• Volume 7, Issue 3-4

      October 1985,   pages  152-409

    • Dr N. S. Satya Murthy

      C N R Rao

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    • Intergrowth structures in inorganic solids: A new class of materials

      C N R Rao

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      An increasing number of inorganic solids forming long-period structures due to recurrent intergrowth of two chemically distinct but structurally related units are getting to be known in recent years. These novel structures have given rise to new chemistry at solid-solid interfaces. Besides intergrowth structures with long-range order, many solids with random intergrowth (similar to stacking faults in polytypes) are known. Ordered integrowth gives rise to homologous series of structures in many systems. Barium ferrites, the Aurivillius family of oxides and other perovskite-related oxides, siliconiobates, and tungsten oxide bronzes are some of the systems exhibiting ordered intergrowth structures. Both ordered and disordered intergrowths are fruitfully investigated by high resolution electron microscopy. The main emphasis in this article is on intergrowth structures where the component units are compositionally different. These systems are obviously most fascinating since compositional change occurs across each interface (intergrowth plane), unlike in polytypic materials where the composition remains constant. Even in ordered intergrowth structures, there is always some disorder. If order in an intergrowth structure does not prevail over large distances, but occurs only over shorter stretches (say, a few repeats of the sequence), it becomes difficult to describe the solid except in terms of the gross composition and where possible, the unit cell dimensions. Structures with occasional intergrowths are found in a variety of materials such asβ-alumina, Magnéli phases, silicates, ferrites and several other oxide systems. In addition to examining the structural features of various intergrowths, the origin of the intergrowth phenomenon is discussed.

    • Quasi crystals—an overview

      G Venkataraman

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      This paper gives a broad overview of recent developments concerning quasi crystals. After briefly recapitulating various experimental facts, the architecture of quasi periodic systems in various dimensions is discussed. The various density-wave models are then reviewed. This is followed by a discussion of the extra long wavelength models which appear in such systems. Some comments are then made about possible relationship to recent theories of glass which stress local icosahedral order. The paper concludes with an outlook for the future.

    • Insertion/extraction of lithium and sodium in transition metal oxides and chalcogenides

      J Gopalakrishnan

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      Lithium and sodium can be reversibly inserted into a variety of transition metal oxide and chalcogenide hosts at low temperatures. The reaction is essentially topochemical involving electron-transfer and accompanying diffusionless transformation of the anion array in many cases. Alkali metal insertion/extraction reactions provide new routes for the synthesis of novel solids exhibiting unusual structure and properties. It appears that while the structure of the insertion compound is determined by the host structure and the size of the alkali metal ion, the extent of insertion/extraction is decided by the redox characteristics and electronic conductivity of the host.

    • Structure and superconductivity studies on ternary equiatomic silicides,MM′Si

      G V Subba Rao K Wagner Geetha Balakrishnan J Janaki W Paulus R Schöllhorn V S Subramanian U Poppe

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      A series of ternary equiatiomic transition metal silicidesMM′Si (M=Zr, Nb, Ta;M′=Ru, Re) and Zr1/2M1/2 RuSi (M=Nb, Ta) have been synthesized and characterized. Guinier x-ray data clearly indicate that the compoundsMM′Si (M=Nb, Ta;M′=Ru, Re) crystallize with the orthorhombic TiFeSi-structure. This is contrary to one of the literature reports ascribing the hexagonal ZrRuSi structure toMReSi,M=Nb, Ta. ZrRuSi and Zr1/2M1/2RuSi (M=Nb, Ta), however, crystallize with the ZrRuSi structure. All the compounds are metallic and exhibit small and negative Seebeck coefficients. Superconductivity has been discovered inMM′Si,M=Nb, Ta;M′=Ru, Re and the transition temperatures, as measured by the low frequency a.c. susceptibility technique, range from 2·7 to 5·1 K. Zr1/2Nb1/2RuSi is not superconducting above 2·2 K. The observed properties are explained in terms of the electronic charge transfer effects and crystal structures adopted by these metal excess ternary silicides.

    • 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.

    • Magnetic relaxation in rare earth garnets

      C M Srivastava B Uma Maheshwar Rao N S Hanumantha Rao

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      An attempt has been made to review the ferromagnetic resonance and relaxation data on rare earth iron garnets and in the light of it, to analyze the data on Sm and Eu thin films and Gd and Dy bulk polycrystalline samples obtained in our laboratory. A phenomenological approach adopted to explain the temperature and composition dependence of the line width andgeff appears to explain the complex spectra in most cases.

    • Magnetic, Mössbauer and electrical properties of Ti-substituted Ni0·3Zn0·7Fe2O4

      D C Khan M Misra

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      The effect of addition of Ti4+ in small amounts (1/2, 1, 2, 5, 10% by weight of TiO2) under oxidising conditions on magnetisation, Mössbauer spectra and resistivity of Ni0·3Zn0·7Fe2O4 is studied. Analysis of the magnetic data on the basis of 3 sublattice Yafet-Kittel model gives the transfer rate of Fe3+ ions between theA andB sublattices, the distribution of Ti4+ and vacancies between them and the five exchange constants of Ni-Zn ferrite. The observed Mössbauer spectra at room temperature are quadrupole doublets and those at liquid nitrogen temperature are magnetic sextets with large relaxation effects. The resistivity shows an increase by about two orders of magnitude due to Ti4+ addition.

    • Effect of high pressure on chalcogenide glasses

      G Parthasarathy E S R Gopal

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      The effect of high pressures on the various properties of the chalcogenide glasses is reviewed. The properties discussed include the mechanical, electrical, optical and magnetic properties. The phenomena of the crystallization of the chalcogenide glasses under high pressure is also discussed.

    • Neutron investigation of ferrites

      L Madhav Rao

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      This article reviews the work carried out at Trombay in a variety of single and mixed spinel ferrites. The use of polarised neutrons in the elucidation of cation distribution and magnetic structures in powder specimens is emphasised. Magnetic form factor studies in single crystal specimens of Fe3O4 and MnFe2O4 are described. Evaluation of dominant exchange interactions in a few powder specimens in their paramagnetic phase using the cold neutron scattering technique is described. Measurements of the acoustic magnon dispersion in MnFe2O4 and Li0.13Fe2.87O4 are outlined.

    • Studies of materials using polarised neutrons

      P Radhakrishna

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      The scattering cross-sections of a polarised neutron beam, incident on a magnetic sample, are a function of the polarisation of the beam, and of the nuclear and magnetic structure of the sample. An analysis of the polarisation of the scattered neutrons is valuable in isolating magnetic contributions.

      The spin distribution, obtained by Fourier inversion of the magnetic structure factors calculated from the cross-sections, has been studied in ferromagnetic metals, intermetallic compounds and alloys. Applications of the method to the study of anti-ferromagnets and canted ferromagnets are outlined. Paramagnetic metals, solid organic radicals and transuranium elements have spin distributions in which orbital and other effects can be compared with theory.

      The value of polarisation analysis and some of the new technical developments which have made it possible are described. Applications to inelastic phenomena are mentioned.

    • Neutron quasi-elastic scattering in materials research

      B A Dasannacharya

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      The paper reviews the quasi-elastic neutron scattering activities at the Bhabha Atomic Research Centre and demonstrates how this technique gives information on rates and geometries of stochastic motions in liquids and solids.

    • Neutron scattering from fractals

      S K Sinha

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      We describe briefly the concept of fractal dimension as applied to both mathematical and statistical fractals. We then discuss the scattering of radiation from fractals and describe the results of small angle neutron scattering studies of the aggregation of small particles in fractal clusters.

    • X-ray spectroscopy and local structures in glasses

      K J Rao B G Rao

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      X-ray spectroscopy which is a combination of two techniques, namely x-ray absorption near edge structure (xanes) and extended x-ray absorption fine structure (EXAFS) analyses, is a unique technique for the study of local structures in glasses. Availability of synchrotron radiation sources has made the technique quite attractive and useful because the photon flux from synchrotrons is very intense and polarized. In this article, a brief summary ofxanes andEXAFS techniques is given along with a few applications to the study of local structures in glasses.

    • Compositionally modulated magnetic films

      R Krishnan

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    • Band theory analysis of shock velocity-particle velocity relations for metals

      S K Sikka B K Godwal R Chidambaram

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      The systematics of the shock constants in shock velocity-particle velocity relations for metals have been examined by energy band theory methods. The causes of non-linearity of this relation at high pressure are discussed in terms ofsd electron transfer.

    • Neutron beam research in magnetism: The Indian scene

      N S Satya Murthy

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