J. Janaki

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.

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.

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.

Electromigration of oxygen in YBa2Cu3O7-δ

Electromigration (EM) in metals is used to produce directed particle fluxes within the material and can be used to change the composition of a given material or to purify it. EM experiments were performed on sintered YBa2Cu3O7- delta (YBCO) rods at 200 degrees C and 350 degrees C and the results were subsequently analysed by magnetic susceptibility and X-ray diffraction. Explorative trials showed that oxygen does migrate under the influence of the field in this material and motion is directed towards the cathode. Another remarkable feature during electromigration was that towards the cathode. Another remarkable feature during electromigration was that a decomposition reaction occurred at the anode, resulting in the formation of YBCO (tetragonal), CuO, Y2O3 and BaCO3 phases. It appears that the effects of electromigration will have a bearing on the stability of this material during use.

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.

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

Thermogravimetric study of superconducting YBa2Cu3O9−x in hydrogen

Abstract The thermogravimetric curve of YBa 2 Cu 3 O 9−x in hydrogen exhibits five steps, the first two corresponding to the reduction of copper oxide to copper by about 683 K (as identified by XRD on quenched samples) followed by three more steps predominantly due to hydration-dehydration behaviour of BaO. The need for blank corrections and the choice of 1223 K as the temperature for the determination of oxygen stoichiometry are discussed.

Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

Abstract The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd 1.4− x Dy x Ce 0.6 Sr 2 RuCu 2 O 10 ( x =0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity ( T c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions ( T M2 ranging from 95 to 106 K and T M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c / a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.

Crystal structure of solid C70

Detailed analysis of the room temperature X-ray powder diffraction data of pure solid C70 is reported. C70 prepared by slow evaporation from toluene solution adopts an hcp structure (space groupP63/mmc) witha=10·53(1) A andc=17·24(1) A. C70 sublimed on to Si wafer adopts an fcc structure witha=14·89(1) A. The occurrence of both the hcp and fcc phases is rationalized in terms of cohesive energy calculations. Theoretical calculations of the diffraction pattern for the hcp structure, taking into account (a) orientational disorder amongst the molecules (b) presence of stacking faults and (c) a fraction of the sample to be amorphous/microcrystalline is seen to provide very good agreement with the experimental diffraction pattern.

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.