Similar to well-studied transition metal oxide systems such as SrCoO3−δ29, bulk SFO exhibits two topotactic phases: the cubic perovskite SrFeO3 and the brownmillerite SrFeO2.53031. Perovskite SrFeO3 has a bulk lattice parameter of 3.851 Å. The oxygen-deficient brownmillerite SrFeO2.5 structure has unit cell parameters of a = 5.672 Å, b = 15.59 Å, and c = 5.527 Å. This structure can be reduced to pseudotetragonal with unit cell parameters of = 4.011 Å, = 3.898 Å, and = 3.908 Å3132. Figure 1 shows the X-ray diffraction (XRD) patterns of the films treated with different conditions. The out-of-plane lattice parameters of SFO thin films are shown to increase with decreasing oxygen pressure during anneal, as evidenced by the progressively smaller 2θ values of the SFO film peak when comparing O2 anneal, no anneal and vacuum anneal samples, respectively. A broad XRD 2θ scan indicates that films are c-axis oriented with no detectable mixed phases present (not shown here). The out-of-plane SFO film lattice parameter is ~3.83 Å for the O2 annealed film, ~3.85 Å for the film with no anneal, and ~3.97 Å for the vacuum annealed film. Reciprocal space mapping (RSM) scans around the (103) peak of the SFO films are compared in Fig. 1b–d. In all three films, it can be seen that the in-plane lattice parameter of SFO remains strained to the Nb:STO substrate, indicating a = b ~3.905 Å. The out-of-plane lattice parameters progressively increase from 3.83 Å to 3.97 Å with reducing oxygen content in annealing. This corresponds to a relative unit cell volume increase of approximately 3.6% when comparing anneal in vacuum vs. oxygen. Oxygen vacancies donate electrons to the empty 3d-orbitals of Fe, reducing the ion from Fe4+ to Fe3+ and increasing the ionic radius of the Fe ion14. Hence, the lattice parameter is directly related with the oxygen content. These results agree with SFO materials reported by Yamada et al.32.