VARIATION OF THE SUPERCONDUCTING AND CRYSTALLOGRAPHIC PROPERTIES AND THEIR RELATION TO OXYGEN STOICHIOMETRY OF HIGHLY HOMOGENEOUS YBA2CU3OX

A great number of YBa2Cu3Ox (if6.2 ≤ x ≤ 7.0) samples were investigated by a.c. susceptibility and field cooling magnetization measurements, X-ray and microprobe analyses, microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content x was determined by the reduction of the samples in a hydrogen flow. To reach the (x-T-p) equilibrium, well-oxidized YBCO samples were annealed in pure oxygen flow (10-1 atm ≤ P(O2) ≤ 1 atm) at temperatures between 200 and 1000 °C and subsequently quenched into liquid gallium. Other samples were prepared under 80 atm O2 (200 °C ≤ T≤ 1000 °C). We were able to produce macroscopically very homogeneous oxygen deficient material with sharp χa.c. transitions in the entire range of critical temperatures. The evolution of the Meissner fraction is non-monotonic with a relative maximum near x = 6.6, a concentration where the orthorhombicity 2 (b - a) (a + b) also deviates from monotonic behaviour. © 1990.