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 .

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 4 h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2 a × √2 a × 2 c 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.