Structural evolution of epitaxial SrCoOx films near topotactic phase transition

Abstract: Control of oxygen stoichiometry in complex oxides via topotactic phase transition is an interesting avenue to not only modifying the physical properties, but utilizing in many energy technologies, such as energy storage and catalysts. However, detailed structural evolution in the close proximity of the topotactic phase transition in multivalent oxides has not been much studied. In this work, we used strontium cobaltites (SrCoOx) epitaxially grown by pulsed laser epitaxy (PLE) as a model system to study the oxidation-driven evolution of the structure, electronic, and magnetic properties. We grew coherently strained SrCoO2.5 thin films and performed post-annealing at various temperatures for topotactic conversion into the perovskite phase (SrCoO3-δ). We clearly observed significant changes in electronic transport, magnetism, and microstructure near the critical temperature for the topotactic transformation from the brownmillerite to the perovskite phase. Nevertheless, the overall crystallinity was well maintained without much structural degradation, indicating that topotactic phase control can be a useful tool to control the physical properties repeatedly via redox reactions.

• The paper demonstrates that epitaxial SCO films undergo a sharp, reversible topotactic phase transition with a 4% lattice contraction upon oxygen uptake.
• The methodology employs pulsed laser epitaxy and in situ annealing under controlled oxygen pressure to maintain epitaxial coherence and reproducibility.
• Findings reveal concurrent insulator-to-metal and non-ferromagnetic to ferromagnetic transitions, highlighting the material's potential for solid-state redox applications.

Structural Evolution in Epitaxial SrCoOx Films Near Topotactic Phase Transition

Introduction

The study investigates the atomic-scale structural, electronic, and magnetic evolution in epitaxial SrCoOx (SCO) thin films across a topotactic phase transition. These perovskite-related multivalent oxides exhibit reversible oxygen intercalation/removal at relatively low temperatures, making them promising candidates for energy-related devices such as solid oxide fuel cells, batteries, and sensors. Notably, the interplay between structural rearrangement and physical properties during topotactic oxidization in such materials is not fully elucidated at the microstructural level. By employing pulsed laser epitaxy (PLE) to synthesize coherently strained SCO films and controlled post-annealing procedures, the study systematically probes the simultaneous evolution of crystal structure, conductivity, and magnetism near the brownmillerite-to-perovskite transformation boundary.

Experimental Design and Methodology

The authors synthesized 30-nm-thick epitaxial brownmillerite SrCoO$_{2.5}$ (BM-SCO) films on (001) LSAT substrates to maintain coherent strain throughout the annealing regimen. Post-annealing was performed in situ at incrementally varying temperatures under a static O$_2$ pressure (500 Torr), enabling topotactic oxidation to perovskite SrCoO$_{3-\delta}$ (P-SCO) without vacuum breaks, thus mitigating contamination and facilitating reproducibility. X-ray diffraction (XRD), DC transport (Van der Pauw geometry), and SQUID magnetometry were used to monitor crystallographic phase, resistivity, and magnetic ordering, respectively.

Structural Evolution and Crystallinity

XRD analysis reveals that BM-SCO films retain their original structure up to 175 ℃, as evidenced by sharp diffraction peaks characteristic of the brownmillerite phase and the absence of intermediate or impurity phases. Annealing above 200 ℃ triggers a rapid, direct conversion into the perovskite phase without detectable intermediate states. The transformation induces a ∼4% reduction in the out-of-plane lattice constant due to oxygen uptake, indicative of vacancy filling and concomitant lattice contraction.

The full width at half maximum (FWHM) of XRD rocking curves increases substantially at 150–175 ℃, reflecting a rise in mosaicity as the film approaches the transformation. Above 200 ℃, P-SCO films exhibit restored high crystallinity comparable to their as-grown brownmillerite counterparts, with no evidence of strain relaxation or phase segregation. This demonstrates that the topotactic redox process preserves epitaxy and microstructural order, emphasizing the robustness of oxygen-driven phase conversion.

Electronic Transport

Transport measurements show a sharp insulator-to-metal transition coincident with the onset of the perovskite phase at annealing temperatures above 200 ℃. Films annealed below this threshold display reduced resistivity but retain insulating character, while those processed at ≥200 ℃ demonstrate metallicity consistent with full oxidation and the presence of Co$^{4+}$. The thermal activation energy $E_a$ in insulating phases decreases markedly with increasing annealing temperature, dropping to 140 meV for strained BM-SCO/LSAT (lower than in bulk or other substrate configurations) and falling dramatically at the insulator-metal boundary. The intermediacy of the sample annealed at 175 ℃ (with stoichiometry x ≈ 2.75) correlates with a significantly suppressed activation barrier, underscoring the mobility enhancement via partial oxygen intercalation.

Of note, films annealed at the highest temperatures (400 ℃) show upturned resistivity, implying oxygen loss via thermal desorption and the onset of oxygen vacancy defect formation even under a high O$_2$ background.

Magnetic Properties

Ferromagnetism emerges abruptly in samples annealed at 200 ℃ and above, paralleling the perovskite metallic phase. Lower temperature annealing (≤175 ℃) leaves the material in a non-ferromagnetic, insulating state, despite significant oxygen intercalation and increased conductivity. The magnetization of fully oxidized films is slightly suppressed relative to ozone-grown P-SCO analogues, likely due to subtle nonstoichiometry or residual defects. Films annealed at 400 ℃ exhibit reduced ferromagnetic moment, mirroring the increased resistivity and probable oxygen depletion.

Implications and Perspectives

The study demonstrates that epitaxial SCO thin films maintain structural coherence and high crystallinity through reversible, low-temperature topotactic phase transformations. This robust transformation is coupled to dramatic changes in electronic and magnetic ground states, permitting precise, repeatable control over physical properties without permanent structural damage or secondary phase formation. The low activation barriers and rapid switching characteristics highlight the practical utility of these oxygen sponge materials for solid-state redox devices. Moreover, the unique preservation of crystallinity through substantial stoichiometric modulation positions these systems as potential platforms for nonvolatile memory, solid oxide electrolytes, or active catalytic layers where repeated redox cycling is required.

The findings open avenues to further atomistic studies (e.g., via in situ STEM/EELS or operando spectroscopies) of topotactic redox kinetics and to the design of complex oxide heterostructures with switchable functionalities. Understanding how strain, dimensionality, and cationic ordering influence transformation kinetics and critical temperatures in other transition metal oxides could yield significant advances in both fundamental materials physics and applied device technologies.

Conclusion

This work provides a comprehensive examination of the microstructural, electronic, and magnetic transitions in coherently strained SrCoOx thin films across a low-temperature topotactic phase boundary. The reversible and robust transformation between electronically and magnetically distinct phases, occurring with minimal crystallographic degradation, highlights the utility of topotactic redox processes for controlling multifunctionality in complex oxides. These results provide an essential framework for future development of oxide-based energy storage/conversion devices and functional materials engineering.