  It is known that solid-state reaction in high-pressure oxygen can stabilize
high-oxidation phases of Y-Ba-Cu-O superconductors in powder form. We extend
this superoxygenation concept of synthesis to thin films which, due to their
large surface-to-volume ratio, are more reactive thermodynamically. Epitaxial
thin films of $\rm{YBa_2Cu_3O_{7-\delta}}$ grown by pulsed laser deposition are
annealed at up to 700 atm O$_2$ and 900$^\circ$C, in conjunction with Cu
enrichment by solid-state diffusion. The films show clear formation of
$\rm{Y_2Ba_4Cu_7O_{15-\delta}}$ and $\rm{Y_2Ba_4Cu_8O_{16}}$ as well as regions
of $\rm{YBa_2Cu_5O_{9-\delta}}$ and YBa$_2$Cu$_6$O$_{10-\delta}$ phases,
according to scanning transmission electron microscopy, x-ray diffraction and
x-ray absorption spectroscopy. Similarly annealed $\rm{YBa_2Cu_3O_{7-\delta}}$
powders show no phase conversion. Our results demonstrate a novel route of
synthesis towards discovering more complex phases of cuprates and other
superconducting oxides.
