Electronic structure, magnetic, and dielectric properties of the edge-sharing copper oxide chain compound NaCu2O2

We report an experimental study of NaCu2O2, a Mott insulator containing chains of edge-sharing CuO4 plaquettes, by polarized x-ray absorption spectroscopy (XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and pyroelectric current measurements. The XAS data show that the valence holes reside exclusively on the Cu2+ sites within the copper-oxide spin chains and populate a d orbital polarized within the CuO4 plaquettes. The RMXS measurements confirm the presence of incommensurate magnetic order below a NEel temperature of T-N = 11.5 K, which was previously inferred from neutron powder-diffraction and nuclear-magnetic-resonance data. In conjunction with the magnetic-susceptibility and XAS data, they also demonstrate an orbital selection rule for RMXS that is of general relevance for magnetic-structure determinations by this technique. Dielectric property measurements reveal the absence of significant ferroelectric polarization below TN, which is in striking contrast to corresponding observations on the isostructural compound LiCu2O2. The results are discussed in the context of current theories of multiferroicity.