Abstract
The Jahn–Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn–Teller ions can induce a cooperative distortion, as exemplified by LaMnO3(refs.2,3). Although many examples occur in octahedrally4or tetrahedrally5coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar anion coordination, as found in infinite-layer copper6,7, nickel8,9, iron10,11and manganese oxides12. Here we synthesize single-crystal CaCoO2thin films by topotactic reduction of the brownmillerite CaCoO2.5phase. We observe a markedly distorted infinite-layer structure, with ångström-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn–Teller degeneracy of thedxzanddyzorbitals in thed7electronic configuration along with substantial ligand–transition metal mixing. A complex pattern of distortions arises in a\(2\sqrt{2}\times 2\sqrt{2}\times 1\)tetragonal supercell, reflecting the competition between an ordered Jahn–Teller effect on the CoO2sublattice and the geometric frustration of the associated displacements of the Ca sublattice, which are strongly coupled in the absence of apical oxygen. As a result of this competition, the CaCoO2structure forms an extended two-in–two-out type of Co distortion following ‘ice rules’13.
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The data presented in the figures and other findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We thank W.-S. Lee for discussions. The work at SLAC and Stanford was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (contract number DE-AC02-76SF00515) and the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative (grant number GBMF9072, synthesis equipment and initial development). Electron microscopy at Cornell was support by the Department of Defense Air Force Office of Scientific Research (number FA 9550-16-1-0305) and the Packard Foundation, and made use of the Cornell Center for Materials Research Shared Facilities which are supported through the NSF MRSEC programme (DMR-1719875), with the Thermo Fisher Helios G4 UX focused ion beam also supported by NSF (DMR-1539918). The Thermo Fisher Spectra 300 X-CFEG was acquired with support from PARADIM, an NSF MIP (DMR-2039380), and Cornell University. M.A.S. acknowledges additional support from the NSF GRFP under award number DGE-1650441. The 3A beamline at PLS-II is supported in part by MSIT. B.-G.C. is currently affiliated to Korea Research Institute of Standards and Science (KRISS). D.J. acknowledges funding by the Alexander-von-Humboldt foundation via a Feodor Lynen postdoctoral fellowship. Raman spectroscopy measurement was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-2026822. TOF-SIMS characterization was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, and using instrumentation within ORNL’s Materials Characterization Core provided by UT-Battelle, LLC under contract number DE-AC05-00OR22725. The computational work for this project was performed on the Sherlock cluster in the Stanford Research Computing Center.
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W.J.K. and H.Y.H. conceived and designed the experiments. M.A.S., B.H.G. and L.F.K. performed the STEM and EELS measurements and analysis. C.J. performed the DFT calculations. C.J., B.M. and T.P.D. performed the cluster calculations. D.J. performed Raman spectroscopy measurements. W.J.K. grew the samples, which were characterized by W.J.K., K.L., D.J. and M.O. W.J.K. and B.-G.C. performed and analysed the synchrotron GIXRD measurements. A.V.I. performed TOF-SIMS measurements. W.J.K., T.P.D. and H.Y.H. wrote the manuscript, with input from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Structural characterizations for CaCoO2.5and CaCoO2.
a, Atomic-resolution HAADF-STEM image along the [100]tzone-axis projection of CaCoO2.5showing alternate stacking of tetrahedral and octahedral layers.b, X-ray diffraction reciprocal space map of CaCoO2around the (−103) SrTiO3diffraction peak, indicating that the film is relaxed from the substrate.c, Empirical relationship between perovskite and infinite-layer lattice parameters.c-axis lattice parameters for various transition metal oxide compounds are plotted for the perovskite phase and the infinite-layer phase after topotactic reduction. The dashed line is a linear fit for all the data points in the plot. Note that the CaFeO2有相对较大的cinfinite layer/cperovskiteassociate with out-of-plane displacement of both FeO4and Ca layers48.
Extended Data Fig. 2 EELS measurements of CaCoO2.
a, Co-L3,2edge; the blue (red) solid line indicates EEL spectra for CaCoO2.5(CaCoO2).b, Ca-L3,2edge EELS shows that there are no substantial changes in the spectra before (CaCoO2.5, blue) and after reduction (CaCoO2, red).c, A plot of the intensity ratioI(L3)/I(L2) of the Co-L3,2edge for different Co compounds with different oxidation states. Note that the dashed line indicates a polynomial fit curve for four different compounds from ref.27(CoCO3, CoSO4, Co3O4, and CoSi4).I(L3)/I(L2) of the CaCoO2.5and CaCoO2films are depicted with blue and red circles, respectively.d, O K-edges EELS data. Spatially averaged O K-edge spectra of CaCoO2(CaCoO2.5) in red (blue). The partially transparent, solid lines indicate the raw, background-subtracted data, and the dashed lines indicate the Gaussian filtered spectra. Upon reduction of the CaCoO2.5电影CaCoO2, we observe a suppression of the distinct pre-peak at ~ 529 eV in the region of the O K-edge associated with hybridization between O 2pand transition metaldorbitals consistent with a nominal electronic transition from 3d6to 3d7. This is similar to the pre-peak suppression observed upon reduction from perovskite to infinite-layer phase in the related nickelates49. We further see the emergence of a shoulder in the CaCoO2spectrum at ~ 530 eV, which is similar to a feature attributed to ligand hole states in doped infinite-layer nickelates49. This feature is also consistent with published spectra acquired from SrCoO3-δ, which has negative charge transferred state37. An O K-edge spectrum of the SrTiO3substrate is included in black for comparison.
Extended Data Fig. 3 Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and ABF-STEM measurements of CaCoO2.
a, Depth profiles of H+and other ions from both CaCoO2.5and CaCoO2thin film on SrTiO3substrate (with ~ 2 nm SrTiO3capping layer) were measured with secondary-ion mass spectrometry. The Co ion signals from both CaCoO2.5and CaCoO2thin films were employed as a marker for the interface position. TOF-SIMS measurements show that the H+concentration for CaCoO2is similar to the background level of the as-synthesized CaCoO2.5thin film.b, ABF-STEM image along the [100]tzone-axis projection with overlaid Co, Ca, and O atoms.c, Intensity line profiles for the blue and the orange dashed lines inb. The intensities of the line profiles are from inverted imageb. The blue (orange) solid line indicates the line profile for the Co column (Ca and O column). The peak positions are the relative distances noted at the bottom of the imageb.d, Atomic distances between Ca and Ca (black triangles), Co and Co (green diamonds), Ca and O (red squares), and Ca and Co (blue circles) layers are plotted. Note that the atomic layer numbers indcorrespond to those inb. Error bars are taken as the full-width at half-maximum of the intensity peaks inc.
Extended Data Fig. 4 Powder XRD simulation andc-lattice parameter determination.
Lattice structure models fora, 2\(\sqrt{2}\)at\(\times \)2\(\sqrt{2}\)at\(\times \)ctandb, 2\(\sqrt{2}\)at\(\times \)2\(\sqrt{2}\)at\(\times \)2ct. The second structure model is lattice doubled from the first model by stacking a half-unit-cell shifted layer along the in-plane direction. Powder XRD simulation results for bothc, 2\(\sqrt{2}\)at\(\times \)2\(\sqrt{2}\)at\(\times \)ctandd, 2\(\sqrt{2}\)at\(\times \)2\(\sqrt{2}\)at\(\times \)2ctmodels. Note that the XRD simulation for the 2\(\sqrt{2}\)at\(\times \)2\(\sqrt{2}\)at\(\times \)2ctmodel has a distinct half-order peak along thec-lattice direction. We first founde, the CaCoO2(103)tXRD peak as a reference peak. Based on this reference peak position, we performθ–2θscans along the expected CaCoO2(0.75, 0.25, 0.5) position.f, No XRD peak was observed at the expected CaCoO2(0.75, 0.25, 0.5) peak position, indicating that CaCoO2does not have ac-axis doubling of the simple tetragonal unit cell.
Extended Data Fig. 5 DFT calculations for CaCoO2.
a, Plan-view of the relaxed crystal structure for CaCoO2from DFT + U calculations with U = 2 eV, U = 3 eV, U = 4 eV, U = 5 eV, and U = 6 eV.b, Calculated band dispersion of CaCoO2(DFT + U for U = 5 eV). Green highlightsdxz(anddyz) projections. The inset shows high-symmetry points in the tetragonal Brillouin zone.c, Resistivity versus temperature of CaCoO2thin film. The inset shows that the resistivity is well fitted with an Arrhenius plot with an estimated (transport) gap of 0.337+0.001 eV. The spin-dependent partial density of states (PDOS) ofd, Co(2)ande, Co(3)dorbitals from DFT + U (U = 5 eV). The spin-dependent PDOS of Co(2)shows the degeneracy lifting of thedxz/yx轨道。
Extended Data Fig. 6 Total energy calculation for CaCoO2with\(\sqrt{2}\times \sqrt{2}\times 1\)and\(2\sqrt{2}\times 2\sqrt{2}\times 1\)supercell.
a, DFT+U (U = 5 eV) calculations for the total energy under purely Q2-JT-distortions in the\(\sqrt{2}\times \sqrt{2}\times 1\)supercell.b,\(2\sqrt{2}\times 2\sqrt{2}\times 1\)supercell with different distortion amplitudes. Approaching #10, the structure is approaches the experimentally refined structure.c, Normalized total energy for the structures depicted inb. Three different first-principle calculations are used forc(Methods).
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Kim, W.J., Smeaton, M.A., Jia, C.et al.Geometric frustration of Jahn–Teller order in the infinite-layer lattice.Nature615, 237–243 (2023). https://doi.org/10.1038/s41586-022-05681-2
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DOI:https://doi.org/10.1038/s41586-022-05681-2
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