Square-lattice antiferromagnetic oxides with spin S=1/2 provide the basis for many dramatic and interesting phenomena, include high temperature superconductivity observed in copper oxides. We have recently synthesized two new layered antiferromagnets Sr2NiO3X (X = F, Cl) in which S=1/2 is provided by Ni(III) ions. These have very different magnetic behaviour. The X=F compound displays spin-glass features, while the X=Cl compound orders antiferromagnetically at TN ~ 40 K. We would like to determine the nature of the ground state order (X=Cl) and magnetic correlations (X=F) and understand why the two compounds behave differently. Our objectives are to isolate the magnetic diffraction peaks and diffuse scattering at low temperature, and to follow the temperature dependence through any ordering transitions.