Phase transformations in YBa$_2$Cu$_3$O$_y$ ($y\le$ 6.5) under low-temperature treatment

The influence of low-temperature annealing conditions on the properties and structure of YBa$_2$Cu$_3$O$_y$ (123) with $y\le$ 6.5 has been investigated. It has been shown that, at $T$ = 200$^\circ$C, the transition to the orthorhombic phase and the separation into phases with different oxygen concentrations occur slowly. With an increase in the temperature, the rate of phase transformations increases significantly. The phase transition of tetragonal YBa$_2$Cu$_3$O$_{6.3}$ to the orthorhombic phase due to the oxygen ordering leads to the appearance of superconductivity. There is a direct evidence that, at temperatures $T\le$ 300$^\circ$C in the absence of water vapor, the 123 compound is separated into phases with different oxygen concentrations. The phase separation is accompanied by the formation of an oxygen-enriched phase with a critical temperature $T_c\approx$ 90 K (at $T$ = 200$^\circ$C) or $T_c\approx$ 76 K (at $T$ = 300$^\circ$C). The presence of water vapor in the low-temperature annealing atmosphere significantly accelerates all the processes occurring in the system; however, the incorporation of water into the structure of the 123 compound leads to its transition into the 124-type phase.