The First Vanadate–Carbonate, K2Mn3(VO4)2(CO3): Crystal Structure and Physical Properties

Mixed potassium–manganese vanadate–carbonate, K2Mn3(VO4)2(CO3), represents a novel structure type; it has been synthesized hydrothermally from the system MnCl2–K2CO3–V2O5–H2O. Its hexagonal crystal structure was determined by single-crystal X-ray diffraction with a = 5.201(1) Å, c = 22.406(3) Å, space group P63/m, Z = 2, ρc = 3.371 g/cm3, and R = 0.022. The layered structure of the compound can be described as a combination of honeycomb-type modules of [MnO6] octahedra and [VO4] tetrahedra, alternating in the [001] direction with layers of [MnCO3] built by [MnO5] trigonal bipyramids and [CO3] planar triangles, sharing oxygen vertices. The K+ ions are placed along channels of the framework, elongated in the [100], [010], and [110] directions. The title compound exhibits rich physical properties reflected in a phase transition of presumably Jahn–Teller origin at T3 = 80–100 K as well as two successive magnetic phase transitions at T2 = 3 K and T1 = 2 K into a weakly ferromagnetic ground state, as evidenced in magnetization, specific heat, and X-band electron spin resonance measurements. A negative Weiss temperature Θ = −114 K and strongly reduced effective magnetic moment μeff2 ∼ 70 μB2 per formula unit suggest that antiferromagnetic exchange interactions dominate in the system. Divalent manganese is present in a high-spin state, S = 5/2, in the octahedral environment and a low-spin state, S = 1/2, in the trigonal-bipyramidal coordination.