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

The superconducting critical temperature (Tc) 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 Tc 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.