Layered La2MSb2O9 (M = Co, Ni, and Cu): Synthesis, Crystal Structure, Magnetism, and Optical Properties

Single crystals of La2MSb2O9 (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 P1̅ (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 Sb2O10 dimers. The La3+ ions are located in an interlayer space. La2CoSb2O9 and La2NiSb2O9 are isostructural. La2CuSb2O9 has a number of structural features that indicate a different structural type. The absorption spectra of La2MSb2O9 are a superposition of d-d transitions of M2+ ions in tetragonally distorted MO6 octahedra and MO5 square pyramids (MO4 square in the case of M = Cu) and charge-transfer transition between O 2p and M 3d states. Magnetic ions in La2MSb2O9 (M = Co and Ni) are antiferromagnetically coupled in one-dimensional chains and undergo an antiferromagnetic-type transition at TN = 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 La2CoSb2O9.