EVIDENCE FOR PHASE-STABILITY AND WEAK LINKS IN POLYCRYSTALLINE HGBA2CUO4+DELTA

Polycrystalline HgBa2CuO4 + delta (hereafter denoted as Hg-1201) super conductors were prepared by solid state reaction of stoichiometric qua ntities of HgO and Ba2CuO3 precursors in an evacuated quartz tube. The ''as-synthesized'' Hg-1201 samples had a T(c) of 95 K, as defined by the onset of diamagnetism. The crystal symmetry was tetragonal with la ttice parameters, a = 3.876(2) angstrom, and c = 9.515(3) angstrom, si milar to reported values. These results were highly reproducible with several batches. Finely ground Hg-1201 powders were stable upon overni ght exposure to ambient air; long-term exposure (approximately 10 days ) of the powders led to surface deterioration and minor changes in the X-ray pattern, with no observed change in the bulk T(c). Low field ma gnetization studies showed complete shielding. A Meissner signal appro ximately 30% of (-1/4 pi), and clear evidence for weak coupling betwee n the randomly oriented superconducting grains. In large magnetic fiel ds, the hysteresis of the magnetization (proportional to the intragrai n current density, assuming a Bean model) decreased rapidly with appli ed field and temperature. These features and direct studies of the tim e dependent magnetization give unambiguous evidence for substantial fl ux creep effects.