Structure and magnetic susceptibility of MnNb 3O 6

Abstract

The mixed valence state niobium compound MnNb 3O 6 was found while studying phase formations in the system MnO-Nb 2O 5-NbO. It is isostructural with A x Nb 3O 6, x ≤ 1 and A = Na, Ca. Single crystals were obtained by heating MnC 2O 4 · 2H 2O and Nb 2O 5 in a flow of H 2 at 1300 °C. Monophasic samples were also prepared by heating stoichiometric mixtures of MnO, Nb 2O 5 and Nb in niobium ampoules under Ar(g) at 1100 °C. The crystal structure of MnNb 3O 6 ( Immm, Z = 4, a = 7.1057(5), b = 10.1420(6), c = 6.5341(5) Å) was refined, using singlecrystal MoKα X-ray diffraction data, to a weighted R value of 0.018 for 329 unique reflections. The structure contains undulating layers of NbO 6 octahedra of the type α 2[ NbO 6/2 ]in the ac-plane, with the octahedra sharing edges along [001]and corners along [100]. Between the layers there are columns along [001]of edge-sharing square MnO 8 prisms alternating with columns containing Nb 2O 8 clusters with an Nb-Nb distance of 2.6163(5) Å. The magnetic susceptibility shows a Curie-Weiss behaviour: χ M = C/(T+θ) with θ ≈ −22 K and μ eff = 6.0(1) μ B for T ≥ ca. 35 K, with a small deviation from this dependence at lower temperatures, indicating Mn 2+ ions with localised magnetic moments and antiferromagnetic interactions.