  Certain pyrochlore oxides are among the best oxygen catalysts in alkaline
media. Hence, exploring epitaxial films of these materials is of great
fundamental and technological interest. Unfortunately, direct film growth of
one of the most promising pyrochlores, Bi$_2$Pt$_2$O$_7$, has not yet been
achieved, owing to the difficulty of oxidizing platinum metal in the precursor
material to Pt$^{4+}$. In this work, in order to induce oxidation of the
platinum, we annealed pulsed laser deposited films consisting of epitaxial
$\delta$-Bi$_2$O$_3$ and co-deposited, comparatively disordered platinum. We
present synchrotron x-ray diffraction results that show the annealed films are
the first epitaxial crystals of Bi$_2$Pt$_2$O$_7$. We also visualized the
pyrochlore structure by scanning transmission electron microscopy, and observed
ordered cation vacancies in a bismuth-rich film but not in a platinum-rich
film. The similarity between the $\delta$-Bi$_2$O$_3$ and Bi$_2$Pt$_2$O$_7$
structures appears to facilitate the pyrochlore formation. These results
constitute a new approach for synthesis of novel pyrochlore thin film oxygen
catalysts.
