Reaction of the oxygen-deficient YBa₂Cu₃O₆ phase with water

The oxygen deficient high-T_c material, YBa₂Cu₃O₆, decomposes rapidly when in the presence of water solution or water vapor. Accordingly, upon reaction with water, it decomposes into Ba(OH)₂, Cu₂O, CuO, a metastable "Y₂BaCuO₅" phase and a series of unidentified amorphous compounds. Although the majority of the decomposition products are insoluble in water, Ba(OH)₂ leaches away into solution where it subsequently reacts with atmospheric CO₂ to form sparingly soluble BaCO₃ crystals. Interestingly, the "Y₂BaCuO₅" phase forms only in the initial stages of corrosion and disappears after long water exposure times. On the other hand, authentic Y₂BaCuO₅ samples are very stable in the presence of water. Thus, the chemical reactivity of the "Y₂BaCuO₅" phase generated via decomposition of the high-T_c material is different from that of the authentic phase. In addition, comparisons of the relative reactivity of YBa₂Cu₃O₇ and YBa₂Cu₃O₆ samples reveal that the oxygen deficient material is more reactive than the fully oxygenated compound. This unusual reactivity trend suggests that factors other than copper formal valence dominate the chemical reactivity of the high-T_c phase.