(a) Charge order on the Ni site leads to displacements of the surrounding oxygen ions, directly observed by high resolution diffraction measurements. (b) Magnetic order:+/- are spins in/out of plane, dots are unordered sites.
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The layered hexagonal magnet AgNiO2 realizes a unique example of a triangular antiferromagnet with both spin and orbital moment (Ni3+ with one eg electron) and with metallic conductivity.
Both the spin and orbital order are frustrated in a triangular geometry and the electronic ground state when electrons are mobile is not well understood. Using high-resolution diffraction on Osiris and HRPD we have uncovered a supercell crystal structure where expanded NiO6 octahedra are surrounded by a honeycomb network of contracted sites (see figure). Band-structure calculations indicate that this structural modulation is naturally explained by a spontaneous charge order on the Ni sites: expanded sites are electron rich Ni2+ and contracted sites are electron depleted Ni3.5+. Our results show that in weakly-delocalized metallic systems the orbital degeneracy can be lifted by charge order as opposed to the Jahn-Teller distortions found in insulating systems. At base temperature the honeycomb sites are nonmagnetic and the expanded Ni sites order in an unusual collinear pattern of alternating stripes on a triangular lattice (see figure).
E Wawrzyñska, R Coldea (University of Bristol), EM Wheeler (University of Oxford), II Mazin, MD Johannes (Naval Research Lab, Washington, USA), T Sörgel, M Jansen (Max-Planck Institut für Festkörperforschung, Stuttgart, Germany), RM Ibberson, PG Radaelli (ISIS)
Research date: December 2007
E Wawrzyñska et al., Phys. Rev. Lett., 99 157204 (2007).
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