THERMAL-TREATMENT OF YBA2CU3-XALXO6-CRYSTALS IN DIFFERENT ATMOSPHERESAND NEUTRON-DIFFRACTION STUDY OF EXCESS OXYGEN PINNED BY THE AL SUBSTITUENTS(DELTA SINGLE)

YBa2Cu3-xAlxO6+delta single crystals with different Al contents (x = 0 .06, 0.14 and 0.19) were heat treated in different atmospheres, The ox idized as-grown crystals show a macroscopically tetragonal tweed-struc ture. The oxygen content delta in the Cu(1) layer of these oxidized sa mples is significantly lower than 1, which results from the Al coordin ation disturbing the ordering in the chain-oxygen sublattice. Neutron- diffraction data show that upon reduction at 1044 K the oxygen content delta within the Cu(1) layer remains at 0.25 due to pinning of excess oxygen by the aluminum at x = 0.19. Crystals annealed in reducing atm ospheres at temperatures above 973 K develop orthorhombic long-range o rder after reoxidation at 673 K, probably due to short-range clusterin g of the AlOn defects. The obtained orthorhombicity of the reoxidized material decreases with decreasing temperature of the reduction treatm ent, No change in the structural state is obtained when the reducing t reatment is performed below 950 K; the reoxidized samples remain macro scopically tetragonal. Annealing the samples at 1073 K in oxygen resto res the tweed structure. An evolution of the tweed structure with anne aling time towards orthorhombic long-range order was not observed.