Magnetic ordering in the layered oxyhalide Sr2NiO2Cl2

Abstract

The full characterisation of the high-pressure-synthesised, metastable layered oxyhalide Sr2NiO2Cl2 is reported. It is comprised of infinite NiO2 layers along with double rock salt Sr2Cl2 layers and is closely related to the n = 1 Ruddlesden-Popper phases. At ambient temperature, it crystallises in the tetragonal space group I4/mmm with 2 formula units per unit cell with the basal lattice parameters a = b = 4.03417(2) Å and the stacking direction lattice parameter, c = 15.1058(1) Å. A tiny cusp in the temperature dependence of the magnetic susceptibility in a previous report suggested that this oxyhalide underwent long-range magnetic order and therefore contained high-spin Ni2+ ions. Powder neutron diffraction has confirmed that Sr2NiO2Cl2 is a localised-moment oxyhalide, adopting the high-spin S = 1 configuration with fully occupied dxz, dyz and dxy orbitals and partially occupied dz2 and dx2-y2 orbitals. The Ni2+ magnetic moments order antiferromagnetically below ≈ 180 K in a G-type arrangement on a √2a × √2a × c expansion of the nuclear cell with the propagation vector (½, ½, 0) and with a saturated long-range ordered magnetic moment of 1.57(7) μB per Ni2+ cation which is in line with previously calculated computational results and showing that the ligand field around the Ni2+ cation is not sufficiently anisotropic to drive it into the low-spin, diamagnetic configuration found for this d8 cation in square planar or highly elongated octahedral coordination.