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We report a high-resolution neutron diffraction study of the crystal andmagnetic structure of the orbitally-degenerate frustrated metallic magnetAgNiO2. At high temperatures the structure is hexagonal with a singlecrystallographic Ni site, low-spin Ni3+ with spin-1/2 and two-fold orbitaldegeneracy, arranged in an antiferromagnetic triangular lattice with frustratedspin and orbital order. A structural transition occurs upon cooling below 365 Kto a tripled hexagonal unit cell containing three crystallographically-distinctNi sites with expanded and contracted NiO6 octahedra, naturally explained byspontaneous charge order on the Ni triangular layers. No Jahn-Tellerdistortions occur, suggesting that charge order occurs in order to lift theorbital degeneracy. Symmetry analysis of the inferred Ni charge order patternand the observed oxygen displacement pattern suggests that the transition couldbe mediated by charge fluctuations at the Ni sites coupled to a soft oxygenoptical phonon breathing mode. At low temperatures the electron-rich Nisublattice (assigned to a valence close to Ni2+ with S = 1) orders magneticallyinto a collinear stripe structure of ferromagnetic rows orderedantiferromagnetically in the triangular planes. We discuss the stability ofthis uncommon spin order pattern in the context of an easy-axis triangularantiferromagnet with additional weak second neighbor interactions andinterlayer couplings.

Charge disproportionation and collinear magnetic order in the frustrated triangular antiferromagnet AgNiO2