Jahn-Teller ordering in Kagomé-type layers of compounds A₂A′Mn^III₃F₁₂ (A = Rb, Cs; A′ = Li, Na, K)

For a series of new Mn(III) fluorides the crystal structures have been determined by X-ray diffraction on single crystals: Cs₂LiMn₃F₁₂, space group R3̄, Z = 3 (merohedral twin), a = 7.440(1), c = 17.267(3) Å, R = 1.95%; three isostructural triclinic phases, space group P1̄, Z = 2, Cs₂NaMn₃F₁₂, a = 7.305(1), b = 7.512(1), c = 10.376(2) Å, α = 89.89(3)°, β = 87.32(3)°, γ = 89.89(3)°, R = 2.75%; Rb₂NaMn₃F₁₂, a = 7.115(1), b = 7.482(1), c = 10.160(2) Å, α = 89.92(3)°, β = 86.52(3)°, γ = 89.51(3)°, R = 2.70%; Rb₂LiMn₃F₁₂, a = 6.883(1), b = 7.481(1), c = 10.194(2) Å, α = 89.42(3)°, β = 87.16(3)°, γ = 90.25(3)°. R = 4.87%; monoclinic Cs₂KMn₃F₁₂, space group C2/c, Z = 4, a = 13.112(3), b = 7.571(2), c = 12.672(3) Å, β = 108.79(3)°, R = 5.3%. All structures derive from the Cs₂NaAl₃F₁₂ parent structure and consist of [Mn₃F₁₂] layers of corner-sharing octahedra of the Kagomé-net-type (3+6 octahedra units like in the hexagonal tungsten bronze layers). The [MnF₆] octahedra are strongly elongated by the Jahn-Teller effect with ratios of long to short axes of about 1.15. In all structures the long axes are oriented within the layers in an alternating wind-wheel-like pattern corresponding to an antiferrodistortive Jahn-Teller ordering. Thus, all Mn-F-Mn bridges are asymmetric; the bridge angles are between 136° and 141°. Magnetic investigations of Cs₂NaMn₃F₁₂ and Cs₂KMn₃F₁₂ indicate similar antiferromagnetic exchange interactions (J/k = -2.5 K), but only for the Cs₂K compound has 2D and 3D ordering been found at low temperatures. The results are discussed in context with the Jahn-Teller ordering and the 'frustrated' exchange situation in the Kagomé net.