M. P. Janawadkar

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.

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.

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.

Structure property correlations in superconducting Ti-Nb alloys

Titanium-rich transition metal alloys are metastable in their quenched boc β phase. The instability is relieved by low temperature structural transformations. We have investigated this in a series of Ti-Nb alloys, through the measurements of electrical resistivity (ρ), superconducting transition temperature and upper critical field. Supporting structural evidence has been obtained from transmission electron microscopy (tem) and x-ray studies. It is shown that both ρ and dρ/dT can be used as useful indices of this instability. The enhanced value of resistivity on account of the instability results in the enhancement of upper critical field as shown from dHc2/dT measurements.

Superconductivity and structural instability in EuMo6S8 at high pressures (0–70 kbar)

We report the detailed temperature dependence of the electrical resistivity of EuMo6S8 as a function of pressure up to 70 kbar under quasihydrostatic pressure conditions. The effect of an external magnetic field on the electrical resistivity has been studied at selected temperatures and pressures. Our results, which are qualitatively different from those reported under hydrostatic pressure conditions, can be explained on the basis of a proposed coexistence of the untransformed rhombohedral phase and the transformed triclinic phase of EuMo6S8.

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.

Positron studies on Y1Ba2Cu3O7−x : Charged oxygen vacancies?

Abstract Positron lifetime, Doppler broadened lineshape parameter, I, and the oxygen deficiency, x, have been measured as a function of quench temperature in YBCO. The lifetime characteristic of the oxygen vacancies is seen to be different in the tetragonal and orhorhombic phases and this has been explained in terms of the difference in the charge state of the oxygen vacancies in the two phases.

Superconducting Behavior of Cu1.8Mo6S8 at High Pressures

Low temperature electrical resistance measurements on the Chevrel phase compound (CPS) Cu1.8Mo6S8 upto a quasihydrostatic pressure of 60 kbar are reported. These measurements show a negative dTc/dP, increase in the width ΔTC to several Kelvin, and suppression of the structural phase transition seen at ambient pressures.

Electrical and thermal conductivity of soft solder at low temperatures

The electrical resistivity of soft solder (Pb0.28Sn0.72) has been measured in the temperature range 4.2 K to 300 K. The ‘alloy’ becomes electrically superconducting at a temperature of 6.9 K. Above this, in the entire temperature range, the resistivity could be described, apart from the residual resistivity, by the weighted average of the resistivities of the individual constituents which are derived from the Bloch-Grüneisen relation. The results are in accordance with the phase diagram, which shows a co-existence of two phases in almost the entire range of concentration of the Pb-Sn binary system. It has been shown that the thermal conductivity data on soft solder as well as on Pb0.7Sn0.3, both taken from literature, could be interpreted on the same basis, below and above the ‘superconducting transition temperature’. Recent results on other Pb-Sn systems are discussed in the light of this interpretation.