High‐Pressure Synthesis of A 2 NiO 2 Ag 2 Se 2 (A=Sr, Ba) with a High‐Spin Ni 2+ in Square‐Planar Coordination

Square‐planar coordinate Ni 2+ ions in oxides are exclusively limited to a low‐spin state (S=0) owing to extensive crystal field splitting. Layered oxychalcogenides A 2 Ni II O 2 Ag 2 Se 2 (A=Sr, Ba) with the S=1 NiO 2 square lattice are now reported. The structural analysis revealed that the Ni 2+ ion is under‐bonded by a significant tensile strain from neighboring Ag 2 Se 2 layers, leading to the reduction in crystal field splitting. Ba 2 NiO 2 Ag 2 Se 2 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 NiO 2 square lattice by taking the advantage of the difference in compressibility in oxide and selenide layers.