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

Abstract

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-squareplanar 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.

Discrete copper(i) chalcogenones with metal–metal interaction

“…Dinuclear copper( i ) chalcogenide, [CuE] 2 , is a common building unit in many copper( i )–chalcogenide high-temperature superconducting materials. 1–5 The expansion of the lattice in copper( i )–chalcogenides is driven by the changes in the chalcogenide layer. 6 By substituting Se with S, the Cu–E bond length in the copper( i )–chalcogenide lattice is expected to increase.…”