Effect of structural water on the critical characteristics of highly textured YBa$_2$Cu$_3$O$_{6.9}$

The effect of low-temperature treatment (200$^\circ$C) in a humid argon atmosphere and subsequent annealing (930$^\circ$C) on the critical parameters of a highly textured YBa$_2$Cu$_3$O$_{6.9}$ has been studied. During annealing at $T$ = 200$^\circ$C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at $T$ = 930$^\circ$C and subsequent oxidation, the superconducting characteristics ($j_c$, $B_{1c}$, and $F_p$) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the $\mathbf{c}$ axis $(\perp\mathbf{c})$. The optimum conditions of double annealing substantially increasing the critical current density ($j_c\ge10^4$ A/cm$^2$) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to $\sim$3 T, the critical current density $j_c$ is isotropic despite the conservation of high texture in the samples.