• Volume 3, Issue 2

      July 1981,   pages  1-245

    • Anil Bhusan Biswas (1917–1979)

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    • Some interesting phase transitions in solids

      C N R Rao

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      A few types of phase transitions in solids of interest to the author are discussed after a brief presentation of the general features of phase transitions. The different systems discussed include low-dimensional solids, polytypes, organic solids, plastic crystals, glasses and silver selenide. Spin-state transitions and dipole glasses are also briefly examined.

    • Critical point phenomena, heat capacities and the renormalization group theory of fluctuations

      E S R Gopal

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      In the normal study of matter, the ordered state is considered first, followed by the addition of minor disorder or fluctuations, for instance, studying crystalline solids with some quasiparticle excitations like phonons and magnons. The discovery of the universality of critical point phenomena seems to provide a chance to study a regime dominated by the fluctuations. The Onsager solution of the two-dimensional Ising model, exhibiting a logarithmic singularity in heat capacity, and the Fairbank-Buckingham-Kellers experiments, showing such a singularity in the heat capacity near the superfluid transition of liquid4He, are landmarks in this topic. The recent renormalization group theory shows a way of studying the patterns among the fluctuations. The dependence of the critical exponents upon spatial and spin dimensionalities, the existence of universal amplitude ratios and the other aspects of critical phenomena are briefly discussed.

    • Use of linear weight factor in real-space renormalization group

      D Sengupta P K Ghosh

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      An important problem in all real space renormalization group calculations is the choice of appropriate weight factor for different configurations. In this context, the use of linear weight factor has been restricted due to lack of suitable criterion for selecting the parameterq. An elegant matching procedure has recently been developed to fixq. First we undertake a systematic investigation of the variation of critical constants with cell size. A comparison is also made with the results obtained using an earlier criterion proposed by Subbarao. Secondly, we extend the applicability of the theory to the realm of dynamic problems by studying the exponent of energylike perturbation in 2D Glauber-Ising model. The value of the exponent obtained for square and triangular lattices are 2·147 and 1·918 respectively which are in good agreement with the general consensus (∼2·0).

    • Structural aspects of alpha-omega transformation in group IV transition metals and alloys

      Y K Vohra S K Sikka R Chidambaram

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      The three most common crystal structures encountered in group IV transition metals Ti, Zr, and Hf and alloys based on them, under different temperature, pressure and alloy concentration conditions, are hcp(α), bcc (β) and simple hexagonal (ω). Although the structural relations of α⇌β andβω transformations are well understood, the same is not true for α→ω phase change, which occurs at high pressures. We have done high pressure experiments on Ti-V alloys, followed by electron diffraction to study this. These patterns from pressure treated foils of alloys Ti95 V5 and Ti91 V9 showed the presence ofβ-phase with fourω variants. Some of them showed the existence of all three phases, α,β andω, with the number of variants given by the lattice correspondence matrix, derived through the orientation relations of α →β andβω. This is a clear evidence that the α →ω transformation proceedsvia theβ-phase. The atomic rearrangements required forαω are found to be much smaller if the path is via theβ-phase, rather than the earlier model of Silcock.

    • Statistical theories of nematic liquid crystals

      N V Madhusudana

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      Nematic liquid crystals are composed of molecules with moderate shape anisotropy, and are characterized by purely orientational but no translational long range order. It is necessary to take into account both the anisotropic attractions between the molecules and the excluded volume effects to give a proper statistical theory of this phase. Indeed a reasonably satisfactory semiquantitative description has emerged only in the past two years. The paper reviews recent developments in the field.

    • The tricritical point — a qualitative overview

      S Dattagupta

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      The concept of tricritical points is introduced by way of examples of ordinary liquid mixtures and mixtures of normal He3 and superfluid He4. A prototype Ising spin model is used for a qualitative discussion of the mechanisms behind the occurrence of tricritical phenomena. Mention is also made of the other theoretical developments.

    • Thermodynamic analysis of the metal-insulator transitions in V2O3 alloy systems

      J M Honig H V Keer G M Joshi S A Shivashankar

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      An overview is presented concerning the entropies (ΔS) and enthalpies (ΔH) associated with metal-insulator transitions in the (V1−xMx)2O3 M=Al, Cr, Ti systems, 0⩽x⩽0·12 and in non-stoichiometric V2(1 −y)O3, 0⩽y⩽0·01. The recent literature and new data are analyzed with respect to gradiations in ΔH and ΔS withx ory, and their implications concerning the mechanism of the transitions. A revised phase diagram is presented.

    • Investigation on schottky diodes on amorphous hydrogenated silicon

      SM Pietruszko K L Narasimhan S Guha

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      Schottky barrier diodes with near-ideal characteristics have been fabricated on amorphous hydrogenated silicon prepared by decomposition of a mixture of 10% silane and 90% hydrogen. The interface properties are found to be stable up to heat treatment of 300°C. From a detailed investigation of dark and photovoltaic properties it is concluded that the density of states in the mobility gap is sufficiently small so that there is no significant carrier recombination in the space charge region.

    • ESCA studies of some mixed-valence rare-earth intermetallics

      B D Padalia Varsha Prabhawalkar P D Prabhawalkar E V Sampathkumaran L C Gupta R Vijayaraghavan

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      ESCA (or XPS) studies of some rare-earth intermetallics were carried out using ESCA (3) Mark II and ‘ESCALAB’ machines from V G Scientific. The mixed valence nature of EuCu2Si2 and EuNi2P2 intermetallic compounds has been established. The 3d core-level photoemission spectra of Eu in EuCu2Si2 and EuNi2P2 exhibit two sets of structures corresponding to two different valence states of the rare-earth ions. Each of these sets shows simple spin-orbit splitting. The replicate splitting measured by us agrees well with those calculated by Herbst and Wilkins. The 3d core-level photoemission spectra of Eu is free from the multiplet splitting and satellite structure. The importance of 3d core-level photoemission studies in the understanding of valence fluctuation phenomenon has been emphasized.

    • Oxide dielectrics

      E C Subbarao

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      Three aspects of oxide dielectrics are covered (i) structure engineering: making use of crystal structure and crystal chemistry, the dielectric behaviour can be finely tuned, as illustrated in the systems (Ba, Pb) (Ti, Zr)O3 and (Ba, Pb) (Ti, Nb)O3 studied by Biswas. (ii) Defect structure: control of defect structure of Mn-doped BaTiO3 inhibits the reduction of BaTiO3 in reducing atmospheres at high temperatures thus making the use of base metal electrodes possible in manufacturing ceramic capacitors. (iii) Composites: Diphasic composites of oxide dielectrics with other materials may be utilized for producing voltage stable capacitors and dense or flexible piezoelectrics with outstanding properties.

    • Exchange in insulators

      C M Srivastava

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      The exchange constants obtained using the three sublattice model for ferrites and garnets have been analysed using Anderson’s theory of superexchange. The transfer integrals for the 180° and 90° interactions obtained from the experimental values of exchange constants give approximate values of the covalency parameters which are consistent with the chemical theory of covalency.

    • Correlation between the magnetic properties and the chemical shifts in x-ray absorption spectra of some transition metal compounds

      Chintamani Mande M Y Apte

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      Studies on the chemical shifts of the K x-ray absorption discontinuities of some transition metal compounds recently conducted in our laboratory have revealed that the ΔE vs q curves do not pass through the origin. For such systems one obtains an intercept on the ΔE axis, which seems to depend uponnd, the number of unpairedd electrons. As is well-known the magnetic properties of the compounds of transition metal elements are governed by thed electrons. It is shown that the magnetic properties of the transition metal compounds may be correlated with x-ray absorption spectra.

    • Preparation and characterisation of γ-Fe2O3 as tape recording material

      K Seshan M R Anantharaman Venkatesh Rao A L Shashimohan H V Keer D K Chakrabarty

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      Optimum conditions for the preparation of tape recording quality γ-Fe2O3 by the thermal decomposition of ferrous oxalate dihydrate have been established. Formation of the intermediate Fe3O4 which is most important in forming γ-Fe2O3 takes place only in the presence of water vapour. Various stages of decomposition have been characterised by DTA, TG, DTG, and x-ray powder diffraction. The method for the preparation of acicular γ-Fe2O3 that matches very well with the commercial tape recording material has been developed.

    • Antiferromagnetism in the heisenberg hamiltonian

      C K Majumdar

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      The shortcomings of the classical theories of antiferromagnetism are reviewed. On the basis of known exact results of several antiferromagnetic models, it is argued that the spin correlation functions may have oscillating behaviour anticipated from the classical Neel states even though the exact ground state is quite different from Neel state. It is also found that as the spin becomes large, the classical results become more and more applicable as seen in Fisher’s model and verified experimentally. A few remarks on the broken symmetry are made. The existence of an anisotropic antiferromagnetic model with Neel states as ground state is pointed out.

    • Mossbauer and magnetic studies on the system Fe3−xTixO4

      R Nagarajan C R K Murty

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      Mossbauer measurements have been made on the system Fe3−xTixO4 for 1·0 ≥x ≥ 0. This system is interesting in that it is expected to form a continuous solid solution over the whole range and also occurs naturally. Spectra taken at various temperatures between 300 K and 77 K show complex behaviour. The results can be understood in terms of disorder and consequent formation of superparamagnetic clusters which are supported by our earlier hysteresis and susceptibility studies at various temperatures. It appears that with increasing Ti content domain wall formation is inhibited resulting in effectively monodomain clusters. Considering the work reported by others, we suggest that Mossbauer spectra of multidomain material should show well-resolved magnetic hyperfine spectrum, even close to itsTN.

    • The influence of Cu2+ substitution on the magnetic properties of Fe-Zn and Ni-Zn ferrites

      C M Srivastava M J Patni G Srinivasan T T Srinivasan

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      The influence of a Jahn-Teller ion on magnetization,g-values and linewidth in Znx Fe3−xO4 and Ni1−x Znx Fe2O4 spinel ferrites has been investigated by introducing small quantities of Cu2+ ions. The magnetization of Cu substitution in both the systems but the curie temperature is affected only in FeZn ferrites. The lattice constant in NiZn is significantly reduced while in FeZn there is only a slight increase. The change ingeff on copper substitution,Δgeff, is very different in the two systems. In NiZn,Δgeff has a uniform value, ∼ 0·2, for all values ofx while in FeZn it has a peak atx=0·3 and vanishes atx=0 andx⩾0·5. The ferromagnetic linewidthΔH does not change significantly on copper substitution in both the systems. An explanation based on the perturbation produced due to the local lattice distortion by the Jahn-Teller mechanism explains the observed results satisfactorily.

    • Lattice imaging studies on the structure and non-random disorder in SiC crystals

      S R Singh G Singh

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      A number of SiC crystals having extremely large period unit cells and one-dimensionally disordered structures have been studied by x-ray diffraction and lattice imaging in the electron microscope. It has been observed that structures of extremely large periodicities (>100 nm) can be conveniently studied by lattice imaging technique which simply give continuous streaks in their x-ray diffraction patterns. This has been shown in the case of 150R, 150RA, 900R and another structure whosec-repeat period is more than 100 nm. SiC crystals showing streaks alongc*-direction in x-ray diffraction are found to possess either completely ordered structures with extremely large periodicities or non-random disorder.

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