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Interstitial oxygen order and its competition with superconductivity in La2PrNi2O7+δ

Nature Materials volume 24, pages 1927–1934 (2025)Cite this article

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Abstract

High-temperature superconductivity in pressurized La3Ni2O7 has attracted considerable interest, yet the superconducting phase is rather fragile. Although bulk superconductivity can be achieved by Pr substitution for La, the underlying mechanism is still unclear. A further puzzle is the role of oxygen content: moderate oxygenation enhances superconductivity, whereas high-pressure oxygen annealing suppresses it. Here combining multislice electron ptychography and electron energy-loss spectroscopy, we show that Pr doping mitigates oxygen vacancies and stabilizes a near-stoichiometric La2PrNi2O7 structure. Strikingly, high-pressure oxygen annealing introduces interstitial oxygen atoms that arrange into a stripe-ordered superstructure, which generates excess hole carriers and alters the electronic structure, ultimately suppressing superconductivity under pressure. This contrasts sharply with cuprates, where similar oxygen ordering is known to induce superconductivity. Our findings reveal a competition between interstitial oxygen ordering and superconductivity in bilayer nickelates, providing key insights into the pairing mechanism and guiding principles for engineering more robust superconducting phases.

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Fig. 1: Characterization of as-grown and HPO-annealed polycrystalline samples.
Fig. 2: Atomic structure of the as-grown sample.
Fig. 3: Interstitial oxygen orders in the HPO-annealed sample.
Fig. 4: Phase separation and hole doping from interstitial oxygen order.

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Data availability

The raw data presented in this study are available via Zenodo at https://doi.org/10.5281/zenodo.15666415 (ref. 69) and from the corresponding authors upon reasonable request.

Code availability

The MEP analyses in this study were performed using a publicly available open-source software, fold_slice30, available via GitHub at https://github.com/yijiang1/fold_slice.

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Acknowledgements

This work was supported by the Basic Science Center Project of National Natural Science Foundation of China (NSFC) (number 52388201), the Innovation Program for Quantum Science and Technology (grant number 2021ZD0302502), the NSFC Grant (numbers U22A6005, 52273227, 12025408 and 12404179), the National Key Research and Development Program of Ministry of Science and Technology (grant numbers 2023YFA1406400, 2022YFA1405100 and 2023YFA1406100), and Guangdong Major Project of Basic Research, China (grant number 2021B0301030003). Z.D. acknowledges support from the NSFC’s Young Students Program (grant number 124B2068). Y.W. is partially supported by the New Cornerstone Science Foundation through the New Cornerstone Investigator Program and the XPLORER PRIZE. This work used the facilities of the National Center for Electron Microscopy in Beijing at Tsinghua University, Beijing Laboratory of Electron Microscopy at the Institute of Physics, and the Cubic Anvil Cell Station of the Synergetic Extreme Condition User Facility (SECUF; https://cstr.cn/31123.02.SECUF).

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  1. These authors contributed equally: Zehao Dong, Gang Wang, Ningning Wang, Wen-Han Dong.

Authors and Affiliations

  1. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing, China

    Zehao Dong, Wen-Han Dong, Yong Xu & Yayu Wang

  2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China

    Gang Wang, Ningning Wang, Jinguang Cheng & Zhen Chen

  3. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China

    Gang Wang, Ningning Wang, Jinguang Cheng & Zhen Chen

  4. School of Materials Science and Engineering, Tsinghua University, Beijing, China

    Lin Gu

  5. New Cornerstone Science Laboratory, Frontier Science Center for Quantum Information, Beijing, China

    Yong Xu & Yayu Wang

  6. Hefei National Laboratory, Hefei, China

    Yayu Wang

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Contributions

Y.W., Z.C. and J.C. proposed and supervised the research. Z.D. performed the STEM and EELS experiments and processed the data. G.W. and N.W. synthesized the polycrystalline samples and performed the XRD and transport measurements. W.-H.D. and Y.X. performed the DFT calculations. L.G. contributed to the electron microscopy facilities. Z.D., Z.C. and Y.W. wrote the manuscript. All authors discussed the results and implications throughout the investigation and have given approval to the final version of the manuscript.

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Correspondence to Jinguang Cheng, Zhen Chen or Yayu Wang.

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Extended data

Extended Data Fig. 1 Rietveld refinement of the XRD pattern for the HPO sample.

a-b, Sample is modeled as a two-phase mixture of the pristine Amam phase and the interstitial-oxygen-ordered B2em phase, as established in the main text and Methods. Experimental data are shown as red circles, the calculated pattern as a black line, and the difference curve as a blue line at the bottom. Vertical tick marks indicate the Bragg reflection positions for the B2em phase (upper ticks) and the Amam phase (lower ticks). Panel a displays the full diffraction pattern within the angle range of 20° and 90°. Panel b highlights the diffraction peak (0,0,10) which clearly illustrates the coexisting phases and their respective volume fractions. Details of the refinement are displayed in Supplementary Table 1.

Extended Data Fig. 2 Elemental EELS mapping for Pr dopant distribution.

a-b, Simultaneously acquired HAADF image (left panel), Pr M5-edge intensity map (middle panel), and La M5-edge intensity map (right panel) for as-grown La2.7Pr0.3Ni2O7 (Pr0.3-AG, panel a) and another sample region from as-grown La2PrNi2O7 (Pr1-AG, panel b). Both regions exhibit the same preference for Pr dopants to substitute La atoms in the outer rock-salt LaO layer.

Extended Data Fig. 3 MEP imaging of the HPO-annealed sample along [010] axis.

a, Atomic model for HPO projected along the [010] axis. b, MEP reconstructed image of HPO along the [010] axis. Intercalated layers of interstitial oxygens are denoted by black arrows. Note that the octahedral distortions are manifested as the vertical elongation of oxygen columns upon projection.

Extended Data Fig. 4 Statistics of Ni-O-Ni bond angles.

a, MEP-reconstructed image corresponding to AG in the pristine 327 phase. b, Color-coded map of the planar Ni-O-Ni bond angles of pristine 327 phase. Each square represents a planar oxygen, with its color indicating the deviation of the bond angle from 180°. Reddish squares (negative angles) correspond to planar oxygens distorted downward, while bluish squares (positive angles) indicate upward distortions. c, Statistical histogram of planar Ni-O-Ni bond angles for the pristine 327 phase, where the colors of individual components correspond to the respective components in panel b. The average bond angles and standard deviation is annotated within the panels. d, MEP-reconstructed image corresponding to HPO in the stage-1 period-b ordered phase. e, Color-coded map of bond angles for the stage-1 period-b ordered phase. Interstitial oxygens are represented as black circles. f, Statistical histogram of bond angles for the stage-1 period-b ordered phase, where the colors of individual components correspond to the respective components in panel e. g, MEP-reconstructed image corresponding to HPO in the stage-1 period-2b ordered phase. h, Color-coded map of bond angles for the stage-1 period-2b ordered phase. Interstitial oxygens are represented as black circles. i, Statistical histogram of bond angles for the stage-1 period-2b ordered phase, where the colors of individual components correspond to the respective components in panel h.

Extended Data Fig. 5 Three-dimensional phase separation in the HPO-annealed sample.

a, Projected MEP-reconstructed phase image of HPO sample as shown in Fig. 4a. b-e, Slice images extracted within the yellow dashed rectangle in panel a, from the depth range of 11-13 nm (b), 14-16 nm (c), 17-21 nm (d), and 22-24 nm (e), respectively. Panels b and c display the stage-1 order in the period-b phase, while panel e exhibits the pristine 327 phase. Panel d shows a crossover region between these two phases where octahedral distortions are not well-defined. These results provide clear evidence of a three-dimensional phase separation.

Extended Data Fig. 6 DFT-calculated structural evolutions under pressure.

a-b, Schematics for the relaxed structures (a), lattice constants and Ni-O-Ni bond angles (b) as a function of the hydrostatic pressure for AG phase La2PrNi2O7. The structural phase transition from Amam to I4/mmm is found around 15 GPa. c-d, Schematics for the relaxed structures (c), lattice constants and Ni-O-Ni bond angles (d) as a function of the hydrostatic pressure for HPO phase La2PrNi2O7.23. No structural phase transition is observed up to 100 GPa. An effective Hubbard U = 3.5 eV was applied when optimizing the crystal structures.

Extended Data Fig. 7 Statistics of interstitial oxygen content of the HPO sample.

a, MEP-reconstructed image from Fig. 3e, with the positions of interstitial and planar oxygens highlighted as red points. b, Color-coded map showing the normalized phase values of interstitial oxygens relative to planar oxygens. c, Histogram of phase values for interstitial and planar oxygens, showing a normalized phase value of 0.23 ± 0.07 for interstitial oxygen columns relative to planar oxygens. Accordingly, the resulting oxygen hyper-stoichiometry is also estimated as δ = 0.23 ± 0.07.

Extended Data Fig. 8 DFT + U calculated electronic structures.

a-d, Projected band structures and projected density of states (PDOS) for AG phase La2PrNi2O7 at 0 GPa (a) and 30 GPa (b), HPO phase La2PrNi2O7.23 at 0 GPa (c) and 30 GPa (d), respectively. Experimental crystal structures from Supplementary Table 2 and Supplementary Table 1 were used for panels a and c, respectively. We compared the bands with those from DFT-relaxed structures at 0 GPa, and found no obvious difference. Red arrows indicate the energy of the PDOS peaks at the band edge of bonding dz2 orbitals. Black arrows denote states associated with interstitial oxygens. e-h, Two-dimensional Fermi surfaces for panels a-d, respectively. Different colors represent distinct band indices.

Extended Data Fig. 9 DFT + U simulated oxygen K-edge XAS spectra of AG and HPO phases.

a-c, XAS spectra for three different oxygen sites. Note that the XAS plays the same role as EELS in dipole approximation. The prepeak of the HPO phase (marked by red arrow) shows a reduced energy and slightly increased intensity compared to that of the AG phase (marked by black arrow), consistent with EELS measurements.

Extended Data Fig. 10 Additional information for the prepeak mapping in Fig. 4e.

a, Simultaneously acquired HAADF image for the mapping area. b, Thickness map derived from low energy-loss EELS.

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Dong, Z., Wang, G., Wang, N. et al. Interstitial oxygen order and its competition with superconductivity in La2PrNi2O7+δ. Nat. Mater. 24, 1927–1934 (2025). https://doi.org/10.1038/s41563-025-02351-2

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