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Monitoring the formation of infinite-layer transition metal oxides through in situ atomic-resolution electron microscopy

Nature Chemistry (2024)Cite this article

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Abstract

Infinite-layer transition metal oxides with two-dimensional oxygen coordination exhibit intriguing electronic and magnetic properties due to strong in-plane orbital hybridization. The synthesis of this distinctive structure has primarily relied on kinetically controlled reduction of oxygen-rich phases featuring three-dimensional polyhedral oxygen coordination. Here, using in situ atomic-resolution electron microscopy, we scrutinize the intricate atomic-scale mechanisms of oxygen conduction leading to the transformation of SrFeO2.5 to infinite-layer SrFeO2. The oxygen release is highly anisotropic and governed by the lattice reorientation aligning the fast diffusion channels towards the outlet, which is facilitated by cooperative yet shuffle displacements of iron and oxygen ions. Accompanied with the oxygen release, the three-dimensional to two-dimensional reconfiguration of oxygen is facilitated by the lattice flexibility of FeOx polyhedral layers, adopting multiple discrete transient states following the sequence determined by the least energy-costing pathways. Similar transformation mechanism may operate in cuprate and nickelate superconductors, which are isostructural with SrFeO2.

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Fig. 1: Structural evolution of the SrFeO2.5–SrFeO2 phase boundary captured by in situ HRTEM.
Fig. 2: Atomic shuffle displacement leading to 90° reconfiguration of VO channels—experiment and simulation.
Fig. 3: Atomic-scale mechanism of oxygen release and rearrangement—in situ HRTEM image analysis.
Fig. 4: Energetics of VO formation and migration determining sequence of oxygen removal and rearrangement.

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Data availability

The authors declare that all the theoretical atomic structures generated in this article have been included in Source data. All the raw HRTEM data supporting the major results are included in Supplementary Videos 1–3. Due to the heavy size, the raw format HRTEM videos are available from the corresponding authors upon request. Source data are provided with this paper.

Code availability

The commercially available VASP software is used for the DFT calculations. The simulation settings are presented in Methods. Customized Python code for image simulation is available upon reasonable request.

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Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (no. NRF-2020R1A2C2101735), Samsung Research Funding and Incubation Center of Samsung Electronics under project number SRFC-MA1702-01, Creative Materials Discovery Program (NRF-2019M3D1A1078296) and the KENTECH Research Grant (KRG2022-01-019). J.K.L. acknowledges the support of an NRF grant funded by the Korean government (NRF-2023R1A2C1003900). K.T.K. acknowledges the support of NRF grant funded by the Korean government (NRF-2022R1C1C1005168) and the Learning and Academic research institution for Master’s·PhD students, and Postdocs (LAMP) Program of the grant funded by the Ministry of Education (RS-2023-00301914). W.S.C. acknowledges the support of NRF grant funded by the Korean government (NRF-2021R1A2C2011340 and RS-2023-00220471). The TEM work at Korea Institute of Energy Technology (KENTECH) was supported by KENTECH Center for Shared Research Facilities.

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Author notes

  1. These authors contributed equally: Yaolong Xing, Inhwan Kim, Kyeong Tae Kang.

Authors and Affiliations

  1. Department of Energy Engineering, Korea Institute of Energy Technology, Naju, Korea

    Yaolong Xing, Jinho Byun & Sang Ho Oh

  2. Institute for Energy Materials and Devices, Korea Institute of Energy Technology, Naju, Korea

    Yaolong Xing, Jinho Byun & Sang Ho Oh

  3. Department of Physics, Pusan National University, Busan, Korea

    Inhwan Kim & Jaekwang Lee

  4. Department of Physics, Sungkyunkwan University, Suwon, Korea

    Kyeong Tae Kang & Woo Seok Choi

  5. Department of Physics, KNU G-LAMP Research Center, Kyungpook National University, Daegu, Korea

    Kyeong Tae Kang

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Contributions

S.H.O. conceived the project; Y.X. conducted the TEM experiment and analysis under the supervision of S.H.O.; I.K. J.B. and J.L. performed DFT calculations; K.T.K. and W.S.C. conducted thin film growth; all authors contributed to the interpretation of data and visualization of results; Y.X. and S.H.O. prepared the paper, which was reviewed and edited by all authors.

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Correspondence to Woo Seok Choi, Jaekwang Lee or Sang Ho Oh.

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Extended data

Extended Data Fig. 1 Orientation of 1-D VO channels in the TEM samples of epitaxial SrFeO2.5 (001) film.

a, X-ray θ–2θ diffraction pattern of SrFeO2.5 epitaxial thin film grown on SrTiO3 (001) substrate. b, Schematic illustration of the two orthogonal sampling directions for TEM specimens, which are the [100] (green) and the [010] (purple). c and d, Crystal structure and HRTEM image of SrFeO2.5 along the [100] and the [010] zone axis, respectively. The VO channels seen in end-on projection are indicated by yellow rectangles in the [010] sample. e and f, Schematic illustration of the orientation of VO channels in the TEM specimens prepared along the [100] and the [010] zone axis, respectively. The VO channels are aligned laterally in the [100] and normally in the [010] samples. h is the film thickness of SrFeO2.5 (001) film. l and t is the width and the thickness of TEM specimen, respectively.

Extended Data Fig. 2 Nucleation of SrFeO2 phase within SrFeO2.5 matrix.

a, Atomic structure of SrFeO2.5. The alternate stack of octahedral and tetrahedral layers is indicated by yellow and red dotted lines, respectively. b, Fluctuation of the oxygen arrangement in the tetrahedral layers of SrFeO2.5 prior to nucleation of SrFeO2 as depicted by the interruption of the red lines by yellow lines. This temporal fluctuation in the oxygen arrangement is attributed to oxygen conduction along the VO channels and escape to the vacuum. c, Time-series HRTEM images showing the nucleation of SrFeO2 phase. The nanometer-sized nucleus is indicated by elliptical circle. d, GPA strain maps of the HRTEM images shown in c. SrFeO2 phase is distinguished from SrFeO2.5 matrix as the compressively strain (blue color) in the out-of-plane maps due to the smaller lattice parameter.

Extended Data Fig. 3 Atomic and Electronic structures of SrFeO2.5 and SrFeO2.

a and b, iDPC STEM image and intensity profile of pristine SrFeO2.5 ([110]) and transformed SrFeO2 ([010]) phase, respectively. Compared with SrFeO2.5, all the apical oxygen are missing in SrFeO2 as indicated by arrows. The apical oxygen absent in SrFeO2 is delineated by yellow dashed circles in the atomic structure. c, EELS Fe-L2,3 and d, O-K edge from SrFeO2 (green), SrFeO2.5 tetrahedral layer (orange), and SrFeO2.5 octahedral layer (blue). The chemical shift of the Fe-L2,3 edge of the SrFeO2 to a lower energy ( ~ 1.5 eV from that of SrFeO2.5 octahedral layer) indicates the reduction of the iron valence to 2+ . In the O-K edges the peak a of the SrFeO2, which originates from the transition to the O 2p-Fe 3d orbital hybridized state, shows the shift to a higher energy, resulting in a narrower separation with peak b. Besides, the peak b and c of the SrFeO2, which are attributed to the transition to O 2p-Sr 4d and O 2p-Fe 4p hybridized state, respectively, exhibit a reduced intensity level compared to those of SrFeO2.5. e, Total and projected density of states (DOS) of SrFeO2.5 and SrFeO2. From the DOS, it is apparent that the conduction band position of SrFeO2 is higher by about 2 eV than that of SrFeO2.5. The large band splitting of bonding and anti-bonding state is the common feature of strong hybridization. f, Simulated EELS O-K edges of SrFeO2.5 and SrFeO2. The vertical lines mark the energy of the first peak, guiding the chemical shift between each O-K edge.

Extended Data Fig. 4 Structural characterization of SrFeO2.5 and SrFeO2.

a, HRTEM images of SrFeO2.5 [100] (left), SrFeO2.5 [010] (middle), and SrFeO2 [110] (right). b, Interlayer strontium-strontium d-spacing, dSr-Sr, and c, intralayer iron-iron distance, δFe-Fe, measured directly on the HRTEM images. The dSr-Sr and δFe-Fe maps are displayed in the same order as in the HRTEM images. In the dSr-Sr maps of both SrFeO2.5 [010] and [100], the tetrahedral and octahedral layers are distinguished by red (4.2 Å) and blue (3.7 Å) stripe signals. The dSr-Sr map of SrFeO2 exhibit uniform blue signal corresponding to 3.5 Å. Only the δFe-Fe map of SrFeO2.5 [010] where the VO channels are aligned end-on shows a chequerboard pattern of 2 Å (navy) and 3.9 Å (yellow), where the latter corresponds to the VO channels. The δFe-Fe map of SrFeO2.5 [100] and SrFeO2 [110] show a uniform 2.7 Å (green).

Extended Data Fig. 5 HRTEM image analysis showing morphological development of SrFeO2.5-SrFeO2 phase boundary and strained lattice matching – SrFeO2.5 [100] sample.

a, Time series HRTEM images during phase transformation of SrFeO2.5 [100] sample at 450 °C (Supplementary Video 1). The phase boundary overlaid on each image was determined by the dSr-Sr measurement. b, Map of interplanar spacing of the strontium layers, dSr-Sr. The atomically smooth phase boundary advances via lateral migration of multiple ledges in a step-flow like manner. The phase transformation completes within 30 s. Geometrical phase analysis (GPA) of the HRTEM images displaying: c, In-plane strain maps (εxx); d, Out-of-plane strain maps (εzz); e, Lattice rotation maps (Rxz). The two phases maintain highly commensurate lattice across the phase boundary, as seen by the small but continuous in-plane strain distribution. The contraction of the out-of-plane lattice of SrFeO2 as oxygen is removed from SrFeO2.5 is clearly visible in the out-of-plane strain map. The lattice bending over the ledges accommodates the large mismatch in the out-of-plane lattice parameters between the two phases, resulting in the opposite sign of the lattice rotation in SrFeO2.5 and SrFeO2.

Extended Data Fig. 6 Anisotropy in the phase transformation rate of SrFeO2.5 depending on the orientation of VO channels.

a, Schematic model illustrating three different routes of phase transformation of SrFeO2.5 to SrFeO2 in terms of the orientation of VO channels; transformation of SrFeO2.5 [100] with or without 90°-reorientation, and direct transformation of SrFeO2.5 [010] to SrFeO2 [110]. b, dSr-Sr measurement during phase transformation of three different cases as a function of time. SrFeO2.5 has dSr-Sr of 4.2 Å (tetrahedral layers) and SrFeO2 has dSr-Sr value of 3.5 Å. Red and blue profiles are the measurement from SrFeO2.5 [100] sample undergoing the phase transformation with (red) or without (blue) 90°-reconfiguration, respectively. Orange profile is from SrFeO2.5 [010] sample. The dSr-Sr values in the plot represent the average of measurements from the nine unit cells, with error bars indicating the standard deviation. The SrFeO2.5 [010] sample (orange) shows the highest transformation rate of 1.78 u.c. s-1. The SrFeO2.5 [100] sample without 90°-reorientation shows the lowest transformation rate of 0.64 u.c. s-1. The SrFeO2.5 [100] sample with 90°-reorientation exhibits an intermediate transformation rate of 1.05 u.c. s-1. A distinct step locating at around 3.85 Å can be recognized at blue and red profiles, indicating the existence of transient state. This step is hardly detected in the SrFeO2.5 [010] sample due to the fast transformation.

Extended Data Fig. 7 90°-reconfiguration of SrFeO2.5 [100] ahead of advancing phase boundary.

a, Time sequence HRTEM images showing the phase transformation of SrFeO2.5 [100] to SrFeO2 [110]. White dashed line in each image indicates the phase boundary. b, Magnified images of the phase boundary outlined yellow in a. The structural features inherent to the SrFeO2.5 [010] appeared as a consequence of 90°-reconfiguration, that is, the AFD rotation (β ~±7°) of FeO6 in the octahedral layers and the VO channels (δFe-Fe = 3.9 Å) in the tetrahedral layers. c, Interlayer d-spacing map of the strontium layers, dSr-Sr, for phase identification. Alternating red (tetrahedral layers) and blue (octahedral layers) stripes correspond to SrFeO2.5 while uniform blue region corresponds to SrFeO2 phase. d, Intralayer Fe-Fe distance δFe-Fe map visualizing the normally aligned VO channels. The chequerboard pattern appearing in the SrFeO2.5 [100] ahead of moving phase boundary indicates 90°-reconfiguration which brings the laterally aligned VO channels to the normally aligned one. Uniform green color far below the phase boundary indicates the pristine SrFeO2.5 [100] without reorientation.

Extended Data Fig. 8 Atomic models for 90°-reconfiguration of SrFeO2.5 by atomic shuffle displacements.

a, Twelve successive atomic configurations in the course of 90°-reconfiguration of SrFeO2.5 [100] to [010] by atomic shuffle displacements (Supplementary Video 8). Each oxygen and iron are displaced by different magnitude of displacement vector rather than single shear displacement. The two end configurations with the reaction coordinate (1) and (12) correspond to the SrFeO2.5 [100] and [010], respectively. The atomic models were simulated using the first principles nudged elastic band method by referring to the in-situ HRTEM images. b, Simulated HRTEM images of the atomic models shown in a. The simulated HRTEM images were used to validate the atomic models by cross-correlating with the experimental HRTEM images. Among them, model (1), (6) and (12) were cross-correlated with state (i), (ii) and (iii) in Fig. 2a, respectively. The XCFs higher than 0.9 were obtained for all atomic model structures. c, DFT calculation on the duration time of the twelve structural configurations represented by the reaction coordinate (1) to (12). The structural models for the reaction coordinates were determined by cross-correlation of their simulated HRTEM images with experimental HRTEM images. The reaction coordinate (6) which corresponds to the transient state (ii) stays the longest time. d, Energy landscape (blue) along the reaction coordinates. The orange profile represents the probability of each coordinate in Boltzmann distribution (Methods).

Extended Data Fig. 9 Atomic-scale mechanism of oxygen release and rearrangement – Atomic models from DFT calculation.

a, Atomic structures obtained by DFT calculation (same as in Fig. 3d) corresponding to the transient states observed by in-situ HRTEM during one unit event of phase transformation. b, Simulated HRTEM images using the atomic structures in a. AFD rotation and polar distortion arises in (i) and (ii) state, respectively, indicated by arrows. c, SrO column intensity measurement of all four SrO layers in b. The intensity values in the plot represent the average of measurements from the lateral four SrO columns, with error bars indicating the standard deviation. d, Intensity measurement of each atomic site in tetrahedral layer. Red and blue data points represent the intensity measured at the VO channels and FeO columns, respectively. The intensity variation measured on the simulated HRTEM images match well the experimentally measured one in Fig. 3. e, Tracing of the intralayer Fe-Fe distance (δFe-Fe, defined in b and Fig. 3a) across VO channel with time in experimental data. f, Tracing of the strontium interlayer d-spacing (dSr-Sr, defined in b and Fig. 3a) with time in experimental data (Supplementary Video 11). The measured average values are indicated by black dashed lines, which distinguish the four distinct steps with different duration time. The dSr-Sr measured from the corresponding atomic models simulated by DFT (brown dashed lines) are added for comparison.

Extended Data Fig. 10 Trajectory analysis of atom movements during the phase transformation.

a, Atomic models obtained by DFT and b, HRTEM images captured during the phase boundary migration. These four sets of the atom configuration are the same as those presented in Fig. 3 of the main text. c, Atomic trajectory analysis of atoms of outlined region in b, during the phase transformation. d, Enlarged view of the atomic trajectory of outlined region in c. The “reference” atom marked in c is used to correct the drift, so that its motion trace is minimal. Trajectory was color-corded from the reference (t0) to 4.4 s. The trajectory map reveals the highly discrete nature of strontium, iron, oxygen atom movements during the phase boundary migration from (i) to (iv), characterized by sudden jump of atom position rather than diffusive motion. Such discrete atomic motions involving a large amount of sudden displacement along a specific direction are accounted for by the oxygen conduction via changes in the oxygen coordination around iron atoms within FeOx polyhedral layers.

Supplementary information

Supplementary Information

Supplementary Figs. 1–13, Tables 1 and 2 and References.

Supplementary Video 1

HRTEM video showing the phase transformation from SrFeO2.5 [100] to SrFeO2 [110] at 450 °C. NCSI HRTEM video showing the migration of the phase boundary at 450 °C. The SrFeO2 phase initiates from the film surface and grows towards the interface with the SrTiO3 substrate as the phase boundary advances. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged.

Supplementary Video 2

HRTEM video showing the phase transformation from SrFeO2.5 [010] to SrFeO2 [110] at 450 °C. NCSI HRTEM video showing the migration of the phase boundary at 450 °C. The SrFeO2 phase initiates from the interface with the SrTiO3 substrate and grows towards the film surface as the phase boundary advances. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged.

Supplementary Video 3

HRTEM video showing the nucleation of SrFeO2 at 450 °C. NCSI HRTEM video showing the nucleation stage of the phase transformation from SrFeO2.5 [100] to SrFeO2 [110]. Before the nucleation of SrFeO2 phase, the oxygen configuration in the tetrahedral layers of SrFeO2.5 fluctuates, occasionally exhibiting the contrast similar to the octahedral layers. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged.

Supplementary Video 4

HRTEM video showing the lattice dynamics before the phase transformation. NCSI HRTEM video exhibiting the atomistic details of the phase transformation from SrFeO2.5 [100] to SrFeO2 [110]. Before the phase transformation, active lattice dynamics, such as lateral vibrations of atoms in tetrahedral layers and reorientation of Vo channels, are observed in SrFeO2.5. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged.

Supplementary Video 5

Magnified HRTEM video showing the 90° reorientation of VO channels. NCSI HRTEM video showing the 90° reorientation of VO channels before the phase transformation of SrFeO2.5 [100] sample. Initially, the iron and equatorial oxygen in octahedral layers are almost coplanar, and the iron in tetrahedral layers remains isometric. As the reorientation progresses, AFD rotation evolves in the octahedral layers, with a rotation angle β of approximately ±7˚, and the intralayer iron distance, δFe–Fe, in the tetrahedral layers alternates between 2.0 and 3.9 Å. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged.

Supplementary Video 6

Iron intralayer distance traced during 90° reorientation of VO channels. The intralayer iron–iron distance, δFe–Fe, is measured on the two adjacent unit cells in Supplementary Video 6. The initially uniform δFe–Fe values undergo a split into smaller (δFe–Fe_1) and larger (δFe–Fe_2) values through a transient state (ii), with the latter corresponding to the vertically aligned VO channel.

Supplementary Video 7

Dichromatic pattern showing 90° reorientation of VO channels by atomic shuffle displacement. Animated schematic illustrates the reorientation of VO channels situated between tetrahedral chains through atomic shuffle displacement, which is viewed along the [001] direction. Initially, the positions of iron and oxygen are represented by open circles and open squares, respectively, before the reorientation. After the reorientation, the positions of iron and oxygen are indicated by filled pink circles and squares, respectively. The VO channels are depicted by large open arrows. At the end of the animation, the shuffle displacement of each atom is defined by overlapping the two atomic arrangements and connecting the position of each atom.

Supplementary Video 8

DFT simulated sequential atomic models showing the 90° reorientation of VO channels by atomic shuffle displacement. On the left, animated atomic model shows each reaction coordinate during the atomic shuffle displacement for the 90° reorientation of VO channels, simulated using DFT. On the right, the corresponding HRTEM image simulations are presented.

Supplementary Video 9

High-magnification HRTEM video showing the atomic-scale mechanism of phase transformation. The HRTEM video captures the phase transformation starting from the reoriented SrFeO2.5 phase. To enhance the contrast of the original video, which was recorded at 25 fps, the three successive frames were averaged. Additionally, for clarity, the original video has been slowed down by 4 times.

Supplementary Video 10

DFT-simulated sequential atomic models showing atomic phase transformation mechanism. On the left, the animated atomic model shows the transient states with different polyhedral configurations observed by in situ HRTEM during one unit event of phase transformation, simulated by DFT. On the right, the corresponding HRTEM image simulations are presented.

Supplementary Video 11

Stepwise phase transformation visualized by tracing the strontium interlayer d-spacing during phase transformation. The interlayer strontium d-spacing, dSr–Sr, is traced throughout the phase transformation process. The dashed lines represent the measured average values, accompanied by a standard deviation of approximately ±0.001 Å. These lines delineate the four distinct steps, each lasting on the level of second. Additionally, the dSr–Sr values obtained from corresponding atomic models simulated by DFT are depicted as dashed brown lines.

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Xing, Y., Kim, I., Kang, K.T. et al. Monitoring the formation of infinite-layer transition metal oxides through in situ atomic-resolution electron microscopy. Nat. Chem. (2024). https://doi.org/10.1038/s41557-024-01617-7

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