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

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 A₂ Ni^II O₂ Ag₂ Se₂ (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 Ag₂ Se₂ layers, leading to the reduction in crystal field splitting. Ba₂ NiO₂ Ag₂ Se₂ 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.