V.S. Sastry

Structure and vibrational properties of carbon tubules

The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

SPECIFIC HEAT STUDY OF C60 SINGLE CRYSTALS

Abstract We present the results of measurements of specific heat between 77K and 300K on a collection of C60 single crystals, using quasi-adiabatic heat pulse calorimetry. In the temperature warming cycle the well known orientational transformation occurs at261.5 K, with a peak value of 16 J/gm.-K, both the temperature and magnitude being the highest reported. The transformation occurs at 260.4 K in the cooling cycle, the hysteresis being noticeable due to the extreme sharpness of the peak. Considerable excess specific heat, observed over the contribution due to intramolecular vibrations, is shown to arise from the molecular reorientations between two configurations, corresponding to a pentagonal or hexagonal face of a molecule facing the interhexagonal double bonds of the adjacent molecule. A hysteresis in Cp is also observed below 240K between the warming and cooling cycles, the termination of which at 100 K is identified with the orientational glass transition. Further, heat evolution is observed at many temperatures below 180 K. These features may indicate a complex orientational dynamics of the C60 molecules

Structural transformation in HCP solid C70 at low temperatures

The structural transformation of C 70 powder prepared from ideal h.c.p. grown single crystals has been investigated using X-ray diffraction. Data taken from 300 K to 77 K in steps of 5 K over a span of 1000 h indicate a direct transformation to a monoclinic phase without going through any other intermediate phase. This implies that the freezing of rotations of the C 70 molecules about their major and minor axes can be simultaneous and not necessarily only sequential. The transition is seen to proceed gradually with both the phases coexisting in the range 270 K to 170 K. The coexistence is rationalised in terms of internal dilatory stresses arising due to the nucleation and growth of the monoclinic phase and the consequent progressive suppression of the transformation temperature. Our monoclinic cell parameters support a model in which the polar cap interactions between the molecules in the basal plane and those in the midplane are significant.

Direct evidence for the decomposition of YBa2Cu3O7−x into orthorhombic and tetragonal phases at 200°C

The variations of room temperature resistivity (ρ300), powder a.c. susceptibility (χ′) and x-ray diffraction (XRD) line profiles of (013), (103) and (110); (003), (010) and (100) reflections in the step scanning mode were investigated on single phase samples of Y1Ba2Cu3O7−x (YBCO), as a function of ageing at 200°C up to 860 h. The average off-stoichiometryx=0.22 remained constant throughout the experiment.ρ300 showed a monotonic increase while the superconducting transition curves showed increasing sharpening with ageing time. XRD of the reflections investigated developed satellite structures or additional intensities at 2θ values different from those of main reflections. The results are consistent with the prediction and earlier observation of the decomposition of the off-stoichiometric YBCO into oxygen ordered orthorhombic and vacancy ordered tetragonal phases.

INTERGRANULAR COUPLING FOR CURRENT TRANSPORT IN ‘NON-METALLIC’ SUPERCONDUCTING MATERIALS

The paper points out similarities in the features observed in the resistivity vs temperature behaviour of compacted powder samples of the two low carrier density superconducting systems viz. Chevrel phase compounds (CPC) and the high Tc oxides (HTSC). Quasi-hydrostatic pressures upto 10 GPa were employed on the samples for these studies. Specifically CuM06S8, AgM0S8, YBa2Cu307 and HoBa2Cu307 were studied. The common feature is the broadening of the resistive superconducting transition to a considerable extent in the CPC and the complete loss of transition in the HTSC. It is argued that weak intergranular coupling (IGC) is responsible for the observed behaviour in both these systems and may account for the poor transport current densities observed in bulk materials.