V. Sankara Sastry

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.

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

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.

Structure, transport and magnetism of La0.67Ca0.33Mn1 -xTaxO3

Abstract For the first time, the effect of Ta 5+ substitution at the Mn site of La 0.67 Ca 0.33 MnO 3 compound is reported. A significant increase in lattice parameters and unit cell expansion indicate larger structural modification upon substitution. The ferromagnetic-metallic ground state is modified to a cluster glass insulator for x > 0.03 . The observed suppression of the ferromagnetic transition temperature T c of ∼39xa0K/at% might be the highest reported in Mn-site substituted La 0.67 Ca 0.33 MnO 3 . The well-known effects of changed carrier density, larger average ionic radius at Mn site and dilution of Mn sublattice appear insufficient to explain this strong suppression of T C .

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.

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.

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.