High-Pressure Synthesis of A2 NiO2 Ag2 Se2 (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.

Keywords: high-pressure synthesis; oxychalcogenide; spin states; square lattice; strain.