Origin of the ϕ ∼ ±9° peaks in YBa₂Cu₃O_7−δ films grown on cubic zirconia substrates

The c-axis oriented YBa₂Cu₃O_7−δ films grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates often contain domains whose in-plane alignment is rotated approximately 9° from the cube-on-cube epitaxial relationship, in addition to the more commonly observed 0° and 45° in-plane rotations. We have investigated the origin of this ∼9° orientation using in situ electron diffraction during growth and ex situ 4-circle x-ray diffraction. Our results indicate that the ∼9° orientation provides the most favorable lattice match between the interfacial (110)-oriented BaZrO₃ epitaxial reaction layer, which forms between YBa₂Cu₃O_7−δ and the YSZ substrate. If epitaxy occurs directly between YBa₂Cu₃O_7−δ and the YSZ substrate, i.e., before the BaZrO₃ epitaxial reaction layer is formed, the 0° and 45° domains have the most favorable lattice match. However, growth conditions that favor the formation of the BaZrO₃ reaction layer prior to the nucleation of YBa₂Cu₃O_7−δ lead to an increase in ∼9° domains. The observed phenomenon, which results from epitaxial alignment between the diagonal of a square surface net and the diagonal of a rectangular surface net, is a general method for producing in-plane misorientations, and has also been observed for the heteroepitaxial growth of other materials, including (Ba, K)BiO₃/LaAlO₃. The YBa₂Cu₃O_7−δ/YSZ case involves epitaxial alignment between [111]_{BaZrO₃} and [110]_YSZ, resulting in an expected in-plane rotation of 11.3° to 9.7° for fully commensurate and for fully relaxed (110)BaZrO₃ on (001)YSZ, respectively.