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Strain-induced anion ordering in perovskite oxyfluoride films

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.


INTRODUCTION
Inducing atomic-scale ordering is a promising approach for realizing new or enhanced physical properties in functional materials with mixed cation and anion site occupation. Perovskites, which exhibit the general chemical form ABX3, have been recognized as a versatile family of functional materials due to their structure that accommodates a wide range of chemistries: the A-site cations are coordinated with 12 X-site anions; the B-site cations are coordinated with 6 X-site anions forming an BX6 octahedral structure, which shares corners to form a three-dimensional network. 1,2 In this family, cation substitution is a well-established route to engineer the properties of perovskites as the A-or B-sites can be readily occupied by two or more different elements. 3 Beyond solid solutions, long-range ordering of cations can enhance or endow new functionality into quaternary perovskites by removing disorder or enforcing polarity. [4][5][6] Alloying on the anion-site provides an alternative means of modulating the properties of perovskite oxides, as heterovalent anionic substitution not only alters the electronic state of B-site cation, but also the B-X bonding geometry and ionicity due to the different anionic characteristics, such as electronegativity, polarizability, charge number and atomic size. Previous work has demonstrated that a wide range of oxynitrides, oxyhydrides, and oxyfluorides can be synthesized, in which N 3-, H -, and Fanions are incorporated into the lattice due to their similar radii as O 2-. [7][8][9][10] However, compared to approaches based on cationic substitutions, heteroanionic-based materials design is at a less mature state and fundamental questions remain about how to deterministically control the concentration and site-occupancy of the different anionic species. 11 Epitaxial thin films provide new avenues to enable synthetic control of heteroanionic materials, namely through the formation of interfaces and substrate-induced biaxial strain. For example, biaxial strains of ±3% are now routinely imposed in epitaxial oxide films that are tens of nm in thickness without loss of coherence with the substrate. 12 The development of topochemical approaches [13][14][15] to convert perovskite oxide films to oxyfluorides has enabled the realization of numerous families of epitaxial oxyfluorides including ferrites, [14][15][16][17][18] manganites, 19,20 ruthenates, 21,22 chromates, 23 cobaltites, 24 and nickelates. 25 The thin film nature of these samples has been exploited for functionality not accessible to bulk polycrystalline oxyfluorides, for instance through reversible insertion and removal of F in NdNi(O,F)3-δ films 25 and lateral patterning of SrFeO2.5/SrFeO2F heterostructures through spatially controlled topochemistry. 26 However, the use of substrateinduced strain to alter or direct the F incorporation has yet to be reported.
Here, we demonstrate that epitaxial strain can be used to control F/O ordering in epitaxial SrMnO2.5-δFγ (SMOF) thin films deposited on substrates with different in-plane lattice constants.
By altering the in-plane and out-of-plane B-X bond lengths, this strain has considerable consequences for the relative formation energies of B-F and B-O bonds and thus the site occupancy of the F and O anions. Using a complementary suite of x-ray linear dichroism, x-ray diffraction, polarization-dependent resonant valence-band photoemission, hard x-ray photoemission, and density functional theory calculations, we demonstrate that F occupies the apical-like sites (O1-sites) under compressive strain leading to Mn-F bonds oriented along the out-of-plane direction.
In contrast, F is incorporated on equatorial-like sites (O2, O3-sites) under tensile strain, leading to Mn-F bonds parallel to the in-plane directions. An extreme x-ray linear dichroism at the F and O K-edges results from the anionic configurations within the films. This work highlights a unique opportunity for materials design utilizing topotactic modification of epitaxial films, which enables static strain states that are inaccessible in bulk geometries thus providing a route to metastable ordered structures.

RESULTS AND DISCUSSION
The anisotropic deformation of as-grown SrMnO2.5 (SMO) films upon biaxial strain is shown in Figure 1(a,b). Bulk SMO is orthorhombic, but can be approximated as having a pseudocubic lattice parameter of 3.811 Å 27,28 , which results in a biaxial compressive strain when grown on LaAlO3 (LAO; a = 3.795 Å), and a tensile strain when deposited on (La,Sr)(Al,Ta)O3 (LSAT; a = 3.870 Å) and SrTiO3 (STO; a = 3.905 Å) substrates. For SMO/LAO, the in-plane lattice parameter is compressed, which induces a lattice elongation along c-direction. For the SMO/LSAT and SMO/STO, the in-plane lattice is elongated, and the c-axis parameter is contracted. Figure 1  LSAT, and STO substrates, obtained from diffraction data presented in supplemental Table S1 and Figure S1. The c-axis parameter expands when the film is compressed on LAO (abulk > aLAO) and contracts for the tensile strained SMO grown on LSAT and STO. After fluorination, there is a caxis expansion for all films, consistent with other reports of lattice expansion upon conversion of oxides to oxyfluorides in which the F is incorporated into the perovskite lattice. 15,19,29 Reciprocal space maps, shown in Figure S1, confirm that the SMOF films remain strained to the substrate following the topochemical fluorination reaction. Comparing all three fluorinated films, SMOF/LAO shows a much larger c-axis expansion than SMOF on LSAT and STO. The impact of strain on F incorporation was measured using x-ray photoemission spectroscopy (XPS) to quantify the F concentration within the samples and to confirm that the F extents through the films. In Figure 1(d), the F content of the SMOF films is plotted as a function of the substrate in-plane parameter. The compressively strained SMOF/LAO shows the lowest F concentration, indicating that the large c-axis enhancement in this sample upon fluorination is not the result of an increased F incorporation compared to the films under tensile strain. We attribute the differences in F content to strain-induced differences in the Fdiffusion rate along the pseudocubic [001] direction, as well as the lattice expansion that occurs upon Fincorporation. Given that the F incorporation process proceeds from the surface vertically through the c-axis oriented films, we hypothesize that the smaller in-plane lattice parameters of SMOF/LAO impede vertical F diffusion by increasing the atomic density within the (001) planes, while simultaneously the necessary lattice expansion upon Fincorporation is unfavorable due to the compressive strain imposed by the substrate. Under increasing tensile strain, the energetic and diffusion considerations are opposite to those described above due to the larger in-plane lattice constants, and thus the greater F content inside the films on LSAT and STO. In this way, the lattice expanding effects of Fincorporation results in trends analogous to oxygen vacancy formation in strained perovskites, which is promoted under tensile strain but not under compressive strain. 30,31 Density functional theory (DFT) calculations reveal the relative stabilities of F substituting on different O sites as a function of the pseudocubic lattice parameter and to elucidate the structural implications of F occupying different sites. Among the three O sites, Figure 1(e) reveals that the relative energy of O1-site substitution steadily decreases with decreasing in-plane lattice parameter compared to O2-site substitution. This reveals that O1-site substitution becomes increasingly favored under compressive strain. The relative orientation of Mn-F bonding with respect to biaxial plane is the prime factor in the site-preference as a function of strain. While the Mn-O2 and Mn-O3 bonds are aligned along in-plane directions, the Mn-O1 bond is the only bond aligned along the c-axis (out-of-plane) direction. As compressive biaxial strain decreases the length of the inplane bonds and elongates the out-of-plane bonds, the longer bond length of Mn-F compared to the Mn-O bond prefers to occur along the out-of-plane direction which stabilizes O1-site substitution. Here, we note that the DFT calculations utilize the oxygen vacancy ordered structure of Sr2Mn2O5, while experimentally, we do not have direct evidence of vacancy ordering in the asgrown SrMnO2.5 films. However, the physical argument that compressive strain favors creating Mn-F bonds along the growth direction, or oppositely tensile strain favors in-plane Mn-F bonds, holds for the case of ordered or disordered oxygen vacancy distributions. Similarly, the orientation of the films' orthorhombic axes is unknown. Previous studies have shown that SrMnO2.5 deposited on LSAT and STO grow with the orthorhombic c-axis out-of-the-plane, forming in-plane domains in which the orthorhombic a-axis is rotated by 90°. 32 The orientation of the orthorhombic c-axis on LAO substrates is unknown, but the preference for F occupancy of apical-like sites under compressive strain would remain even if the orthorhombic c-axis of the films lies in-the-plane.
The DFT calculations also reproduce the enhanced c-lattice expansion upon fluorination under compressive strain as plotted in Figure 1(f). The O1-site substitution induces an ~20% larger clattice expansion under fluorination compared to F substitution on the O2 and O3 sites: the c-axis expansion with γ = 0.0625 is 0.07 Å for O1-site substitution and 0.05 Å for O2-site substitution.
We note that while the c-lattice parameter of the compressively strained SMOF film is largest with F occupying the O1 site, the increased rate of lattice expansion under compressive strain occurs regardless of the substitution site, consistent with a general fluorination-induced lattice volume expansion in which Δc becomes larger with reduced ab-plane unit area under compressive strain. We used polarization-dependent resonant valence-band photoemission spectroscopy with linear dichroism to study the Mn-derived contributions to the valence band of the SMOF/LAO film. By tuning the photon energy to the Mn L3 absorption edge, we take advantage of the interference between the direct photoemission process 2p 6 3d n → 2p 6 3d n-1 + eand the decay of a resonantly excited state 2p 5 3d n+1 → 2p 6 3d n-1 + e -. This leads to the enhancement of the 3d photoionization cross sections, thus effectively amplifying the contribution from the Mn 3d states in the valence bands. Linearly polarized x-rays with the electric-field orientation perpendicular (E⊥c) and parallel (E∥c) to the c-axis of the film were used to preferentially probe the in-plane and out-of-plane orbitals. The resonant valence-band linear dichroism map (E∥c -E⊥c) is shown in and Mn 3+ states, respectively. Both features are recognized based on previous resonant photoemission studies on Mn, where Mn 2+ peak is reported to be at 640.9 eV, and Mn 3+ is at 643 eV. 33 The incorporation of F into SrMnO2.5-δFγ via the topochemical reaction used herein leads to a partial reduction of Mn, with previous x-ray absorption spectroscopy experiments revealing the presence of a mixed Mn 2+ /Mn 3+ valence. 19 Thus, the photon-energy dependence of the resonant linear dichroism spectrum shown in the near-EF line-cut plot indicates that the Mn 3d electrons near the Fermi level prefer to occupy states with an in-plane orbital character at the Mn 3+ sites and states with out-of-plane orbital character at the Mn 2+ sites.
DFT calculations reveal that the linear dichroism in the photoemission spectra arises from the orientation of square pyramidal units with respect to substrate. Linearly-polarized x-ray absorption (XA) spectroscopy carried out at the O and F K-edges provide direct evidence for strain-induced changes to the anion site occupation. This approach was recently used to demonstrate anion-site ordering in strained oxynitride SrTaO2N films. 34 Figure 3 shows  To directly compare the absorption asymmetry under the two polarization states, we calculate the x-ray linear dichroism (XLD) difference spectra, defined as E∥c -E⊥c. 36 The XLD spectra of The spectral shape reflects the partial DOS of the unoccupied F 2p and Mn 3d hybridized orbitals, which are shown schematically in Figure S3 with the orbital energy levels and orientations obtained from DFT calculations. Note that for (001)-oriented SrMnO2.5, the 3 2 − 2 and 2px orbitals point out-of-plane and the 3 2 and 2py/2pz orbitals are in-plane as illustrated in Figure   2(b), which is counter to the conventional orbital energetics found in ABO3 perovskite films. 37 We analyze the XLD spectra beginning with the compressively strained SMOF/LAO sample using similar arguments to those recently employed to interpret XLD spectra of strained oxynitride films. 34 For F bonded with Mn 3+ (3d 4 ), the lowest energy excitation is to the empty 3 2 − 2 orbital.
The electron excitation is allowed to hybridized 2 -3 2 − 2 and 2 -3 2 − 2 orbitals contributed by F in O1-and O2/O3-sites, respectively. In the Mn 3+ -F region (a1, a2), the positive XLD of SMOF/LAO indicates larger absorption under the out-of-plane polarization (OPP) condition, where the electric-field vector of the incident x-ray is parallel to the c-axis (E∥c). This corresponds to an electron excitation into the 2 -3 2 − 2 orbital, associated with Mn 3+ bonded to apical-like F. Therefore, F bonded with Mn 3+ prefers to occupy the O1-sites. For F bonded with Mn 2+ (3d 5 ), each orbital is occupied with one electron, and all the electron spins are aligned. When the core electron is excited into the 3d states, it has to share the orbital with another electron with opposite spin, which pushes the absorption leading edge towards higher energy. 38   We next turn to the impact of strain-induced anion ordering on the SMOF electronic behavior.
The occupied valence band DOS, as measured with hard x-ray photoemission spectroscopy (HAXPES), exhibit differences between the compressive and tensile strained films, as shown in Figure 4(a). To verify that the spectral differences arise from the F-site occupancy, the density of states were calculated using DFT with F residing on the O1-site and a 3.8 Å in-plane lattice parameter and F residing on the O2-site and a 3.9 Å in-plane lattice parameter, mimicking the samples under compressive and tensile strain, respectively [ Figure 4(b)]. General agreement is apparent between the measured and calculated spectra down to approximately 7 eV below the Fermi level, as highlighted by the difference spectra between the two strain states presented in the bottom panels of Figure 4(a,b). In particular, spectral features labeled 1-6 all exhibit the same sign at approximately the same energies from the experimental and computational results. Agreement between the HAXPES and DFT results breaks down at binding energies larger than 7 eV, which we attribute to C-based surface contamination on the experimental samples. 39 The that are not captured in the DFT calculations but could also play a role in the measured resistivity.
The carrier effective mass is another factor determining the transport, captured to first approximation by the band curvature. Upon applying compressive strain, we found that the electronic bands of SMO are more dispersive ( Figure S7); this smaller effective mass can potentially explain the decreased resistivity of as-grown SMO under compressive strain compared to tensile strain, shown in Figure 4(c). However, fluorinated systems show more localized bands than SMO, and importantly, we found no significant dependence of the band curvature on biaxial strain. Thus, we speculate that the substantially higher resistivity of SMOF/LAO sample results, at least in part, from the energetic differences in the mid-gap levels for F occupying the apical-like anion site (O1-site) compared to the equatorial-like anion site (O2/O3-sites).

CONCLUSIONS
We have shown that epitaxial strain induces preferential site occupancy for F and O anions in epitaxial SrMnO2.5-δFγ oxyfluoride films grown on LAO, LSAT, and STO substrates. X-ray linear dichroism measurements revealed that under compressive strain, F is more likely to take apicallike O1-sites, while under tensile strain, F preferentially occupies the equatorial-like O2/O3-sites.
The site preferences were further confirmed by the calculated relative energies of different F occupancies as a function of biaxial strain. The anionic site-selective substitutions result in changes to the valence band density of states and lead to an increased resistivity in samples in which F occupies the apical-like sites compared to those in which F resides at the equatorial-like position.
The control of anion ordering in perovskite oxyfluorides using a readily available substrateinduced strain approach provides a new strategy to tailor crystal symmetries and functional properties in mixed-anion materials.

Fluorination.
To synthesize oxyfluorides, we use a fluorination method similar to that described by Katayama. 15 The SMO films and fluoropolymers were placed in an alumina boat wrapped by aluminum foil, then the boat was loaded into a horizontal tube furnace with Ar as the carrier gas.
Our previous efforts have revealed that using polytetrafluoroethylene (PTFE) as fluorine source produces a uniform F distribution within the SMOF films, and the F content can be controlled by fluorination time and temperature. 19,40 In this study, the strained SrMnO2.5 films were fluorinated with PTFE for 4 h at 200 °C. Following fluorination, the samples are kept in air as we have not noticed material degradation in ambient conditions.
Structural and compositional characterization. X-ray diffraction (XRD) was performed before and after fluorination using a Rigaku SmartLab diffractometer. The XRD data was simulated in GenX. 41 Reciprocal space maps were obtained from the fluorinated films using beamline 33-BM at the Advanced Photon Source. The atomic concentrations were obtained from XPS sputter depth profiles carried out throughout the entire film thickness. XPS spectra of Sr 3d, Mn 2p3/2, O 1s, and F 1s were collected for each measurement cycle after 1 min of Ar + sputtering using 500 eV ions.
After 30 sputtering cycles, the depth profile reached the interface of the film and substrate. In converting from sputtering time to depth, we assume the sputtering rate is uniform with ~ 1 nm of film removed during each sputtering cycle, calculated by dividing the total thickness of the film with the number of sputtering cycles to etch through the entire film. The elemental depth profile was obtained by analyzing the measured photoemission spectra in CasaXPS. 42 The F concentration was determined by averaging the F distribution throughout the film as obtained from XPS depth profiles shown in Figure S2; the presented F concentration (γ) was obtained by normalizing the F composition to the average of the Sr and Mn composition. We do not have a measure δ within the films, but the presence of Mn 2+ after fluorination indicates that δ ≤ γ < 2δ. x-rays. The inelastic mean-free path (IMFP) of the emitted electrons limits the information depth of the TEY mode to approximately 5 nm, making it a relatively surface-sensitive probe. 43 In contrast, the LY detection mode is bulk sensitive, capturing information from the entire film thickness by monitoring the absorption-modulated visible luminescence of the substrate. 44,45 The XLD spectra were calculated by the difference spectra, defined as XLD = E∥c -E⊥c. with a kinetic energy cutoff of 550 eV for the plane wave basis set was used to treat the core and valence electrons using the following configurations Sr (4s 2 4p 6 5s 2 ), Mn (3p 6 4s 2 3d 6 ), O (2s 2 2p 4 ), and F (2s 2 2p 5 ). An 8×4×6 Monkhorst-Pack grid was used for SrMnO2.5 with the tetrahedron method while a 4×4×6 grid was used for SrMnO2.5-γFγ oxyfluorides. The atomic structures were relaxed until the forces on each atom were less than 5 meVÅ -1 ; the ab-plane was subjected to the equivalent pseudocubic lattice parameter to mimic the thin film geometry with biaxial strain.

Polarization
The atomic structures of the manganite oxyfluorides were constructed from a 2×1×1 supercell of SrMnO2.5 by substituting a fluorine atom on one of the different oxygen sites (O1, O2, and O3), which forms SrMnO2.5-γFγ with γ = 0.0625. The scenario of F occupying vacancy site is not considered for this study because of its low probability as revealed in our previous work. 19 SrMnO2.5 adopts a unique zig-zag type spin ordering within the ab-plane realized by the square