Thermal stability of YBa$_2$Cu$_3$O$_{7-\delta}$, films deposited on Ni–W textured substrates by laser ablation

The degradation processes of YBa$_2$Cu$_3$O$_{6.77(3)}$ films 600 nm thick deposited on substrates from a textured (100) foil of the Ni–W alloy with the YSZ (Y$_2$O$_3$+ZrO$_2$) (100 nm) and CeO$_2$ (50 nm) buffer layers at 200$^\circ$C in air have been investigated. It has been shown that the degradation proceeds in the same manner as in the bulk ceramic materials with the same oxygen content, namely, initially the single-phase material with the orthorhombic structure laminates into the fractions depleted (the main fraction) and enriched with oxygen. After holding for more than 20 h, the oxygen-depleted fraction is first subjected to the structural phase transformation with the transition of the orthorhombic structure into the cubic one, which is associated with the redistribution of barium and yttrium atoms over their sites in the presence of water vapors.