Recently, with the discovery of unconventional superconductivity and other intriguing magnetic properties, infinite-layer transition metal oxides have garnered tremendous research interests in the oxide community1. Due to the thermodynamically metastable nature of the infinite-layer phase in most transition metal oxides, synthesizing this material has been largely confined to a few kinetically controlled reduction processes of precursor oxide phases, such as perovskite or brownmillerite, using chemical agents. Moreover, the oxygen deintercalation to infinite-layer phase is successful only when the activation barrier for the targeted kinetic transition pathway is sufficiently lowered, thereby suppressing the formation of equilibrium-stable byproducts. Consequently, most reported infinite-layer oxides have been synthesized by utilizing the hydride reduction process of oxides precursor phases since the first successful attempt in strontium ferrite, SrFeO22.
This investigation stemmed from a fundamental question: how does a three-dimensional (3-D) transition metal – oxygen polyhedra (octahedra and tetrahedra) transform into a two-dimensional (2-D) square planar configuration microscopically? Typically, cations strive to coordinate themselves symmetrically with the greatest number of bonds rather than adopting a planar arrangement. A creative and simple ball-and-stick model was suggested to provide insight into the underlying kinetic diffusion pathways of this intriguing transition, manipulating oxygen atoms like playing with Lego blocks3. However, since then, there had been no experimental evidence for the microscopic rearrangements of the ions.
Taking SrFeO2.5 strontium ferrite epitaxial thin film as a model system, our recent work directly visualized the entire reduction process in atomic-scale during the transformation from brownmillerite SrFeO2.5 to the infinite-layer SrFeO2 using in-situ transmission electron microscopy. One of our primary goals  is to capture the highly dynamic motion of individual oxygen ions. For this, high-resolution transmission electron microscopy (HRTEM) under a negative spherical aberration condition3, combined with a highly stable heating holder and high-speed camera, was employed, optimized for capturing such highly dynamic processes. Due to the coherent imaging nature of HRTEM, the imaging window for obtaining directly interpretable images was very narrow. Hence to meet the proper imaging condition, nearly perfect optimization of sample preparation, imaging conditions, and thermal drift control are highly necessary.
Figure 1 90° reconfiguration of oxygen vacancy channels prior to the phase transformation. a, Structural models showing the crystallographic reorientation of SrFeO₂.₅, facilitated by atomic shuffle movements, simulated by density functional theory calculations. b, Time-sequence of the HRTEM images, capturing the 90° reconfiguration process of SrFeO₂.₅ from the [100] to the [010] zone axis, prior to the phase transformation towards SrFeO₂.
Prior to the transformation, we observed an unexpected 90° crystallographic reorientation of SrFeO₂.₅ from the orthorhombic [100] to [010] direction near the phase boundary, mediated by atomic shuffle movements. The experimental observation was further supported by density functional theory (DFT) calculations. This reorientation is driven by the geometrical anisotropy in oxygen diffusion, resulting from the one-dimensional oxygen vacancy channels in SrFeO₂.₅ (Figure 1). This self-reorientation aligns the oxygen diffusion channels perpendicular to the top and bottom surfaces of the TEM lamella specimen, achieving the shortest length and increasing the number of oxygen diffusion channels exposed to the oxygen sink, facilitating the rapid yet sequential oxygen removal and rearrangement. Subsequently, as oxygen is released and migrates, the 3D-to-2D reconfiguration of oxygen is facilitated by the lattice flexibility of FeOx polyhedral layers. This process involves multiple discrete transient states surrounding the multivalent iron ions (Figure 2). The transition is a complex interplay of thermodynamics and kinetics, facilitated by atomic shuffling, oxygen release and migration, and self-regulation of the polyhedra, all occurring along the least energy-costing pathways.
Figure 2 Atomic-scale mechanisms of oxygen movement during the transformation from SrFeOâ‚‚.â‚… to infinite-layer SrFeOâ‚‚. a, Atomic models showing the emergence of four states via oxygen release and migration during phase transformation from SrFeO2.5 to SrFeO2, proposed by DFT calculations. b, Time-sequence HRTEM images corresponding to the four states in a, observed during the transformation to SrFeOâ‚‚.
Additionally, we found that this transition is reversible in a heating-cooling cycle between 450 °C and room temperature. Notably, this reversibility is preserved only when the sample is small in size with sufficiently large surface coverage for oxygen release and reentrance. In such conditions, the large surface-to-volume ratio, coupled with the self-arrangement of oxygen diffusion channels in the SrFeO2.5, enables rapid oxygen release with a shorter annealing time needed to complete the transition, before thermodynamically driven processes set in. However, for larger samples with a low surface-to-volume ratio, the oxygen release and thus the transition rate are highly suppressed. The prolonged annealing favors the emergence of both kinetic (infinite-layer phase) and thermodynamic byproducts (decomposition into binary oxides) in the surface region. The decomposition causes the stoichiometry of the sample to deviate further, preventing the reverse transition. It suggests that the transformation product is strongly influenced by the sample dimension through the competition between kinetics and thermodynamics. This behavior aligns with previous studies using micrometer-sized powder samples, which have presented challenges in synthesizing a single-phase SrFeO2 through thermal reduction alone. Despite these challenges, our findings offer a novel avenue for achieving reversible phase transformation in infinite-layer transition metal oxides, potentially in ultrathin membrane forms.
The atomic-scale dynamic motion of iron and oxygen ions during the topotactic phase transformation toward the infinite-layer structure of SrFeO2 revealed in our study provides important piece of information in realizing the infinite-layer structure with other transition metal oxides. The idea might be applied and modified for different and undiscovered multivalent transition metal ions with similar structural framework. The study also offers insights into understanding the emergent physical properties and functionality of the infinite-layer structures, which is closely related to its low-dimensional crystal structure via structure-property relationship. For example, similar atomistic mechanism may operate in cuprate and nickelate superconductors, which are isostructural with SrFeO2 and sharing similar topotactic transformation characteristics.
References
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Tsujimoto, Y. et al. Infinite-layer iron oxide with a square-planar coordination. Nature 450, 1062–1065 (2007).
Inoue, S. et al. Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides. Nature Chemistry 2, 213–217 (2010).
Jia, C. L., Lentzen, M. & Urban, K. Atomic-resolution imaging of oxygen in perovskite ceramics. Science 299, 870–873 (2003).