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 <i>a</i> = 5.201(1) Å, <i>c</i> = 22.406(3) Å, space group <i>P</i>6₃/<i>m</i>, <i>Z</i> = 2, ρ_c = 3.371 g/cm³, and <i>R</i> = 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 <i>T</i>₃ = 80–100 K as well as two successive magnetic phase transitions at <i>T</i>₂ = 3 K and <i>T</i>₁ = 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 μ_eff² ∼ 70 μ_B² per formula unit suggest that antiferromagnetic exchange interactions dominate in the system. Divalent manganese is present in a high-spin state, <i>S</i> = ⁵/₂, in the octahedral environment and a low-spin state, <i>S</i> = ¹/₂, in the trigonal-bipyramidal coordination