Most materials around us retain their properties once formed, but SrFeO_x (strontium iron oxide) is an exception. This material drastically changes its properties depending on the amount of oxygen it contains — with x ranging from 2 to 3 — making it a highly intriguing subject of study. Professor Woo Seok Choi’s research team in the Department of Physics has developed this material in the form of thin single-crystal films and reported a series of significant findings.

For instance, SrFeO_2.5 adopts a layered structure called brownmillerite, which forms when oxygen is partially removed from SrFeO₃, a material with a perovskite structure. The resulting structure features alternating layers of FeO₆ octahedra and FeO₄ tetrahedra and exhibits a distinctive electrical polarity — that is, a directional asymmetry. In this study, it was revealed that this structure shows ferroelectricity even at ultra-thin scales, down to a single atomic layer (as shown in the Figure below, see the first reference). Ferroelectricity is a key physical property for applications in memory and energy devices. This ferroelectric behavior originates solely in the FeO₄ tetrahedral layers, while the FeO₆ layers in between act like insulating spacers, minimizing interference between neighboring layers. As a result, each thin layer can switch its electric polarization independently — like stacking atomically thin electric switches — opening up exciting possibilities for next-generation ultra-compact memory devices.

Meanwhile, further reducing the oxygen in SrFeO_2.5 produces SrFeO₂, which has a completely different structure known as the infinite-layer structure, where iron atoms are surrounded by oxygen only in two dimensions (see the second reference). This transformation is achieved by high-temperature treatment to remove oxygen atoms. In this study, researchers used real-time electron microscopy to directly observe how oxygen atoms escape along the layers, how iron atoms rearrange, and how the structure evolves step by step — all at the atomic scale. Surprisingly, this transformation happens very quickly, and the rate of change varies depending on the orientation of oxygen diffusion channels. In fact, the crystal structure can even rotate 90 degrees to align these channels in a direction that facilitates easier oxygen release.

Thanks to this flexible yet precisely controlled structural behavior, SrFeO_x is not only electrically active but also holds great potential for future optoelectronic devices involving magnetism, electrical conductivity, and even superconductivity. In essence, SrFeO_x is an oxygen-tunable material whose structure and properties can be dramatically altered by adjusting its oxygen content — offering a key to creating faster, smaller, and more efficient electronic components in the near future.

Reference

[1] Sub-unit-cell-segmented ferroelectricity in brownmillerite oxides by phonon decoupling, Nat. Mater. https://doi.org/10.1038/s41563-025-02233-7 (2025).

[2] Monitoring the formation of infinite-layer transition metal oxides through in situ

atomic-resolution electron microscopy, Nat. Chem. https://doi.org/10.1038/s41557-024-01617-7 (2025).