Interplay between Jahn-Teller instability, uniaxial magnetism, and ferroelectricity in Ca3CoMnO6

Ca3CoMnO6 is composed of CoMnO6 chains made up of face-sharing CoO6 trigonal prisms and MnO6 octahedra. The structural, magnetic, and ferroelectric properties of this compound were investigated on the basis of density-functional theory calculations. Ca3CoMnO6 is found to undergo a Jahn-Teller distortion associated with the CoO6 trigonal prisms containing high-spin Co2+ (d(7)) ions, which removes the C-3 rotational symmetry and hence uniaxial magnetism. However, the Jahn-Teller distortion is not strong enough to fully quench the orbital moment of the high-spin Co2+ ions thereby leading to an electronic state with substantial magnetic anisotropy. The Jahn-Teller distorted Ca3CoMnO6 in the magnetic ground state with up-up-down-down spin arrangement is predicted to have electric polarizations much greater than experimentally observed. Implications of the discrepancy between theory and experiment were discussed.