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

Abstract

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 root 3 x root 3 charge order proposed for 2H-AgNiO2 [E. Wawrzynska et al., Phys. Rev. Lett. 99, 157204 (2007)] will have a monoclinic symmetry if present in the trigonal lattice of 3R-AgNiO2. 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 angstrom. 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.