Abstract
Nd2NiO4+δ is a non-stoichiometric oxide, crystallizing in the Ruddlesden-Popper type framework, exhibiting a wide range of oxygen non-stoichiometry (δ) with a complex structural phase diagram. Excess oxygen atoms insert holes, directly influencing the Ni valence state and associated electronic structure. We report here the existence of a correlated incommensurate structural and antiferromagnetic order below TN ≈ 150 K in highly oxygen-doped Nd2NiO4+δ with δ ≈ 0.23. The crystal and magnetic structures are investigated by polarized and unpolarized single crystal neutron diffraction studies together with synchrotron X-ray powder diffraction and macroscopic magnetization measurements. Due to the long-range ordering of excess oxygen atoms, the real structure of the compound is incommensurately modulated represented by Qn = ±0.813a * ±0.519b * wave vectors. The antiferromagnetic order is characterized by the presence of incommensurate Bragg peaks of type (h ± ε, 0, l/2) with h and l being odd integer numbers and the magnetic incommensurability ε ≈ 0.36. The inplane magnetic correlation length is found to be ξab = 184(1) ˚A, while significantly reduced along the caxis to ξc = 39(2) ˚A, indicating the quasi-two-dimensional (2D) nature of the antiferromagnetic correlations. Our study indicates that the excess oxygen atoms nevertheless strongly enhance the magnetic correlations between the NiO2-planes, resulting in a doubling of the magnetic unit cell along the c-axis. Contrary to Nd2NiO4.1, where the incommensurate periodicity of oxygen and magnetic ordering was found to be identical, the structural and electronic ordering in heavily oxygen-doped Nd2NiO4.23 show two independent modulation vectors. Our study thus unravels a strong correlation between structural and electronic orderings in highly oxygen-doped Ln2NiO4+δ-system.