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We investigated the influence of oxygen over-stoichiometry on apical oxygendisorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperaturerange of 2-300 K by means of synchrotron x-ray powder diffraction, neutronsingle crystal and powder diffraction studies, combined with macroscopicmagnetic measurements. In the investigated temperature range, the compoundcrystalizes in a tetragonal commensurate structure with the P42/ncm space groupwith excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites,coordinated by four apical oxygen atoms. Large and anisotropic thermaldisplacement parameters are found for equatorial and apical oxygen atoms, whichare strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase.Maximum Entropy analysis of the neutron single crystal diffraction datauncovered anharmonic contributions to the displacement parameters of the apicaloxygen atoms, toward the nearest vacant 4b interstitial site, related to thephonon assisted oxygen diffusion mechanism. Macroscopic magnetizationmeasurements and neutron powder diffraction studies reveal long-rangeantiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weakferromagnetic component along the c-axis, while the long-range magneticordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutrondiffraction patterns show the appearance of a commensurate magnetic order at TNwith the propagation vector k = (100) and the emergence of an additionalincommensurate phase below 30 K, while both phases coexist at 2 K. Thecommensurate magnetic structure is best described by the P42/nc`m` Shubnikovspace group. Refined magnetic moments of the Ni and Nd-sites at 2 K are1.144(76) muB and 1.632(52) muB respectively. A possible origin of theincommensurate phase is discussed and a tentative magnetic phase diagram isproposed.

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+d (d ~ 0.11)