  We report the synthesis of SrMnO$_{3-{\delta}}$F$_{{\gamma}}$ perovskite
oxyfluoride thin films using a vapor transport method to fluorinate as-grown
SrMnO$_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which
acts as a fluorine vapor source, was investigated by utilizing polyvinyl
fluoride (PVF), polyvinylidene difluoride (PVDF) and polytetrafluoroethylene
(PTFE) in the reaction. The same process was carried out with polyethylene (PE)
to isolate the role of carbon in the vapor transport process. The F
distribution was probed by X-ray photoemission spectroscopy, which confirmed
the incorporation of F into the films and revealed higher F concentrations in
films exposed to PVF and PVDF compared to PTFE. The c-axis parameter expands
after fluorination, a result consistent with density functional theory
calculations that attribute the volume expansion to elongated Mn-F bonds
compared to shorter Mn-O bonds. Using X-ray absorption spectroscopy, we show
that the fluorination process reduces the nominal Mn oxidation state suggesting
that F substitutes on O sites in the lattice as opposed to filling anion
vacancy sites, a finding further supported by calculated formation energies of
different F site occupancies. These results provide new insights into
topochemical fluorination of perovskite oxides, which should enable future
synthesis and design efforts focused on oxyfluoride heterostructures.
