Effects of substrate temperature on the microstructure of YBa₂Cu₃O_7−δ films grown on (001) Y‐ZrO₂ substrates

High‐quality YBa₂Cu₃O_7−δ films grown on (001) single‐crystal Y‐ZrO₂ substrates by pulsed laser deposition have been studied as a function of substrate temperature using transmission electron microscopy. A transition from epitaxial films to c‐axis oriented polycrystalline films was observed at 740 °C. An intermediate, polycrystalline, BaZrO₃ layer was formed from a reaction between the film and the substrate. A dominant orientation relationship of [001]_YBCO//[001]_{int. layer}//[001]_YSZ and [110]_YBCO//[110]_{int. layer}//[100]_YSZ was observed. The formation of grain boundaries in the films resulted in an increased microwave surface resistance and a decreased critical‐current density. The superconducting transition temperature remained fairly constant at about 90 K.