  We report a high-resolution neutron diffraction study of the crystal and
magnetic structure of the orbitally-degenerate frustrated metallic magnet
AgNiO2. At high temperatures the structure is hexagonal with a single
crystallographic Ni site, low-spin Ni3+ with spin-1/2 and two-fold orbital
degeneracy, arranged in an antiferromagnetic triangular lattice with frustrated
spin and orbital order. A structural transition occurs upon cooling below 365 K
to a tripled hexagonal unit cell containing three crystallographically-distinct
Ni sites with expanded and contracted NiO6 octahedra, naturally explained by
spontaneous charge order on the Ni triangular layers. No Jahn-Teller
distortions occur, suggesting that charge order occurs in order to lift the
orbital degeneracy. Symmetry analysis of the inferred Ni charge order pattern
and the observed oxygen displacement pattern suggests that the transition could
be mediated by charge fluctuations at the Ni sites coupled to a soft oxygen
optical phonon breathing mode. At low temperatures the electron-rich Ni
sublattice (assigned to a valence close to Ni2+ with S = 1) orders magnetically
into a collinear stripe structure of ferromagnetic rows ordered
antiferromagnetically in the triangular planes. We discuss the stability of
this uncommon spin order pattern in the context of an easy-axis triangular
antiferromagnet with additional weak second neighbor interactions and
interlayer couplings.
