Layered La ₂ MSb ₂ O ₉ (M = Co, Ni, and Cu): Synthesis, Crystal Structure, Magnetism, and Optical Properties

Single crystals of La 2 MSb 2 O 9 (M = Co, Ni, and Cu) have been grown by the molten salt synthesis method, and crystal structures were determined. The compounds crystallize in layered triclinic structures, space group P 1̅ (no. 2), which, as far as we know, do not have phase prototypes. The structure is formed by the layers consisting of alternating M–O and Sb–O polyhedron chains. The layers are connected by the Sb 2 O 10 dimers. The La 3+ ions are located in an interlayer space. La 2 CoSb 2 O 9 and La 2 NiSb 2 O 9 are isostructural. La 2 CuSb 2 O 9 has a number of structural features that indicate a different structural type. The absorption spectra of La 2 MSb 2 O 9 are a superposition of d–d transitions of M 2+ ions in tetragonally distorted MO 6 octahedra and MO 5 square pyramids (MO 4 square in the case of M = Cu) and charge-transfer transition between O 2p and M 3d states. Magnetic ions in La 2 MSb 2 O 9 (M = Co and Ni) are antiferromagnetically coupled in one-dimensional chains and undergo an antiferromagnetic-type transition at T N = 15 and 26 K, respectively. Weak ferromagnetic contribution to the total magnetization arising from spin canting in highly distorted Co(Ni)–O polyhedra is observed at low temperatures. A stepwise hysteresis in the magnetization vs magnetic field plot, characteristic for metamagnets, is observed for La 2 CoSb 2 O 9 .