Phase Conversion of Y-Ba-Cu-O Thin Films by Super-Oxygenation and Cu-Enrichment

The superconducting critical temperature (T c ) of hole-doped cuprates tends to increase with their lattice complexity, which is generally correlated with higher states of oxidation. For YBa2Cu3O7−δ (YBCO-123), it is known that solid-state reaction in ultrahigh-pressure oxygen can induce the formation of more complex and oxidized phases such as Y2Ba4Cu7O15−δ (YBCO-247) and Y2Ba4Cu8O16 (YBCO-248). In this study, we apply this super-oxygenation concept of oxide materials synthesis to thin films which, owing to their large surface-to-volume ratio, are more thermodynamically reactive than bulk samples. Epitaxial thin films of YBCO-123 were grown by pulsed laser-ablated deposition on (LaAlO\(_{3})_{0.3}\)(Sr2TaAlO\(_{6})_{0.7}\) substrates, and then annealed in 500 atm of oxygen at 800 ∘C. The high-pressure annealing was done in conjunction with Cu-enrichment by solid-state diffusion, as an additional driving force for phase conversion. Resistivity was measured to determine the T c and to assess the amount of disorder in the films. Transmission electron microscopy and x-ray absorption spectroscopy were used to probe the local lattice structure and oxygen stoichiometry. Data taken on the super-oxygenated films show clear formation of YBCO-247 and YBCO-248, as well as distinct intergrowths of YBa2Cu5O9, a novel phase of YBCO that has three CuO chains per unit cell.