Y. Hariharan

Thermal decomposition of C60

Abstract The influence of heat-treatment of C60 pellets in vacuum at temperatures between 500°C and 900°C has been investigated by a variety of experimental techniques. X-ray measurements show that the diffraction peaks become diffuse at temperatures beyond 700°C. Uv-visible absorption measurements show a sharp diminution in the C60 content of the pellets heat-treated at temperatures beyond 700°C. Resistivity measurements show that the pellets heat-treated at 700 to 900°C are conducting with a negative temperature coefficient of resistivity. From these studies it is inferred that C60 decomposes into amorphous carbon on heat-treatment at temperatures beyond 700°C. Plausible mechanisms for the thermal decomposition of C60 are discussed.

Carbon solubility and superconductivity in MgB2

Abstract Successful replacement of B by C in the series MgB 2− x C x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x =0.30 into the MgB 2 lattice. The superconducting transition temperature, T c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x =0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ D , extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ , obtained from the McMillan equation using the measured T c and θ D , is seen to increase monotonically from 0.8 in MgB 2 to 0.9 in the x =0.50 sample. The ratio of the resistivities between 300 and 40 K versus T c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T c is argued to mainly arise due to large decrease in θ D with C concentration and a decrease in the hole density of states at N ( E F ).

A study of positron distribution and annihilation characteristics in YBa2Cu3O7-x

The positron distribution and lifetimes have been computed for the model structures of orthorhombic YBa2Cu3O7, tetragonal YBa2Cu3O6.5 and tetragonal YBa2Cu3O6, which consist of different ordered arrangements of oxygen vacancies. The positron potential was obtained as a sum of the full lattice electrostatic potential and the correlation potential in the local density approximation. The positron wavefunction and the eigenvalue of the lowest-lying itinerant positron state was obtained by solving the Schrodinger equation in the finite-difference scheme. The positron wavefunction displays maxima at the oxygen vacancy positions and it is seen that the positron probes predominantly the oxygen vacancies in the basal plane. By computing the overlap of the positron density with the core and valence electron densities, the positron lifetimes for YBa2Cu3O7, YBa2Cu3O6.5 and YBa2Cu3O6 are estimated to be 190 ps, 222 ps and 234 ps, respectively. Calculation of the positron wavefunction and lifetime has also been carried out for a positron localised at an isolated oxygen vacancy along the chain in orthorhombic YBa2Cu3O7. The calculated values of lifetimes for different arrangements of vacancies have been compared with the experimental values of positron lifetimes in YBa2Cu3O7-x measured after various heat treatments.

Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study

We have studied single crystals of C70 by Raman scattering and photoluminescence in the pressure range from 0 to 3 1.1 GPa. The Raman spectrum at 3 1.1 GPa shows only a broadband similar to that of the amorphous carbon without any trace of the Raman lines of CT0. After releasing the pressure from 3 1.1 GPa, the Raman and the photoluminescence spectra of the recovered sample are that of the starting C70 crystal. These results indicate that the C70 molecules are stable up to 3 1.1 GPa and the amorphous carbon high pressure phase is reversible, in sharp contrast to the results on solid Cm. A qualitative explanation is suggested in terms of intermolecular versus intramolecular interactions.

Experimental study of the decomposition of Y1Ba2Cu3O7−x into tetragonaland orthorhombic phases

Abstract Infrared absorption spectroscopy and X-ray diffraction measurements have been used to show that a single phase oxygen deficient Y 1 Ba 2 Cu 3 O 7− x (YBCO) when annealed at low temperatures (200°C) is unstable towards decomposition into a mixture 0824 1062 of orthorhombic and tetragonal phases. This is in qualitative agreement with the calculated phase diagram of YBCO. The degradation in the superconducting properties (Meissner signal reduced by a factor of 2) that accompanies this annealing treatment is understood as being a direct consequence of the decomposition.

Pressure induced dimerisation of C70

Solid C70 has been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 750°C. X-ray diffraction measurements on the recovered samples indicate that the initial h.c.p. solid C70 transforms to a rhombohedral structure which recovers to an f.c.c. structure on annealing. The associated changes in the intra molecular vibrational modes have been probed through infrared (IR) and Raman measurements. The IR measurements on these HPHT samples show splitting of some of the pristine modes and occurrence of several new modes. These sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been attributed to the formation of C70 dimers.

Synthesis of high quality 1-2-3 compound through citrate combustion

Abstract The procedure for the preparation of improved quality YBa 2 Cu 3 O 7−x by the citrate incineration method and optimisation of parameters for oxygen annealing are discussed. The characterization of the product of citrate incineration and the experimental control tests employed for assessing the quality of the 1-2-3 compound so produced are also discussed.

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.

Positron annihilation study of oxygen vacancies in Y1Ba2Cu3O7−x

Positron lifetime and Doppler broadened lineshape parameter have been measured as a function of quench temperature in Y1Ba2Cu3O7-x. The mean lifetime, , and the peak parameter of the Doppler broadened lineshape, I, are observed to decrease as the quench temperature (TQ) is lowered from 925 °C to 325 °C. The variations in and I with TQ are observed to correlate with the oxygen deficiency (x) measured at a various quench temperatures. The positron lifetime component, τ2, attributed to trapping at oxygen vacancies, is observed to decrease from (200 ± 1) ps in the tetragonal phase (x = 0.5) to (190 ± 1) ps in the orthorhombic phase (x = 0.0). The variation in positron annihilation parameters with TQ is qualitatively explained in terms of a change in the charge state of oxygen vacancies.

Low temperature x-ray diffraction study of the phases of C70

Results of X-ray diffraction study at 300K, 77K and 10K are presented for C 70 powder prepared by both solution route and by crushing a collection of single crystals. For the sample prepared by solution route the room temperature hep phase is quenched and retained up to 10K. Small changes however are observed in the c/a ratio. For the sample prepared via single crystal route the intensities of the Bragg peaks are several orders of magnitude larger and the line widths considerably reduced enabling more accurate phase analysis to be carried out. At room temperature the sample is predominantly of rhombohedral phase with an admixture of fcc phase which is less than 6%. At 10K the structure is predominantly monoclinic with the fcc fraction becoming vanishingly small. Upon warming back, the structure reverts back to rhombohedral + fcc (∼ 6%) at 300K. The results observed suggest that the equilibrium structure at room temperature could be either rhombohedral or fcc depending on the microstructural defect state of the system and / or impurities in the system and also that the equilibrium structure at low temperature is monoclinic. The results also imply for the first time that there is a path in which the fcc phase transforms directly to monoclinic phase on cooling without the intermediary rhombohedral phase implying a simultaneous freezing of all orientations.