Planar defects in non-stoichiometric SrFeO_3-δ perovskite type thin films studied by analytical scanning transmission electron microscopy

Abstract text (incl. figure legends and references)

Strontium-ferrite (SFO), Sr_nFe_nO3_n-1, provides a fascinating system in which stoichiometric changes can be explored in order to study their impact on the material's crystallographic structure, its properties, and, in particular, on defect formation. For n = 2, SFO adopts a brownmillerite-type structure of alternating layers of FeO₆ octahedra and FeO₄ tetrahedra with iron exclusively in +3 oxidation state. Increasing the oxygen content, with n = 4 and n = 8, SFO reveals tetragonal and orthorhombic structures with iron in +3 and +4 oxidation states (see, e.g., [2]). For the limiting case that n → ∞, SFO is found in the stoichiometric, cubic perovskite structure as SrFeO₃, where Fe is uniquely in +4 oxidation state, forming the well-known FeO₆ octahedra as building blocks of the perovskite structure. From a properties point-of-view, SFO is known to be a mixed ionic-electronic conductor and consequently, tuning the structure and defects provides a mean to control its conductivity.

We present an experimental study of non-stoichiometric SrFeO_3-δ, where the loss of oxygen (δ > 0) must lead to oxygen vacancies, possibly structural defects, and thus resulting in changes in the oxidation state of Fe as well. We used HAADF- and ABF-STEM to develop a structural model of planar defects, which run "packman"-style along {001} planes in the SFO film grown on SrTiO₃, and which are formed due to the oxygen deficiency [2] (see Fig. 1). Atomic-resolution EDX spectroscopy reveals that the defective bands are formed by Fe₂O₂ double layers. The defects strongly resemble the Sr₄Fe₆O_12+δ structure viewed along the pseudocubic [107] direction, which consists of bands of corner-sharing FeO₆ octahedra separated by bands formed of periodically arranged FeO₅ polyhedra identified as trigonal bipyramids and tetragonal pyramids, respectively.

To assess the impact of these chemistry-driven structural defects on the physical properties, EELS was used to measure the Fe L_3,2 and the O K excitation edges. Subtle but significant differences in the fine structure of the O K edge at the defect planes compared to bulk spectra were identified and interpreted by complementing the experimental data with simulations using FEFF9. This analysis suggests an increased electron doping of the Fe 3d e_g bands in the Fe₂O₂ defect layers compared to the bulk SrFeO₃, which as a consequence must affect SFO's electron-hole conductivity. Moreover, analysis of the Fe L_3,2 excitation edge uncovers the expected change of the Fe oxidation state in the defective planes from +4 towards +3.

In summary, STEM imaging combined with EDX and EELS measurements revealed the nature of planar defects present in SrFeO_3-δ thin films grown on SrTiO₃ and their effect on the electron-hole conductivity. Controlling the amount of oxygen vacancies in these perovskite-based SFO thin films thus allows the electronic conductivity of these films to be adjusted.

Figure 1 HAADF STEM: overview (a) and detail (b) of the atomic structure of the planar defects formed in oxygen deficient, perovskite-type SrFeO₃.

References

[1] M. D. Rossell, A. M. Abakumov, Q. M. Ramasse, R. Erni, ACS Nano 7 (2013) 3078-3085.

[2] M. D. Rossell, P. Agrawal, M. Campanini, D. Passerone, R. Erni, Phys. Rev. Mater. 4 (2020) 075001.

[3] P. Agrawal and D. Passerone are kindly acknowledged for providing support for interpretation of the FEFF9 data, and we would like to thank M. Campanini providing the script for peak-pair analysis.