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 octahedrally4 or tetrahedrally5 coordinated 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, iron10,11 and manganese oxides12. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the brownmillerite CaCoO2.5 phase. 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 the dxz and dyz orbitals in the d7 electronic configuration along with substantial ligand–transition metal mixing. A complex pattern of distortions arises in a 22×22×1 tetragonal supercell, reflecting the competition between an ordered Jahn–Teller effect on the CoO2 sublattice 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 CaCoO2 structure forms an extended two-in–two-out type of Co distortion following ‘ice rules’13.
| Original language | English |
|---|---|
| Pages (from-to) | 237-243 |
| Number of pages | 7 |
| Journal | Nature |
| Volume | 615 |
| Issue number | 7951 |
| DOIs | |
| State | Published - Mar 9 2023 |
Funding
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. 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.