Geometric frustration of Jahn–Teller order in the infinite-layer lattice

Woo Jin Kim (), Michelle A. Smeaton, Chunjing Jia, Berit H. Goodge, Byeong-Gwan Cho, Kyuho Lee, Motoki Osada, Daniel Jost, Anton V. Ievlev, Brian Moritz, Lena F. Kourkoutis, Thomas P. Devereaux and Harold Y. Hwang ()
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Woo Jin Kim: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Michelle A. Smeaton: Cornell University
Chunjing Jia: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Berit H. Goodge: Cornell University
Byeong-Gwan Cho: Pohang Accelerator Laboratory, POSTECH
Kyuho Lee: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Motoki Osada: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Daniel Jost: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Anton V. Ievlev: Center for Nanophase Materials Sciences Oak Ridge National Laboratory
Brian Moritz: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Lena F. Kourkoutis: Cornell University
Thomas P. Devereaux: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Harold Y. Hwang: Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory

Nature, 2023, vol. 615, issue 7951, 237-243

Abstract: 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 $$2\sqrt{2}\times 2\sqrt{2}\times 1$$ 2 2 × 2 2 × 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.

Date: 2023
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DOI: 10.1038/s41586-022-05681-2

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