• V Ganesan

      Articles written in Pramana – Journal of Physics

    • Lattice parameter and thermal expansion of CsCl and CsBr by x-ray powder diffraction. I. Thermal expansion of CsCl from room temperature to 90° K

      V Ganesan K S Girirajan

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      The thermal expansion coefficients of CsCl at eight different temperatures between room temperature and 90° K are presented. The lattice parameters obtained from the observed Bragg angles with the use of Bragg formula were extrapolated to 90° using the Nelson-Riley extrapolation scheme to minimize the systematic errors. The true lattice parameters thus obtained at each temperature were used to obtain the thermal expansion coefficients and these results are compared with earlier interferometric measurements.

    • Lattice parameter and thermal expansion of CsCl and CsBr by x-ray powder diffraction. II. Thermal expansion of CsBr from room temperature to 78.2 °K

      V Ganesan K S Girirajan

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      The lattice parameters of CsBr at eight different temperatures from room temperature to 78.2 °K were measured and the true lattice parameters were obtained by the least squares method using the Nelson-Riley extrapolation function. Using these parameters the thermal expansion coefficients of CsBr were estimated at each temperature by fitting them into a cubic polynomial involving temperatureT. TheαT thus obtained were compared with the values of earlier workers who used an interferometric technique and the agreement was found to be good.

    • Precise lattice parameter measurements in CsCl-Br solid solutions between room temperature and 90°K by powder x-ray diffraction

      V Ganesan K S Girirajan

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      Solid solutions of CsCl-Br in five different concentrations were prepared in sealed quartz tubes by heating the mixture to 1123°K for 6–8 hr and quenching to room temperature. X-ray diffractograms were taken at eight different temperatures between room temperature and 90°K for these solid solutions using the YPC50NM powder diffractometer and a continuous flow cryostat. The observed lattice parameters for each sample at each temperature obtained from the powder diffractograms were then extrapolated to give the true lattice parameters using the least square method with Nelson-Riley extrapolation scheme. The values of the true lattice parameters at each concentration and at each temperature were tabulated and the results discussed. It is shown that the lattice parameters vs temperature for some concentrations exhibit an anomalous behaviour.

    • Temperature variation of the Debye-Waller factors of metal and halide ions in CsCl and CsBr powders by X-ray diffraction. I. Debye-Waller factors of Cs+ and Cl ions in CsCl from room temperature to 90°K

      V Ganesan K S Girirajan

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      The temperature variation of the Debye-Waller factors of Cs+ and Cl ions in CsCl powder has been studied using X-ray powder diffraction. A continuous flow cryostat has been used to record the diffractograms and the integrated intensities of the Bragg peaks at different temperatures have been obtained. The integrated intensities of the odd and even reflections have been analysed following the structure of the CsCl compound and the Debye-Waller factors of the Cs+ and Cl ions have been estimated. The results have been verified by structure factor least squares refinement. Theoretical shell model lattice dynamical calculations have been done using a 7-parameter model in the harmonic approximation and the values compared with the present X-ray measurements.

    • Temperature variation of the Debye-Waller factors of metal and halide ions in CsCl and CsBr powders by X-ray diffraction. II. Debye-Waller factors of Cs+ and Br ions in CsBr from room temperature to 78.2°K

      V Ganesan K S Girirajan

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      The temperature variation of the Debye-Waller factors of Cs+ and Br ions in CsBr powder has been studied using powder X-ray diffraction. The integrated intensities of the Bragg peaks at different temperatures have been obtained. These results have been verified by structure factor least squares refinement program. Theoretical shell model lattice dynamical calculations have been done using a 7-parameter model in the harmonic approximation and the values compared with the present X-ray measurements. The observed intensities have been corrected for first order thermal diffuse scattering.

    • Temperature variation of the Debye-Waller factors of metal and halide ions in CsCl-Br solid solutions between room temperature and 90°K by powder X-ray diffraction

      V Ganesan K S Girirajan

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      Solid solutions of CsCl-Br in different molal concentrations were prepared and X-ray diffractograms taken. The integrated intensities of the Bragg peaks have been estimated and structure factors were obtained. The crystal lattice in each of these solid solution samples is made up of a random distribution of Cs+, Cl and Br ions depending upon the molal concentration. A pseudo-atom model has been proposed in which the Cs+ ions occupy (000) position and a pseudo-atom occupies (1/2 1/2 1/2) position. The presence of incoherence scattering in these solid solutions has been considered by applying the necessary correction to the atomic scattering factors. The integrated intensities have been analysed and the temperature variation of the Debye-Waller factors of the metal (Cs+) ion and the pseudo ion (CB) have been estimated.

    • Critical current density of a sample of melt grown Y1.2Ba1.8Cu2.4Ox

      V Ganesan R Srinivasan R Gopalan T Rajasekaran

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      Melt grown samples of Y1.2Ba1.8Cu2.4Ox have been prepared and studied for their current carrying capacity. The composition was chosen to include Y2BaCuO5 (211) particles in the YBa2Cu3Ox (123) phase. The critical current density (Jc) of these samples was studied as a function of magnetic field using magnetization technique. The micrographic investigation shows well aligned grains in this material. The magnetic hysteresis measurements were done using a MPMS SQUID magnetometer up to the fields of 5.5 T. TheJc was estimated from the remanent magnetization using Bean model. Isothermal magnetization hysteresis loops at low fields reveal the presence of only one kind of hysteresis loops (corresponding to intragrain magnetizations). This is a valid proof that the weak links are greatly eliminated in these samples prepared by MG process. TheJc behaviour as a function of magnetic field has two components, a rapidly decaying exponential function of field and the other component that predominates at higher fields. This could be explained if we assume that the sample contains two phases of superconductors, one having a lowHc2 becoming normal at fairly medium fields of the order of a few kilogauss will act as pinning centres for the other phase having higherHc2 and hence higherJc at high fields.

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