  Anionic ordering is a promising route to engineer physical properties in
functional heteroanionic materials. A central challenge in the study of
anion-ordered compounds lies in developing robust synthetic strategies to
control anion occupation and in understanding the resultant implications for
electronic structure. Here, we show that epitaxial strain induces preferential
occupation of F and O on the anion sites in perovskite oxyfluoride SrMnO2.5-dFg
films grown on different substrates. Under compressive strain, F tends to take
the apical-like sites, which was revealed by F and O K-edge linearly polarized
x-ray absorption spectroscopy and density functional theory calculations,
resulting in an enhanced c-axis expansion. Under tensile strain, F tends to
take the equatorial-like sites, enabling the longer Mn-F bonds to lie within
the plane. The anion ordered oxyfluoride films exhibit a significant orbital
polarization of the 3d electrons, distinct F-site dependence to their valence
band density of states, and an enhanced resistivity when F occupies the
apical-like anion site compared to the equatorial-like site. By demonstrating a
general strategy for inducing anion-site order in oxyfluoride perovskites, this
work lays the foundation for future materials design and synthesis efforts that
leverage this greater degree of atomic control to realize new polar or
quasi-two-dimensional materials.
