ramesh

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      In recent years, solar cell technology has advanced significantly and is nearing commercial viability. Practical solar cells that are capable of converting the solar radiation directly into electricity are now available. It is, however, imperative to couple them with appropriate power storage systems. This article is a brief review of the current status of electrochemical power systems such as storage batteries and fuel-electrolysis cell hybrids which could be successfully utilized to this end.

    • Optical properties of bismuth granules in a glass matrix

      G C Das R Das D Chakravorty

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      Optical absorption characteristics for ultra-fine bismuth particles having dimensions around 10 nm and dispersed in both silicate and vanadium phosphate glass matrices have been investigated in the wavelength range 300 to 700 nm. Bismuth particles in vanadium phosphate matrix show an absorption peak around 440 nm whereas in silicate glass matrix they give two peaks in the ranges 500 to 530 nm and 420 to 430 nm respectively. The peak positions in all the glass-bismuth metal systems are predicted in fair agreement with experiment by Maxwell-Garnett (mg), as extended by Polder and van Santen (mg-pvs) and Bruggeman (br) effective medium theories. It is observed, however, thatmg-pvs andbr models give the best fit to experimental data over the entire wavelength range studied.

    • Transition metal chalcogenides exhibiting quasi-one-dimensional behaviour

      J Gopalakrishnan K S Nanjundaswamy

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      This paper presents a survey of transition metal chalcogenides (mainly sulphides and selenides) that exhibit unidimensional structural features and electronic properties arising therefrom. The survey indicates that linear, single-atom, chains of transition metals are formed in chalcogenides by sharing faces of MX6 (X=chalcogen) trigonal prisms or octahedra as well as corners or edges of MX4 tetrahedra. Besides these single-atom chain compounds, chalcogenides possessing multiple-atom chains are known among the early members of the transition series when the transition metal is in a low formal oxidation state. Typical examples of this class are Ti5Te4 and TlMo3Se3.

    • Near-neighbour impurity effect on the spin-state transitions in LaCoO3 at low temperature (12<T<300 K)

      N Y Vasanthacharya P Ganguly

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      The low-temperature magnetic susceptibility behaviour of LaCoO3 prepared under different conditions as well as substituted samples such as LaCo0.95M0.05O3 (M=Al, Ga, Cr, Fe, Mn, Ni) and La0.98Sr0.02CoO3 have been investigated in the temperature range 12–300 K. Earlier interpretations of the magnetic susceptibility have been reexamined. In the case of LaCoO3 samples containing Al, Ga, Cr and Fe impurities spin-state transitions involving a temperature independent activation energy (∼0.01 eV) are observed atT<200 K. Analysis of the data indicates that either the excited state has an intermediate-spin (t2g5eg1) configuration or only half the Co ions are involved in the activated transition to the high-spin (t2g4eg2) configuration. Al3+, and Cr3+ increases the activation energy considerably. Substitution of L ≠ 0 ions such as Mn3+ and Ni3+ or Co4+ (low-spin) seems to introduce ferromagnetic interactions and stabilizes the paramagnetic state. LaCoO3, when Co is substituted by Mn (5%) or La is substituted by Sr (2%) show giant magnetic moments. When Co is substituted by Ni (5%) a ferromagnetic ground state is observed.

    • Chemical amplification — A novel approach to ultratrace analysis

      S R Rajagopalan

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      The lowest concentration that can be estimated with 100% error in chemical analysis by instrumental technique is referred to as detection limit. It is equal to the ratio of twice the background signal to the magnitude of analytical signal corresponding to a concentration of unity. Detection limit (dl) could be improved if the analytical signal alone could be selectively amplified without affecting the background. It is shown that this could be achieved by chemical amplification. Atomic amplification of two or three orders of magnitude could be obtained by combining two chemical amplifications in series. It is demonstrated that by using heteropoly acid formation as the first amplification reaction and determining molybdenum in the heteropoly acid by catalyticdc polarographic wave, a detection limit for the hetero atom of less than a ppb can be obtained. It is pointed out that the approach of selectively amplifying analytical signals by tandem chemical amplification reactions makes it possible to perform ultratrace analysis (ppb level) with classical instrumental analytical techniques which havedl at ppm level.

    • Electroless and electrodeposition of nickel boron composites

      Indira Rajagopal

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      It is shown that nickel boron composite coatings can be obtained by electroplating nickel from a bath containing dispersed boron particles. This deposit heated to 300°C forms Ni-Ni3B composite. Further heating to 400°C converts it into Ni-Ni2B composite. Mechanical properties, corrosion resistance and wear resistance Ni-Ni2B composite are better than nickel or the electroless Ni-B composite in the as-deposited condition. The properties of heat-treated electroplated Ni-B composite are similar to that of heat-treated electroless Ni-B composites.

    • Hydrodynamics of cholesteric liquid crystals in the coarse-grained limit

      G S Ranganath

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      The hydrodynamical behaviour of cholesteric liquid crystals has been considered in the limit of low amplitude and low frequency distortions and motions. It is shown that there are interesting analogies with superfluid-hydrodynamics, such as the fountain effect, thermal superconductivity and temperature wave propagation. In certain situations, there is an unusual formation of a boundary layer at low velocities, and in certain others the properties resemble those of percolation in porous media. Results concerning some special phenomena peculiar to cholesteric liquid crystals are also presented. Finally it is pointed out that there should be two types of second sound in chiral smectic C.

    • Microcrystals of cooperative-behaviour materials

      M S Multani R Vijayaraghavan M R Srinivasan P Ayyub V R Palkar V Saraswati A V Gurjar

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      Properties of materials exhibiting cooperative phenomena are likely to be modified on restricting the lattice size. Several such microcrystalline materials have been studied to infer general underlying principles. Ferroelectricity, ferromagnetism, superconductivity, and superfluidity in limited lattice sometimes not only show modified behaviour but also new behaviour not present in bulk. Microcrystals are considered to be free, coupled, grain-boundary-separated, or domain-wall-separated.

    • Material-tissue interface

      M S Valiathan

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      The interface between prosthetic materials and body tissues has become important thanks to the extensive use of bioimplants and artificial internal organs. The long-term function and survival of implanted prostheses depend on the stability of the material-tissue interface. The methods in current use for the fixation of implanted prostheses are mainly based on mechanical linkages which are inherently unstable. The manifestations of instability are seen in clinical phenomena such as prosthetic thrombosis and failure of skin-prosthetic linkage. A less vulnerable approach to stablising material-prosthetic interface would be the development of chemical bonding which has already been accomplished at the bone-bioglass ceramic level. The approach may have wider relevance to the linkage of polymeric materials to body tissues.

    • Materials research at national aeronautical laboratory (contributions of S Ramaseshan)

      S R Valluri S R Rajagopalan

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      Materials research programmes of the National Aeronautical Laboratory and Prof. Ramaseshan’s contributions to these are presented in this article.

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