ENHANCEMENT OF T-C OF CYCU1-YBA2CA2CU3OX FROM 67 K TO 120 K BY REDUCTION TREATMENTS

The T-c of the CyCu1-yBa2Ca2Cu3O8+delta (y approximate to 0.5) phase, synthesized under high pressure, has been raised from 67 K to about 12 0 K by heat-treatments under reducing conditions. An increase of both a and c lattice parameters occurs concomitantly with that of T-c (befo re: a = 3.858 (1), c = 14.766 (9) Angstrom, after: a approximate to 3. 862(1), c approximate to 14.825 (3) Angstrom). Subsequent heat-treatme nts under oxygen atmosphere lead, reversibly, to a decrease of T-c and of the lattice parameters. Consequently, the increase of T-c has been associated with an oxygen loss. Measurements of the resistivity under high pressure indicate that after reduction the overdoped as-prepared samples go towards the optimal doping. A structural model is proposed about the position of the mobile oxygen in the basal plane of the str ucture.