• Volume 36, Issue 2

      February 1991,   pages  133-243

    • A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

      A Jayaraman G A Kourouklis

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      The effect of pressure on the Raman modes in TeO2 (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD44 symmetry toD24 symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl2-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO2 is emphasized, to unravel the structure of the various high pressure phases in the system.

    • Calculated elastic constants of dilute alloys based on bcc metals

      P D Semalty P N Ram

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      The elastic constants of dilute alloys based on bcc metals have been calculated using the Green’s function method obtaining explicit expressions for change in elastic constants in terms oft-matrix. The crystal impurity problem is discussed within an impurity model containing central and non-central force constant changes extended up to second neighbours of the impurity. The effect of volume change on elastic constants and a contribution from electron pressure term are considered. Numerical results for changes in three elastic moduli have been presented for a number of dilute alloys based on Mo, Nb, W, Ta and V.

    • Libratory modes of water molecules in tutton salts

      Shashi P Gupta Y Kumar B N Khanna

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      The existence of rocking, wagging and twisting librations of different types of water molecules in tutton salts has been established from their ir spectra. These bands have been identified, following the intensity criterion of Miyazawa. The present assignments indicate that the bands due to libratory motion of H2O III in NH4Mn-, KNi-, KCo and KMn- and of H2O I in KMn- and KCu- tutton salts are out-of-plane H-bonded. The strength of the hydrogen bonding is the weakest in the (NH4)2Fe(SO4)2.6H2O.

    • Dielectric properties of BaTiO3 and La doped BaTiO3 ceramics

      N S Panwar B S Semwal

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      The dielectric constant and loss tangent of barium titanate and barium titanate doped with 0.4% (mole) La ceramics have been studied in the frequency range 22 KHz-70 MHz at room temperature and in the temperature range 20°C–170°C at 22 KHz using aQ-meter. It is observed that the dielectric constant at room temperature of the samples increases nearly a hundred fold on doping. The maximum of the dielectric constant broadens and shifts to a lower temperature in the La doped samples. The loss tangent is also relatively high in the doped sample. The dielectric constant and loss tangent of the doped samples decrease faster with frequency than that of the undoped samples.

    • Strong, large distance impurity dipole interaction in KNbO3 single crystals

      S G Ingle J G Dupare

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      It is found that the unrelaxed impurity dipoles can arrange themselves linearly in the structure joining each other end-to-end in pseudocubic [110] direction at the tetragonal to the orthorhombic phase transition. It is shown that this alignment precedes the domain formation at the phase transition, which implies quick movements of the dipoles in the structure, and a strong dipolar interaction. The experiments with the application of dc fields to the crystals showed that the dipolar interaction becomes stronger with the field. The dipoles can see each other across the existing domain walls implying the large distance nature of the interaction. The observation of impurity clusters arranged in pseudocubic [110] direction confirmed the large distance nature of the interaction. It is concluded that this strong, large distance interaction is very interesting in as much as such an interaction of dipoles forms the basis of ferroelectricity.

    • Relaxation intensity in some polyacrylates

      Manisha Gupta Sangita Madhuri Mathur Jagdish Shukla

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      Empirical formula suggested by Kita and Koizumi for evaluation of relaxation intensity in a limited range of frequency around the relaxation frequency for the Cole-Cole type distribution has been tested for poly butyl acrylate (PBA), Poly butyl methacrylate (PBMA) and poly isobutyl methacrylate (PiBMA).

      The relaxation intensity Δε is expressed in terms ofε″M, the dielectric loss maxima andW, the frequency separation for half, two thirds or three quarters ofε″M, in the form Δε=ε″M/[(C1/W)+C2+C3W], where the numerical constantsC1,C2,C3 are given for the respective type of relaxation.

    • Electronic structure calculation of CuMn alloy

      P K Khowash D C Khan

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      We have used a fully self-consistent multiple scatteringXα method within the local density formalism to study the charge distribution, bonding characteristics and the density of states in CuMn alloy. The charge distribution shows almost no ionic character but significant hybridization ofs andd states is observed near the Fermi level. The crystal field splittings, ionization energies and the excitation energies are calculated and compared with experiments wherever available.

    • Fluorescence studies of CS2 and SO2 at 121.6, 73.6–74.4 and 58.4 nm

      I A Prajapati S M Ahmed Vijay Kumar

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      The fluorescence spectra of CS2 and SO2 have been studied at three incident photon wavelengths of 121.6, 73.6–74.4 and 58.4 nm and relative production cross sections for different product states have been measured. The CS(A1Π→X1Σ+) system between 240 and 290nm has been obtained when CS2 is photoexcited at 121.6nm whereas CS2+(B2Σu+X2Πg) and CS2+(A2ΠuX2Πg) systems have been produced between 276 and 295 and 437 and 555nm respectively when excited by both the incident photon wavelengths of 73.6–74.4 and 58.4nm. The fluorescence spectra of SO2 obtained at 121.6 and 73.6–74.4nm include the vibrational bands of SO(A3Π→X3Σ) and SO(B2ΠX3Σ) systems from 240 to 268 and 268 to 442nm respectively whereas the emission spectrum at 58.4nm, has contributions from the two SO systems and SO+(A2Π→X2Π) system. In all these emission spectra, the fluorescence bands of different systems have been analyzed and their relative production cross sections have been measured. The results obtained in the present investigations have been compared with a few recent reliable measurements reported in literature.

    • Point contact tunnelling studies on ceramic YBCO with scanning tunnelling microscope tips

      H Srikanth M Rajeswari A K Raychaudhuri

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      A detailed study of point contact tunnelling into ceramic YBCO with electrochemically etched tips of Pt, Nb and W is reported. The superconducting gap parameter (Δ) has been extracted fromIV and dI/dVV curves using various procedures. Our results indicate a gap value of about 20 meV. We observe that the zero bias conductance is strongly dependent on the junction resistance. The normal state conductance varies linearly with bias voltage and the conductance curves are asymmetric with respect to polarity of the bias voltage. With contacts of very high junction resistance, we observe G(0)/G(100 mV) has a value as low as 1/6. This may be the lowest value reported so far.

    • Thermopower anisotropy in magnetic films of Co and Ni plated in presence of an external magnetic induction

      Benu Sinha Amar Nath Nigam

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      Electroplated films of cobalt and nickel were grown on copper substrate both in absence and presence of external magnetic induction ranging from 25 to 72 gauss. The latter films showed a magnetic behaviour and an anisotropy in thermopower with respect to the direction of magnetisation.

    • An automated ac-magnetic susceptibility apparatus

      A Chakravarti R Ranganathan A K Raychaudhuri

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      We report here both hardware and software for an ac susceptibility measurement system, namely the design and construction of the cryostat, coil system, sample rod assembly and also the automation of the sample rod movement, bridge control and nulling etc with the help of an inexpensive Z-80A microprocessor via a home-made IEEE-488 interface. The variable parameters are temperature, magnitude of the rms field and frequency. An entirely new dynamic bridge nulling algorithm with continuous sample movement, which eliminates to a large extent problems related to time-dependent drifts, has been developed. We also present some experimental data collected with this system.

    • Observation of rf SQUID characteristics in YBCO bulk at 77 K

      Neeraj Khare A K Gupta S K Arora V S Tomar V N Ojha

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