  We investigated the influence of oxygen over-stoichiometry on apical oxygen
disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature
range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron
single crystal and powder diffraction studies, combined with macroscopic
magnetic measurements. In the investigated temperature range, the compound
crystalizes in a tetragonal commensurate structure with the P42/ncm space group
with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites,
coordinated by four apical oxygen atoms. Large and anisotropic thermal
displacement parameters are found for equatorial and apical oxygen atoms, which
are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase.
Maximum Entropy analysis of the neutron single crystal diffraction data
uncovered anharmonic contributions to the displacement parameters of the apical
oxygen atoms, toward the nearest vacant 4b interstitial site, related to the
phonon assisted oxygen diffusion mechanism. Macroscopic magnetization
measurements and neutron powder diffraction studies reveal long-range
antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak
ferromagnetic component along the c-axis, while the long-range magnetic
ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron
diffraction patterns show the appearance of a commensurate magnetic order at TN
with the propagation vector k = (100) and the emergence of an additional
incommensurate phase below 30 K, while both phases coexist at 2 K. The
commensurate magnetic structure is best described by the P42/nc`m` Shubnikov
space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are
1.144(76) muB and 1.632(52) muB respectively. A possible origin of the
incommensurate phase is discussed and a tentative magnetic phase diagram is
proposed.
