Geometric frustration of Jahn-Teller order and ice rules in the infinite-layer cobaltate

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, plays a key role in many symmetry-lowering crystal deformations. While many examples occur in octahedrally or tetrahedrally coordinated transition metal oxides, due to their high orbital degeneracy, this effect has yet to be manifested for square-planar coordination, as found in infinite layer Cu, Ni, and Fe oxides. Within this quasi-2D structure, how orbital degeneracy would affect the crystal and electronic structure is an open experimental question. To answer this, we stabilized a new type of infinite-layer cobaltate which has orbital degeneracy within square-planar coordination. We observe a dramatically distorted infinite layer structure, with Angstrom-scale displacements of the cations from their high-symmetry positions. The details of crystalline structure of infinite-layer cobaltate and its possible spin state will be discussed.