Possible charge disproportionation in 3R-AgNiO₂ studied by neutron powder diffraction

Using neutron-scattering techniques, we have investigated the nuclear and the magnetic structures of the triangular antiferromagnet 3R-AgNiO₂. The symmetry analysis based on the group theory suggests that the √3×√3 charge order proposed for 2H -AgNiO₂ will have a monoclinic symmetry if present in the trigonal lattice of 3R-AgNiO ₂. The Rietveld refinement shows that symmetry reduction in the NiO₂ layer is consistent with the prediction of the group theory. The pair density function consistently shows that the nearest-neighbor Ni-O bonds split into two groups separated by approximately 0.1 Å. The antiferromagnetic Bragg peaks observed below T_N=25 K can be described by the propagation vector k= (0,1,0) of the monoclinic unit cell. The similarities of the local structure and the antiferromagnetic order strongly suggest that the novel charge order observed in 2H-AgNiO₂ also exists in 3R-AgNiO₂.