High- versus Low-Spin Ni(2+) in Elongated Octahedral Environments: Sr(2)NiO(2)Cu(2)Se(2), Sr(2)NiO(2)Cu(2)S(2), and Sr(2)NiO(2)Cu(2)(Se(1–x)S(x))(2)

[Image: see text] Sr(2)NiO(2)Cu(2)Se(2), comprising alternating [Sr(2)NiO(2)](2+) and [Cu(2)Se(2)](2–) layers, is reported. Powder neutron diffraction shows that the Ni(2+) ions, which are in a highly elongated NiO(4)Se(2) environment with D(4h) symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μ(B) per Ni(2+) ion. The adoption of the high-spin configuration for this d(8) cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr(2)NiO(2)Cu(2)S(2), which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni(2+), and accordingly, there is no evidence for magnetic moment on the Ni(2+) ions. Examination of the solid solution Sr(2)NiO(2)Cu(2)(Se(1–x)S(x))(2) shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr(2)NiO(2)Cu(2)Se(2) up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni(2+) coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr(2)NiO(2)Cu(2)Se(2) and Sr(2)NiO(2)Cu(2)S(2), suggest that simple high-spin and low-spin models for Ni(2+) may not be entirely appropriate and point to further complexities in these compounds.