High‐Pressure Synthesis of A2NiO2Ag2Se2 (A=Sr, Ba) with a High‐Spin Ni2+ in Square‐Planar Coordination

All in a spin: A high‐pressure reaction allows the synthesis of A₂NiO₂Ag₂Se₂ (A=Sr, Ba) with a square‐planar NiO₂ sheet. These materials (and Cu/Mn analogues) have extremely long Ni−O (Cu/Mn−O) bonds owing to an extensive strain from AgSe layers, enabling an otherwise missing S=1 square‐planar state. The distinct compressibility between oxide and chalcogenide layers accounts for this phenomenon.

Abstract

Square‐planar coordinate Ni²⁺ ions in oxides are exclusively limited to a low‐spin state (S=0) owing to extensive crystal field splitting. Layered oxychalcogenides A₂Ni^IIO₂Ag₂Se₂ (A=Sr, Ba) with the S=1 NiO₂ square lattice are now reported. The structural analysis revealed that the Ni²⁺ ion is under‐bonded by a significant tensile strain from neighboring Ag₂Se₂ layers, leading to the reduction in crystal field splitting. Ba₂NiO₂Ag₂Se₂ exhibits a G‐type spin order at 130 K, indicating fairly strong in‐plane interactions. The high‐pressure synthesis employed here possibly assists the expansion of NiO₂ square lattice by taking the advantage of the difference in compressibility in oxide and selenide layers.

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