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. 1-3 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 highsymmetry positions. The details of crystalline structure of infinite-layer cobaltate and its possible electronic ground state will be discussed.

[1] T. Siegrist et al., Nature 334, 231–232 (1988).

[2] M. A. Hayward, et al., J. Am. Chem. Soc. 121, 8843–8854 (1999).

[3] Y. Tsujimoto et al., Nature 450, 1062–1065 (2007).