• R Vijayaraghavan

      Articles written in Bulletin of Materials Science

    • Fine particle physics and technology; some results

      M S Multani N G Nanadikar V R Palkar A K Pansare A Gurjar R Vijayaraghavan

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      The new techniques of fine particle synthesis are shown to lead to better ceramic materials such as yttrium iron garnet (YIG) and ferroelectric lead zirconate titanate (PZT). The densities are greater than 99% of the theoretical density and the grain size can be easily controlled from 2 microns upwards. For YIG this means higher values of the nonlinear spin-wave excitation thresholdhc. For PZT the result is higher values of the tensor componentd33 and poling voltage. It is also shown that the fine particle systems and the fine-grain microstructure lead to interesting physics results. For PZT the lowering of the peak dielectric constant, the increasing transition temperatures and the decreasing values ofd33 with decreasing grain size can be understood in terms of the changing low-frequency cut-off of the soft lattice vibration mode. Another interesting result is that for 2000 Å size particles of α-Fe2O3 the anisotropy constant is found to be 1·38 × 102 ergs/cm3 from Mössbauer measurements.

    • 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.

    • 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.

    • Structural and magnetic investigations of some new boron containing rare earth intermetallic compounds

      S K Malik S K Dhar R Vijayaraghavan

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      By alloying boron with RPd3 (R=rare earth) compounds, new ternary alloys of the formula RPd3Bx (0 ≤x ≤ 1) have been prepared. The parent RPd3 compounds crystallise in the cubic AuCu3 type structure. The addition of boron does not change the structure but results in an expansion of the lattice. Therefore, it is likely that the small boron atoms occupy the vacant body centred position in AuCu3 type structure. It is also observed that compounds of the composition RRh3B can be formed in the cubic structure for all rare earths R though the parent RRh3 compounds exist only for R=Ce (AuCu3 type) and La, Sm, Nd and Gd (all hexagonal CeNi3 type). This points out the role of boron in stabilising new crystallographic phases. The results of susceptibility measurements on some of the RPd3B and RRh3B compounds are presented. In particular, it is noted that while GdPd3 orders antiferromagnetically withTN=6 K, GdPd3B does not order magnetically down to 4.2 K.

    • Amorphous state ferroelectricity, magnon scattering and phase stability in microparticle materials

      M S Multani P Ayyub V R Palkar M R Srinivasan V Saraswati R Vijayaraghavan DO Shah

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      Microparticles and micrograin ceramics show features distinct from the usual-sized polycrystalline materials. Amorphous state material combined with microparticle size for Pb(Zr0.51 Ti0.49)O3 mimics the dielectric behaviour of crystalline ferroelectricity in ABO3 compounds. Fine-grained Y3Fe5O12 (yig) synthesized by pressure sintering ofyig microparticles exhibit spinwave relaxation due to transit time across grain diameter. Applying microemulsion techniques for microparticle synthesis,γ-Fe2O3 has been synthesized. The phase stability fractionγ-Fe2O3/α-Fe2O3 is found to be a function of particle size. At very small sizesγ-Fe2O3 becomes amorphous, leading to interesting Mössbauer studies.

    • Magnetic properties of RRh3B2(R=La, Ce, Nd, Gd) compounds

      D Rambabu S K Dhar S K Malik R Vijayaraghavan

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      The11Bnmr results on RRh3B2(R=La, Ce, Nd and Gd) are reported. For CeRh3B2, specific heat and electrical resistivity are reported. From a comparative study of the11Bnmr Knight shifts and the spin lattice relaxation times of these compounds it is shown that in CeRh3B2, there is strong hybridization of 4f states with the conduction electrons. A local moment on Ce with admixture of 4f and 5d–6s orbitals is suggested.

    • Role of oxygen content in the structural and the superconducting properties of high temperature superconducting ErBa2Cu3Oy

      G K Bichile D G Kuberkar A K Rajarajan S S Shah L C Gupta R Vijayaraghavan

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      Samples of high temperature superconducting system ErBa2Cu3Oy prepared by the standard ceramic technique were found to have single phase. To vary the oxygen content in the samples, the mother compound was reduced by vacuum-annealing at different temperatures for a fixed period. The oxygen content was measured by gravimetric and iodometric titration procedures. The structural changes induced due to the variations in the oxygen content were studied by X-ray methods.

    • Cooperative effect of Zn and Ga doping in YBa2Cu3O7

      N C Mishra A K Rajarajan K Patnaik R Vijayaraghavan L C Gupta

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      The creation of local magnetic moments at the plane and chain Cu-sites in YBa2Cu3O7 due to incorporation of Ga3+ and Zn2+ has been studied with respect to the superconducting transition temperature and structural parameters of the compound. The result indicates that the local moments are created due to slowing down of spin and charge fluctuations in the planes and chains respectively due to doping.

    • On the electron superconductors Nd2−xCexCuO4−y

      G Balakrishnan R Vijayaraghavan

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      The compounds Nd2−xCexCuO4−y forx=0 to 0.18 have been synthesized and characterized. Superconductivity is observed forx=0.14 to 0.16. The normal state resistivity and d.c. magnetic susceptibility of these compounds have been measured in the range 4.2 to 300 K. Both the superconducting and non-superconducting compounds exhibit a Curie-Weiss behaviour at high temperatures, giving an effective moment for the Nd ion close to the free-ion value.

    • Influence of transition metals substitution on the superconductor Bi4Ca3Sr3Cu4Oy

      E V Sampathkumaran R Vijayaraghavan

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      The results of our investigation on the specimens Bi4Ca3Sr3Cu4−xTxOy (T=Fe, Co, Ni,x⩽0.5) synthesized in air are presented. Fe and Co substituents result in the formation of Bi2Sr2CuOy-type of phase, with considerable depression ofTc of the main phase. However, Ni is completely soluble with Cu in this concentration range without significant depression of superconducting transition temperature (Tc). This difference in the solubility behaviour of Fe and Co on the one hand and Ni on the other is explained taking into account ionic charge and coordination number mismatches.

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