Coexistence of spin ordering on ladders and spin dimer formation in a new-structure-type compound Sr₂Co₃S₂O₃

We report on the syntheses and characterizations of single crystalline and polycrystalline Sr₂Co₃S₂O₃ with a novel crystal structure type. It contains Co-O 2-leg rectangular ladders and necklace ladders. The two ladders share common legs and construct a hybrid spin ladder. A rare meridional heteroleptic octahedral coordination is found for the Co²⁺ ions in the 2-leg ladder. Within the necklace ladders, the Co²⁺ ions are in trans-octahedral coordination. An antiferromagnetic order is observed at T_N ~ 267 K, while a broad maximum in magnetic susceptibility is found below T_N. This relatively high ordering temperature among Co-based ladder compounds is related to the highly anisotropic mer-coordination of the Co²⁺ ions. The trans-octahedrally coordinated Co²⁺ ions, on the other hand, corresponds to the possible short-range magnetic correlations through dimers with an effective . This results in a rare situation that spin ordering and spin dimers coexist down to 2 K.