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High-Pressure Synthesis of A2 NiO2 Ag2 Se2 (A=Sr, Ba) with a High-Spin Ni2+ in Square-Planar Coordination.

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 NiII 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.