Abstract
The recent discovery of superconductivity in La3Ni2O7−δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2,3,4,5,6 and theoretical efforts7,8,9,10,11,12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La3Ni2O7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.
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Data availability
The raw 4D-STEM data presented in the main text are available in Zenodo at https://doi.org/10.5281/zenodo.10947322 (ref. 60).
Code availability
The code for multislice electron ptychography is adapted from the previously published versions in Zenodo at https://doi.org/10.5281/zenodo.4659690 (ref. 61).
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Acknowledgements
We thank G.-M. Zhang, Y. Peng and S. Zhang for helpful discussions, and T. Zhang for assistance during the experiments. This work was supported by the Basic Science Center Project of NSFC (no. 52388201), the National Key Research and Development Program of China (MOST) (grant nos. 2023YFA1406400, 2023YFA1406500, 2022YFA1403000 and 2023YFA1406002), the National Natural Science Foundation of China (grant nos. U22A6005, 52273227, 12174454, 12274207 and 52250402), the Innovation Program for Quantum Science and Technology (no. 2021ZD0302502), the Basic and Applied Basic Research Major Programme of Guangdong Province, China (grant nos. 2021B0301030003 and 2021B1515120015), the Guangzhou Basic and Applied Basic Research Funds (grant no. 2024A04J6417) and the Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices (grant no. 2022B1212010008). 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.
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Z.C. and Y.W. initiated and supervised the research. Z.D. performed the simulations and the STEM experiments. P.L. contributed to the setup of energy-filtered 4D-STEM from K3 camera. L.G. contributed to electron microscopy and discussions. M.H., J.Y.L., H.S. and M.W. performed the single crystal growth and transport measurements. J.L. and Y.L. performed the DFT calculations. Z.D., Y.W. and Z.C. wrote the paper with inputs from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Comparison of experimental results utilizing HAADF, iDPC, ABF and MEP.
a-d, Experimental imaging results on the same region from La3Ni2O7-δ, by HAADF (a), iDPC (b), ABF (c), and MEP (d), respectively. Consistent to the simulation results in the main text, the oxygen atoms are only visible in iDPC, ABF, and MEP results. However, the oxygen atoms in the iDPC and ABF images are blurry and thus difficult to quantify. Only in the MEP image are the oxygen atoms captured clearly.
Extended Data Fig. 2 Statistics for Ni-O bond lengths.
a, Illustration of the definition of three inequivalent bond lengths, d1, d2, and d3, corresponding to the distance between Ni and planar O, outer apical O, and inner apical O, respectively. b-d, Statistics of Ni-O bond lengths for the three regions in the main text, namely, regions with δ = 0.04 (b), 0.17 (c) and 0.34 (d). The mean values and standard deviations of bond lengths are shown in legends. d2 increases as the concentration of oxygen vacancy increases, while d1 remains nearly constant. The variation in d3 is mainly attributed to the randomness on inner apical sites.
Extended Data Fig. 3 Details of the large-area O-K edge EELS map.
a, STEM-HAADF image displaying the sample geometry near the region we have performed EELS mapping. b, The same prepeak map as Fig. 3e. The black, blue, and red rectangles select representative regions with strong, medium, and weak prepeak. c, Average O-K EELS for the previously selected regions with the same color. The variation in prepeak intensity is considerable from our EELS results.
Extended Data Fig. 4 O-p orbital resolved band structure and EELS spectra from DFT + U calculations.
a-b, DFT + U calculated band structure for La3Ni2O7, including the contributions from different O-p orbitals, for U = 0 (a) and U = 3.5 eV (b) respectively. Different colors represent the contributions from inequivalent oxygen sites within each unit cell. c, Calculated O-p projected DOS for different O sites and different values of U. The inclusion of U shifts the filled bands further below the Fermi energy (highlighted with black dashed arrows), while leaving the empty bands nearly unchanged. d, Calculated O-K edge absorption spectra for different O sites and different values of U, which is Gaussian blurred according to the experimental energy resolution ΔE = 0.6 eV. The spectra are insensitive to U, since the empty bands above Fermi energy is barely modified. For the inner apical O, the prepeak originates mostly from O-pz states in the upper antibonding γa band. For the outer apical O, while the low-energy unoccupied states are also dominated by the pz orbital, its weight is much smaller than the inner apical one. For the planar O, the prepeak is governed by the O-px,y states from the α and β bands mainly of dx2-y2 character.
Extended Data Fig. 5 Transport characterization of the single crystal.
a, Resistance of the La3Ni2O7-δ single crystal versus temperature under ambient pressure. b, Resistance of the single crystal La3Ni2O7-δ versus temperature under high pressure at 22.4 GPa, exhibiting a superconducting transition.
Extended Data Fig. 6 Energy filtering of 4D-STEM dataset on a clean sample surface with a thickness of ~ 20 nm.
a, Low energy-loss EELS on the examined sample region. The inelastic scattering mainly stems from plasmonic scattering, which constitutes ~20% of the total scattered signal in this case. b-c, One of the diffraction patterns from our 4D-STEM datasets that is filtered (b) or unfiltered (c), respectively. The filtered dataset is less blurred than the unfiltered one. Scale bars, 10 nm−1. d-e, MEP reconstruction results employing the filtered 4D-STEM data (d) and the unfiltered 4D-STEM data (e). The oxygen atoms in the filtered image are sharper compared to the unfiltered ones, which enables a better quantification for oxygen contents. Scale bars, 4 Å.
Extended Data Fig. 7 Statistics for La atoms and the estimation for uncertainty.
a, Projected MEP image presented in the gray scale. b, Linecut along the blue curve in a. The peaks correspond to La atoms, which is assumed to be free from defects. The spatial fluctuation along the line represents the uncertainty in phase. c-e, Histogram for the phases of La and outer apical O from the region with δ = 0.04 ± 0.11 (c), δ = 0.17 ± 0.12 (d) and δ = 0.34 ± 0.22 (e), respectively. The phase values are normalized to the average phase of La atoms, as described in Methods, with the mean value and standard deviation labeled in each panel. The phases of outer apical O are identical within experimental uncertainty, so we can safely neglect the vacancies on this site. Besides, the estimated uncertainty in phase is around 7% for O and around 4% for La.
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Dong, Z., Huo, M., Li, J. et al. Visualization of oxygen vacancies and self-doped ligand holes in La3Ni2O7−δ. Nature 630, 847–852 (2024). https://doi.org/10.1038/s41586-024-07482-1
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DOI: https://doi.org/10.1038/s41586-024-07482-1
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