Mechanisms of interaction of YBa$_{2}$Cu$_{3}$O$_{y}$ with water vapor at low-temperature annealing

The interaction of YBa$_{2}$Cu$_{3}$O$_{y}$ (123) with water vapor at temperatures $T\le$ 150$^\circ$ has been studied. It has been shown that, with an increase in temperature, the mechanism of its interaction with water changes. Near room temperature, the main process is hydrolytic decomposition. At $T\sim$ 100$^\circ$C, the absorption of water is significantly reduced, because the role of hydrolysis becomes less important and water penetrates the structure weakly and is incorporated into oxygen vacancies mainly in the form of OH–-groups, which leads to the transition of YBa$_{2}$Cu$_{3}$O$_{y}$ from the tetragonal to orthorhombic phase. With an increase in temperature to 150$^\circ$C, the absorption of water increases again. In this case, the main mechanism is the penetration of water to the 123 structure, which leads to splitting of Cu–O chains and a phase transition from the 123 to pseudo-124 structure. The role of different mechanisms of interaction with water essentially depends on the oxygen content in the 123 structure. At a low oxygen index ($y$ = 6.3), the role of hydrolysis is more important, and, at $y\ge$ 6.5, the incorporation of water into the structure prevails. It has been revealed that, at $T$ = 150$^\circ$C, after absorption of water, YBa$_{2}$Cu$_{3}$O$_{6.96}$ becomes a proton conductor.