Perovskite‐Derived Layered Crystal Structure in SrCo0.26Fe0.74O3‐δ Thin Films

Oxygen coordination and vacancy ordering play an important role in dictating the functionality of complex oxides. In this work, an unconventional layering of oxygen ions in a mixed conductor SrCo1‐xFexO3‐δ (SCFO) thin film grown epitaxially on SrTiO3 (STO) is reported. Scanning transmission electron microscopy (STEM) reveals alternating layers of oxygen deficiency along the growth direction, with the oxygen‐rich layer correlated with the neighboring Co,Fe‐site intensity, and contraction of the Sr–Sr distance. Density functional theory (DFT) calculations and STEM image simulations support the emergence of periodic (Co,Fe)O6 and (Co,Fe)O4/(Co,Fe)O5 layers, an ordering that is also sensitive to the Co:Fe ratio.

Here, we report a new oxygen-ordered perovskite-derived structure in epitaxial SCFO thin films grown by pulsed laser deposition (PLD) that differs from the structures described above.The alternating oxygen deficiency in B-site layers leads to alternate expansion and contraction of the lattice planes along the out-of-plane direction, observed using STEM imaging.We use DFT to obtain relaxed perovskite structures with different oxygen arrangements, comparing the results with STEM image simulations of possible structures.The oxygen layering observed in our SCFO thin films is accompanied by a change in B-site coordination from BO 6 to BO 4 /BO 5 as proposed in two possible crystal structures whose oxygen deficiency  is 0.5 or 0.625.

Results and Discussion
SCFO forms a solid solution with a P or BM structure, and can be reversibly transitioned between these structures by controlling the oxygen stoichiometry. [4]We previously investigated the growth of SCFO on STO substrates by PLD as a function of composition x and oxygen pressure (P O2 ). [39]For films grown at P O2 = 150 mTorr, we observed X-ray peaks characteristic of P when x ≥ 0.74 (i.e., Fe-rich) or BM when x < 0.74.However, electron microscopy revealed a "layered P" structure plus a small amount of BM at the grain boundaries when x = 0.74, and a majority of BM with a small coexistence of "layered P" and P when x = 0.43.Furthermore, the films included a small (few percent) fraction of CoO y pillars embedded in the SCFO matrix.The Sr:(Co+Fe) ratio in the film remained close to 1.0 despite the segregation of Co into the CoO y pillars, and the volume fraction of pillars diminished with increasing Fe content, suggesting that the formation of the "layered P" does not necessarily correlate with the formation of the CoO y pillar phase.In contrast, films grown at low P O2 (20 mTorr) did not exhibit a CoO y phase.Here, we focus on analyzing the "layered P" crystal structure in films synthesized at 150 mTorr. [39]he "layered P" of an x = 0.74, P O2 = 150 mTorr film viewed along the [110] axis of the pseudocubic unit cell is shown in Figure 1.Simultaneous annular dark field (ADF) and integrated differential phase contrast (iDPC) STEM images (Figure 1c) reveal layers parallel to the substrate.The ADF STEM image from the film shows an intensity variation of the transition metal (Bsite) but does not provide direct imaging of the oxygen atom columns.Instead, the corresponding iDPC image shows that the layers of dim and bright B-site atom columns correspond to oxygen-deficient and oxygen-rich layers, as shown in Figure 1c,d, implying that the oxygen stoichiometry change is predominantly present in the layers containing B-site and O, rather than in the layers of Sr and O.The alternating intensities of the B-site atom columns are also accompanied by expanded and contracted Sr-Sr distances of 455 ± 0.3 pm and 319 ± 0.3 pm along the out-of-plane direction, giving a distance ratio of 1.42 (Figure 1e).The periodic intensity of the B-site layers yields a difference of ≈20% between dim and bright atom columns (Figure 1f) that is too large to be explained even by complete ordering of Fe and Co between the atom columns.Instead, the large variation in B-site atom column intensity is attributed to the large static displacements induced by V O ordering, as in the case of Nb-doped Sr(Co,Fe)O 3- . [38]rom now on we identify this structure as layered perovskite (LP), which comprises the majority of the x = 0.74 film and has a uniform appearance throughout the film (Figure S1, Supporting Information).The average out-of-plane lattice parameter of the LP determined from STEM images was 3.87 Å, which is comparable to the out-of-plane lattice parameter obtained by X-ray diffraction (XRD) (Figure 2a), 3.81 ± 0.4 Å.A simple model shows that LP and P are difficult to distinguish by XRD (Figure S2, Supporting Information), explaining why the XRD by itself suggests that the film consists only of P. In this x = 0.74 sample, a small amount of BM was also observed near low-angle grain boundaries (Figure S3, Supporting Information); but is too little to yield a detectable peak in XRD.
Reciprocal space mapping (RSM) (Figure 2b) shows that the films are fully strained in-plane to match the STO substrate (a = 3.905 Å), consistent with the STEM images.The SCFO films have smaller lattice parameters than the STO substrate (bulk P-SCO and P-SFO lattice parameters are 3.836 [40] and 3.850 Å [24] respectively) and therefore the SCFO is under in-plane tensile strain.The layered structure observed here differs from that of La 0.6 Sr 0.4 CoO 3- films on STO subject to in-plane tensile strain, where layers of oxygen ordering oriented out-of-plane are believed to accommodate the strain. [41]he presence of the LP structure is dependent on the Co:Fe ratio.The x = 0.43 film consists of a majority BM phase that produces its characteristic (0010) superlattice peak in XRD (Figure 2a), unlike the x = 0.74 sample that consists mainly of LP.However, microscopically, cross-sectional HAADF (high angle ADF) STEM image (Figure S4, Supporting Information) of the x = 0.43 film indicates that three different structures coexist in a single film: BM, LP, and P.Even though STEM reveals the presence of LP and P in addition to BM, the main peaks (near the substrate peaks) of these three phases are not readily distinguishable in XRD.Therefore, XRD suggests only BM for the x = 0.43 sample whereas STEM indicates that BM, LP, and P are all present.The LP structure itself appeared the same for x = 0.43 and x = 0.74 according to the STEM images.
The LP structure observed in the x = 0.74 and x = 0.43 films differs from typical oxygen-deficient perovskite structures with random V O distribution, but it is not likely to be one of the Ruddlesden-Popper phases because the B-site cation positions observed in STEM images are not interleaved.Furthermore, the oxygen ordering and periodic intensity change in the out-of-plane direction are inconsistent with the SrFeO 2.75 or SrFeO 2.875 structures that have been reported. [33,34,42,35]The mechanism of oxygen ordering in LP is different from La 0.5 Sr 0.5 CoO 3- [43,44,45,46] because SCFO does not have two different A-site cations.A structure with alternating B-site intensities in its STEM image was reported for SrCo 0.7 Fe 0.2 Nb 0.1 O 2.72 , [38] attributed to vacancy ordering leading to octahedral tilting.
Figure 3 summarizes the experimental and theoretical structures reported on SrBO 3- compounds with respect to two variables,  from 0 to 0.5 and B = Co, Fe, or Mn. [4,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]In order to understand the ordering of oxygen ions in our SCFO films and compare with the structures reported in Figure 3, we considered the simulated STEM images of eight different structures named SrBO 3 , P L -SrBO 2.75 , BM-SrBO 2.75 , 314-SrBO 2.625 , BM-SrBO 2.625 , P L -SrBO 2.5 , BM-SrBO 2.5 , and SrBO 2 , as depicted in Figure 4.These structures were selected because they have periodic  oxygen deficiency in the B-coordinated layers along the [001] direction (i.e., film growth direction).We use the term P L to distinguish the structures from the one with random V O distribution and to underscore a specific ordering of oxygen ions in an oxygen-deficient P.
In Figure 4, SrBO 3 was a reference cubic perovskite without oxygen deficiency.P L -SrBO 2.75 was derived from a stoichiometric P structure but consists of layers of alternating pyramidal and octahedral B-site coordination.We excluded the structure of bulk Sr 4 Fe 4 O 11 ( = 0.25) [47] because it has apical V O in its A-site layers instead of the B-site layers.P L -SrBO 2.5 consisted of octahedral and square planar coordination layers, also derived from removing oxygen from a stoichiometric P structure.(An alternative SrBO 2.5 consisting of layers of square planar coordination of BO 4 would not have a difference in oxygen content between the two B-site layers.)The 314-SrBO 2.625 structure was obtained from that of Sr 3 Y 1 Fe 4 O 10.5 [48] by replacing the Y with Sr.It consists of alternating layers composed of octahedral and a mixture of pyramidal and tetrahedral coordination.BM-SrBO 2.5 is a typical brownmillerite structure with alternating planes of octahedrally and tetrahedrally coordinated B-sites.BM-SrBO 2.625 was constructed by adding one oxygen to one of the planes of the tetrahedrally coordinated B-sites of BM-SrBO 2.5 , and BM-SrBO 2.75 was obtained by adding another oxygen in the remaining tetrahedrally coordinated layer.Last, in SrBO 2 , the two alternating layers consist of BO 6 and BO 2 .
DFT calculations were performed to relax the eight structures with B = Co and Fe while constraining the in-plane lattice to be epitaxially matched to that of STO.Neutral oxygen vacancies were assumed during simulation because it is energetically favorable for the transition metal to reduce upon increasing oxygen deficiency rather than form charged vacancies. [47,49]The out-of-plane lattice parameters of the strained structure (Table S1, Supporting Information), unit cell volume and lattice parameter difference from the fully relaxed state (Tables S2 and S3, Supporting Information), and formation energies of the strained state (Table S4, Supporting Information) are summarized in the Supporting Information.SrCoO 3- structures in general showed a lower formation energy compared to SrFeO 3- .When B = Co, there is an increase in lattice parameter with increasing , whereas there is less change in lattice parameter with increasing  when B = Fe.
We then simulated the corresponding ADF and iDPC images using multislice-based [50] STEM imaging with the structures obtained from DFT.The comprehensive results are shown in Figure S5 (Supporting Information), and images corresponding to selected structures SrFeO 3 , 314-SrFeO 2.625 , BM-SrFeO 2.625 , P L -SrFeO 2.5 , and BM-SrFeO 2.5 are shown in Figure 5a.Even before simulating the STEM images, we were able to exclude the possibility of BM-SrBO 2.625 and SrBO 2 , because the former showed different B cation positions from those in the STEM images and the latter was highly unstable based on DFT calculations (Table S3, Supporting Information).We can also rule out the possibility of BM-SrBO 2.5 , because the XRD did not show the characteristic BM superlattice peaks in Figure 2a, and BM could be clearly distinguished as a different structure in Figure S3 (Supporting Information).Lastly, the ordering observed in iDPC of BM-SrBO 2.75 in Figure S5 (Supporting Information) is lacking in our experimentally observed images, eliminating this structure from the candidates.This leaves P L -SrFeO 2.75 , 314-SrFeO 2.625 , and P L -SrFeO 2.5 as the most plausible structures.
We further compare the structures by determining the Sr-Sr distance ratio between the alternating planes along the out-ofplane direction (Figure 5b) and the B-site intensity deviation from the mean (Figure 5c) from the simulated STEM images.Out of the 8 structures, both 314-SrBO 2.625 and P L -SrBO 2.5 most resemble the experimental results in Figure 1c in three respects.First, the alternating Sr-Sr distance length ratio of these two structures is ≈1.2, at the upper end of all the structures except for BM-SrBO 2.5 and SrBO 2 .Second, the B-site intensity variation is ≈10%, which is the next highest after the unstable SrBO 2 .Finally, the [110] and [1 10] projections are equivalent ([1 10] projections are shown in Figure S5, Supporting Information).This is an important criterion because all of the LP seen in the STEM images has the same appearance; a structure with inequivalent [110] and [1 10] projections would be expected to show the two different variants in different parts of the sample.Kinematic X-ray diffraction calculations indicate that the superlattice reflection intensity is ≈2.0% of the main peak for 314-SrBO 2.625 and 4.5% for P L -SrBO 2.5 compared to 7.5% of BM-SrBO 2.5 , i.e., superlattice peaks of 314-SrBO 2.625 and P L -SrBO 2.5 are expected to be present but would be weaker than those of BM-SrBO 2.5 .However, in the x = 0.74 film, superlattice peaks were absent despite the clear periodicity of the Sr-Sr distance.We speculate that this is due to the mosaicity of the crystallites or disorder in the structure that would lower the peak intensities below the calculated values. [51]e also consider the stability of the proposed layered structures with respect to a P unit cell with randomly placed V O , namely R-SrBO 2.625 that was generated by introducing three V O in a 2 × 2 × 2 perovskite supercell analogous to Ref. [52].314-SrFeO 2.625 is stable by −87.4 meV f.u.−1 (f.u.refers to formula unit) compared to R-SrFeO 2.625 , whereas 314-SrCoO 2.625 is unstable by +61.9 meV f.u.−1 compared to R-SrCoO 2.625 , suggesting that Fe stabilizes the ordering of V O .(We did not make a comparison for  = 0.5 because of the large number of possible configurations of R-SrBO 2.5 .) Considering the comparisons between experimental data and simulations, we cannot say decisively that the LP phase is an exact match for 314-SrBO 2.625 or P L -SrBO 2.5 .Although 314-SrBO 2.625 is energetically more stable than P L -SrBO 2.5 based on DFT, we do not observe the expected slight modulation of the B-site cation positions in the iDPC image shown in Figure 5a, suggesting that the structure is closer to that of P L -SrBO 2.5 .However, the composition may have fewer V O than P L -SrBO 2.5 because Fe typically favors a higher oxidation state compared to Co.It is possible that the experimental result has characteristics of both 314-SrBO 2.625 or P L -SrBO 2.5 , i.e., a structure with alternating layers of octahedral and mixed B-site coordination.The discrepancy between the observed Sr-Sr distance ratio ≈1.4 and the calculated value ≈1.2 may be affected by epitaxial strain: the Sr-Sr distance ratio calculated from bulk unstrained 314-structured SrFe 0.25 Co 0.75 O 2.63 is ≈1.1, [37] smaller than our calculated value of ≈1.2 for tensilestrained supercells.
We marked our best description of the LP structure on Figure 3, highlighting that the ordering of oxygen ions observed in the thin films of this work is atypical among the series of BM Sr(Co,Fe)O 2.5 solutions.We represent its  as a range between that of the two best-fit structures, 314-SrBO 2.625 and P L -SrBO 2.5 .As a final comment on the LP structure, in Ref. [39] we reported that ionic liquid gating can introduce oxygen into the layered structure causing it to separate between the layers, yielding distorted layers with wider spacing (Figure S6, Supporting Information).Upon application of electrostatic driving force, the oxygen-deficient layers may provide sites for oxygen insertion, producing linear defects between the planes due to Coulomb repulsion. [39]

Conclusion
In summary, we report an oxygen-deficient layered perovskite structure in SCFO films grown by combinatorial PLD.The perovskite-derived structure of SCFO with x = 0.74 shows clear evidence of layers with alternating oxygen deficiency parallel to the film growth direction, resulting in the expansion (oxygendeficient), and contraction (oxygen-rich) of the out-of-plane Sr-Sr distance, concurrent with the B-site ADF intensity deviation (dim and bright, respectively).This can be described as the layering of BO 6 and BO 4 /BO 5 coordinated B-site cations resembling 314-SrBO 2.625 or P L -SrBO 2.5 , neither of which has been previously identified in thin films.The layered structure is dominant and consistent throughout the film when x = 0.74 and is present as a minority phase at a different Co:Fe ratio, x = 0.43.The stability of this crystal structure, and the remaining ambiguity in the exact oxygen ordering encourages further investigation of its origin, as well as the effects of the interplay between kinetics and thermodynamics in PLD associated with the oxygen pressure during growth, and the prospect of achieving yet more ordered structures in ABO 3- .This study also motivates the exploration of the anisotropic magnetic, electrical, and transport properties of the oxygen vacancy ordered perovskite structure.

Figure 1 .
Figure 1.a) STEM image overviewing the x = 0.74 SCFO (film), STO (substrate), and their interface.The alternating layers in the SCFO are visible throughout the film thickness.b) Enlarged region of (a) at the interface showing the epitaxial growth.c) Simultaneously collected ADF and iDPC STEM images revealing oxygen ordering in alternate B-site atom column layers of the x = 0.74 film.d) Cropped region of (c), with the arrow in the iDPC pointing out the absence of oxygen in corresponding layers of the ADF image.e) Sr-Sr distance map, and f) B-site (Co and Fe site) intensity deviation map obtained from the ADF image in (c), clearly highlighting the different but consistent characteristics of the dim and bright layers.

Figure 2 .
Figure 2. a) XRD results of SCFO films grown at 150 mTorr with x = 0.74 and 0.43.b) RSM of (103) reflection of the as-grown x = 0.74 film.Vertically aligned substrate and film peaks indicate that the film is fully strained.

Figure 3 .
Figure 3. Map of observed SrBO 3- structures as a function of oxygen deficiency (, x axis) and B-site cation chemistry (y axis).Orange and teal colors in the map represent bulk and thin films, respectively.