| Shrnutí: | <p>Sr<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub>, comprising alternating [Sr<sub>2</sub>NiO<sub>2</sub>]<sup>2+</sup> and [Cu<sub>2</sub>Se<sub>2</sub>]<sup>2–</sup> layers, is reported. Powder neutron diffraction shows that the Ni<sup>2+</sup> ions, which are in a highly elongated NiO<sub>4</sub>Se<sub>2</sub> environment with D<sub>4<em>h</em></sub> symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2<em>a</em> × √2<em>a</em> × 2<em>c</em> expansion of the nuclear cell with an ordered moment of 1.31(2) μ<sub>B</sub> per Ni<sup>2+</sup> ion. The adoption of the high-spin configuration for this <em>d</em><sup>8</sup> 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<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>S<sub>2</sub>, 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<sup>2+</sup>, and accordingly, there is no evidence for magnetic moment on the Ni<sup>2+</sup> ions. Examination of the solid solution Sr<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>(Se<sub>1–<em>x</em></sub>S<sub><em>x</em></sub>)<sub>2</sub> 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<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub> 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<sup>2+</sup> 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<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub> and Sr<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>S<sub>2</sub>, suggest that simple high-spin and low-spin models for Ni<sup>2+</sup> may not be entirely appropriate and point to further complexities in these compounds.</p>
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