Evidence for phase stability and weak links in polycrystalline HgBa₂CuO_{4 + δ}

Polycrystalline HgBa₂CuO_{4 + δ} (hereafter denoted as Hg-1201) superconductors were prepared by solid state reaction of stoichiometric quantities 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 lattice parameters, a = 3.876(2) A ̊, and c = 9.515(3) A ̊, similar to reported values. These results were highly reproducible with several batches. Finely ground Hg-1201 powders were stable upon overnight exposure to ambient air; long-term exposure (~ 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 magnetization studies showed complete shielding. A Meissner signal ~ 30% of (- 1 4π), and clear evidence for weak coupling between the randomly oriented superconducting grains. In large magnetic fields, the hysteresis of the magnetization (proportional to the intragrain current density, assuming a Bean model) decreased rapidly with applied field and temperature. These features and direct studies of the time dependent magnetization give unambiguous evidence for substantial flux creep effects.