Topotactic Reduction: From Brownmillerite to Infinite-Layer Strontium Ferrite

Isostructural to high-temperature superconducting cuprates, other infinite-layer materials like nickelates, cobaltates, and ferrites have attracted significant interest in recent years. A popular route to synthesizing these infinite layers in thin films is growing perovskite or brownmillerite films and removing oxygen through topotactic reduction [1].<br/>In strontium ferrite, there has been substantial work dedicated to understanding both the as-grown brownmillerite phase, which is notable for high ionic conductivity and interesting magnetic structure [2,3], as well as the reduced infinite layer phase, which is an antiferromagnetic Mott insulator studied for its correlated electronic, magnetic, thermal, and optical properties [4-8]. However, relatively little is known about the mechanism of the reduction process, which can depend on the orientation of oxygen vacancy channels in the brownmillerite films [4,9].<br/>In this work, we grow epitaxially-strained brownmillerite SrFeO_2.5 thin films on various substrates using pulsed laser deposition and reduce them to the infinite layer phase, SrFeO₂, through low-temperature topotactic reduction using CaH₂ reducing agent. During this reduction, we track the evolution of structure and topography, probing the spatial distribution of the brownmillerite and infinite-layer phases as a function of reduction time and strain. We also study the strain dependence of the final infinite layer phase. This work sheds light on the oxygen diffusion pathways relevant to the topotactic reduction process to enable further refinement and control of the transformation between the phases.<br/><b>References </b><br/>[1] Z. Meng et al., <i>Adv. Functional Mater. </i>(2023) 2305225<br/>[2] A. Nemudry et al., <i>Chem. Mater. </i><b>10 </b>(1998) 2403-2411<br/>[3] J. C. Waerenborgh et al., <i>J. Solid State Chem. </i><b>205 </b>(2013) 5-9<br/>[4] C. Tassel at al., <i>Chem. Soc. Rev.</i><b>41 </b>(2012) 2025-2035<br/>[5] T. Kawakami et al., <i>Nat. Chem. </i><b>1 </b>(2009) 371-376<br/>[6] S. Ju and T. Y. Cai, <i>Appl. Phys. Lett. </i><b>94</b> (2009)<br/>[7] M. Raham, Y. Z. Nie, and G. H. Guo, <i>Inorg. Chem. </i><b>52 </b>(2013) 12529-12534<br/>[8] C. Tassel et al., <i>J. Am. Chem. Soc. </i><b>130 </b>(2008) 3764-3765<br/>[9] S. Inoue et al., <i>Nat. Chem. </i><b>2 </b>(2010) 213-217