Strain-Induced Anion-Site Occupancy in Perovskite Oxyfluoride Films

Abstract

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-δFγ 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 2 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.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

“…The bond valence sum analysis and our understanding of strain suggest that F occupancy of equatorial X 1 sites is certainly possible in Bi 2 TiO 4 F 2 and is consistent with the conclusions of Needs et al However, this contrasts with several other n = 1 Aurivillius and Ruddlesden-Popper oxyfluorides including Bi 2 NbO 5 F, Sr 2 ScO 3 F, and Sr 2 MnO 3 F, which are reported to have F occupancy of apical anion sites despite similar arguments involving stacking strain between [Bi 2 O 2 ] 2+ and Nb­(O,F) 2 layers ( a p = 3.95 Å and a p = 4.08 Å for F – in equatorial X 1 or apical X 2 sites, respectively) or between SrO and Mn­(O,F) 2 layers (ideal a SrO ≈ 3.65 Å; for Sr 2 ScO 3 F a p = 4.12 Å and a p = 4.19 Å for F – in equatorial or apical sites, respectively; for Sr 2 MnO 3 F a p = 3.86 Å and a p = 3.99 Å for F – in equatorial or apical sites, respectively). It is striking that Bi 2 TiO 4 F 2 has relatively small stacking strain compared to these examples, suggesting the possibility that anion distribution might be tuned by strain engineering, for example . Wider consideration of the bonding in Bi 2 TiO 4 F 2 , beyond the purely ionic model, may also be relevantparticularly given the importance of pπ dπ B –O bonding (often reflected in shorter B –O bonds) that can influence the stereochemistry in oxyfluorides .…”

“…The final arrangement, (C), is interesting because it results in a polar Ti­(O 1/2 F 1/2 ) ap (O 2/2 F 2/2 ) eq corner-linked octahedra, that is, either mer- or fac- [TiO 3 F 3 ] units (Figure c). The bond valence sum and GII analysis described above for Bi 2 TiO 4 F 2 suggest that while arrangement (A) is most favored, the difference between arrangements (A) and (B) is small and so (C) might occur over short length scales or could be accessed by strain engineering as demonstrated in strontium manganese oxyfluoride films …”

“…It is striking that Bi 2 TiO 4 F 2 has relatively small stacking strain compared to these examples, suggesting the possibility that anion distribution might be tuned by strain engineering, for example. 69 Wider consideration of the bonding in Bi 2 TiO 4 F 2 , beyond the purely ionic model, may also be relevant—particularly given the importance of pπ dπ B –O bonding (often reflected in shorter B –O bonds) that can influence the stereochemistry in oxyfluorides. 19 Similar factors also influence the N 3– /O 2– order in oxynitrides.…”

“…The bond valence sum and GII analysis described above for Bi 2 TiO 4 F 2 suggest that while arrangement (A) is most favored, the difference between arrangements (A) and (B) is small and so (C) might occur over short length scales or could be accessed by strain engineering as demonstrated in strontium manganese oxyfluoride films. 69 …”

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

“…TM oxyfluorides (OF) possess unique characteristics, including optical properties, catalytic activity, and electrical conductivity [ 58 , 99 , 100 ]. It is interesting to note that the intrinsically weak conductivity of metal–fluorides can be improved by introducing metal–oxygen (M–O) bonds, because the high ionicity of metal–fluorine (M–F) bonds can be well adjusted by high covalent M–O bonds.…”