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In recent studies of La 2Ϫx Sr x NiO 4 it has been suggested that ordering of the dopant-induced holes occurs only commensurately at special values of x, such as 1 3 and 1 2 . Commensurate order of both charge and spin densities has also been found in a crystal with xϭ0.20. The present neutron scattering study of an xϭ0.225 crystal demonstrates that the spin and charge order can also be incommensurate. In fact, the incommensurability is temperature dependent, as observed previously in La 2 NiO 4.125 , indicating that such behavior is intrinsic to the doped NiO 2 layers and not dependent on ordering of the dopant ions. A careful analysis of the unusual variation of peak widths as a function of momentum transfer perpendicular to the planes shows that the charge-and spin-density modulations are tied to the lattice, with the shift in phase of the charge modulations from one layer to the next equal to exactly one in-plane lattice spacing. A comparison of results for a number of samples shows that the charge and spin ordering temperatures vary linearly with hole concentration, with charge order always occurring at higher temperature, clearly indicating that the ordering is driven by the charge.

We report a detailed neutron-scattering study of the ordering of spins and holes in oxygen-doped La 2 NiO 4.133 . The single-crystal sample exhibits the same oxygen-interstitial order but better defined chargestripe order than that studied previously in crystals with ␦ϭ0.125. In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, T m ϭ110.5 K. On cooling through T m , the wave vector ⑀, equal to half the charge-stripe density within an NiO 2 layer, jumps discontinuously from 1 3 to 0.2944. It continues to decrease with further cooling, showing several lock-in transitions on the way down to low temperature. To explain the observed lock-ins, a model is proposed in which each charge stripe is centered on either a row of Ni or a row of O ions. The model is shown to be consistent with the l dependence of the magnetic peak intensities and with the relative intensities of the higher-order magnetic satellites. Analysis of the latter also provides evidence that the magnetic domain walls ͑charge stripes͒ are relatively narrow. In combination with a recent study of magnetic-field-induced effects, we find that the charge stripes are all O centered at TϾT m , with a shift towards Ni centering at TϽT m . Inferences concerning the competing interactions responsible for the temperature dependence of ⑀ and the localization of charge within the stripes are discussed. ͓S0163-1829͑98͒00902-3͔

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La2Ni04. q25 exhibits a cooperative ordering of dopant-induced holes and Ni spins below a transition temperature of 110 K. There is also an ordering of interstitial oxygens that occurs near room temperature. We present a comprehensive analysis of experimentally observed superlattice intensities in terms of structural models, justifying previous identifications. The model for the interstitial order involves a 3a x 5b x 5c unit cell with an ideal interstitial density of b = -per formula unit.In the spin and charge ordered state, the magnetic moments are sinusoidally modulated within the Ni02 planes. The moments point transverse to the modulation direction, with nearest neighbors antiparallel, and have a maximum amplitude that is ) 80'Fp of that observed in undoped LaqNi04.A corresponding modulation of atomic positions within Ni02 planes involves breathing-mode distortions consistent with a modulation of the charge density. The results are compared with theoretical models and with experimental observations on related systems.