SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE 127-K HGBA2CACU2O6.22 SUPERCONDUCTOR - T-C VARIATIONS UPON DIFFERENT ATMOSPHERE ANNEALINGS

The synthesis of the HgBa2CaCu2O6+delta(Hg-1212) superconductor under various experimental conditions in sealed silica tubes is reported. Optimization of synthesis conditions allowed the preparation of bulk superconducting Hg-1212 with T-c onset at 127 K and lattice parameters a = 3.858 Angstrom, c = 12.68 Angstrom. The neutron powder diffraction analysis yielded delta = 0.22 (4). As a result of different atmosphere annealings a relationship between the decrease of T-c and the increase of the lattice parameter a is deduced. A 300 degrees C/10 h treatment in 99% N-2/1% O-2 reduces T-c to 123 K and increases the alpha parameter to 3.862 Angstrom. An additional 10 h reduction in a 100% N-2 flow lowers T-c to 98 K while alpha increases to 3.865 Angstrom. Subsequent oxidation at 300 degrees C for 10 h in O-2 flow restores the lattice parameters and T-c onset to their original values. The T-c dependence on annealing conditions is shown to be reversible.