Nb₂O₂F₃: A reduced niobium (III/IV) oxyfluoride with a complex structural, magnetic, and electronic phase transition

A new niobium oxyfluoride, Nb₂O₂F₃, synthesized through the reaction of Nb, SnO, and SnF₂ in Sn flux, within welded Nb containers, crystallizes in a monoclinic structure (space group: I2/a; a = 5.7048(1)Å, b = 5.1610(1)Å, c = 12.2285(2)Å, β = 95.751(1)°). It features [Nb₂X₁₀] units (X = O, F), with short (2.5739(1) Å) Nb-Nb bonds, that are linked through shared O/F vertices to form a 3D structure configurationally isotypic to ζ-Nb₂O₅. Nb₂O₂F₃ undergoes a structural transition at ∼90 K to a triclinic structure (space group: P1¯; a = 5.1791(5)Å, b = 5.7043(6)Å, c = 6.8911(7)Å, α = 108.669(3)°, β = 109.922(2)°, γ = 90.332(3)°). The transition is described as a disproportionation or charge ordering of [Nb₂]⁷⁺ dimers: (2[Nb₂]⁷⁺ → [Nb₂]⁶⁺ + [Nb₂]⁸⁺), resulting in doubly (2.5000(9) Å) and singly bonded (2.6560(9) Å) Nb₂ dimers. The structural transition is accompanied by an unusual field-independent "spin-gap-like" magnetic transition.