High- versus Low-Spin Ni²⁺ in Elongated Octahedral Environments: Sr₂NiO₂Cu₂Se₂, Sr₂NiO₂Cu₂S₂, and Sr₂NiO₂Cu₂(Se_1–xS_x)₂

Sr₂NiO₂Cu₂Se₂, comprising alternating [Sr₂NiO₂]²⁺ and [Cu₂Se₂]^2- layers, is reported. Powder neutron diffraction shows that the Ni²⁺ ions, which are in a highly elongated NiO₄Se₂ 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 Ni²⁺ 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 Sr₂NiO₂Cu₂S₂, 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 Ni²⁺, and accordingly, there is no evidence for magnetic moment on the Ni²⁺ ions. Examination of the solid solution Sr₂NiO₂Cu₂(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 Sr₂NiO₂Cu₂Se₂ up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni²⁺ 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, Sr₂NiO₂Cu₂Se₂ and Sr₂NiO₂Cu₂S₂, suggest that simple high-spin and low-spin models for Ni²⁺ may not be entirely appropriate and point to further complexities in these compounds.