Evidence of correlated incommensurate structural and magnetic order in highly oxygen-doped layered nickelate Nd₂NiO4.23

Nd2NiO4+delta is a nonstoichiometric oxide, crystallizing in the Ruddlesden-Popper-type framework and exhibit-ing a wide range of oxygen nonstoichiometry delta 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 T-N approximate to 150 K in highly oxygen-doped Nd2NiO4+delta with delta approximate to 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 Q(n) = +/- 0.813a* +/- 0.519b* wave vectors. The antiferromagnetic order is characterized by the presence of incommensurate Bragg peaks of type (h +/- epsilon, 0, l/2), with h and l being odd-integer numbers and the magnetic incommensurability epsilon approximate to 0.36. The in-plane magnetic correlation length is found to be xi(ab) = 184(1) angstrom but is significantly reduced along the c axis to xi(c) = 39(2) angstrom, indicating the quasi-two-dimensional 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. In contrast to Nd2NiO4.1, in which the incommensurate periodicity of oxygen and magnetic ordering was found to be identical, the structural and electronic orderings in heavily oxygen-doped Nd2NiO4.23 show two independent modulation vectors. Our study thus unravels a strong correlation between structural and electronic orderings in the highly oxygen-doped Ln(2)NiO(4+delta) system.