HIGH-PRESSURE SYNTHESIS AND HEAT-TREATMENTS OF THE HGBA2CA4CU5O12+DELTA AND HGBA2CA5CU6O14+DELTA PHASES

The syntheses of the n = 5 and n = 6 members of the HgBa2Can-1CunO2n+2 +delta family are reported. Bulk samples containing either Hg-1245 or Hg-1256 as major phases were synthesized using the high-pressure high- temperature technique at 920 degrees C and 18 kbar. The as-synthesized phases (a = 3.8547(3) Angstrom, c = 22.156(5) Angstrom for n = 5 and a = 3.8516(1) Angstrom, c = 25.232(6) Angstrom for n = 6) exhibited T- c with onsets of about 100 K. They were characterized by X-ray powder diffraction, transmission electron microscopy and AC susceptibility me asurements. Heat treatments increased the T-c onset to 110 K for n = 5 and to 107 K for n = 6. These phases with high n are more inert to th ermal annealings under different atmospheres than the members with sma ller n. Only strong oxidizing {P(O-2) greater than or equal to 500 bar } or reducing {Ar/400 degrees C} conditions can induce significant T-c variations.