T.S. Radhakrishnan

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 ).

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.

Possible magnetic phase separation in Ru-doped La0.67Ca0.33MnO3

Abstract X-ray diffraction, resistivity, AC susceptibility and magnetization studies on La0.67Ca0.33Mn1−xRuxO3 (0⩽x

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.

Transport and thermal properties of La0.67Ca0.33Mn1−xMxO3 (M=Fe, Zr and Hf)

Mn site substitution with Fe, Zr and Hf (1 at.%) were carried out in colossal magneto-resistive material La 0.67 Ca 0 33 MnO 3 . It is found from the resistivity, ac susceptibility and specific heat (C p ) measurements that Zr and Hf substitutions suppress the transition temperature (T c ) more effectively than Fe substitution. The excess entropy under the C p curve associated with the magnetic transition also substantially lowered compared to that of undoped and Fe-doped systems. From the consideration of the excess entropy under the C p curve and the ionic radii, it is speculated that both Zr 4+ and Hf 4+ ions predominantly substitute Mn 3+ site. Larger suppression of T c is attributed to local strain produced due to larger ion substitution as well as hole filling due to replacement of trivalent ion (Mn 3+ ) with tetravalent ions (Hf 4+ and Zr 4+ ).

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

Oxygen ordering and superconductivity in YBa2Cu3O7-δ

Samples of YBa2Cu3O7-δ in which concentration of oxygen was varied by annealing at different temperatures between 200 and 900C followed by quenching to 77 K have been investigated by carrying out measurements of oxygen stoichiometry, room temperature resistivity, superconductivity and crystal structure. It is shown that the overall oxygen stoichiometry alone does not adequately characterize the superconducting and normal state behaviour;Tc, ΔTcand room temperature resistivity also vary with the heat treatment conditions. This implies a dependence of the physical properties on the details of the distribution of the oxygen atoms. The results show a definite correlation betweenTcand resistivity hitherto not reported.