Cycloidal Magnetic Order Promoted by Labile Mixed Anionic Paths in M2(SeO3)F2 (M = Mn2+, Ni2+).

Two M2(SeO3)F2 fluoro-selenites (M = Mn2+, Ni2+) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO2F2]4-chains of edge-sharing octahedra with a rare topology of alternating O-O and F-F μ2 bridges. The interchain corner-sharing connections are assisted by the mixed O vs F anionic nature and develop a complex set of M-X-M superexchanges as calculated by LDA+U. Their interplay induces prominent in-chain antiferromagnetic frustration, while the interchain exchanges are responsible for the cycloidal magnetic structure observed below TN ≈ 21.5 K in the Ni2+ case. For comparison the Mn2+ compound develops a nearly collinear spin (canted) ordering below TN ≈ 26 K with ferromagnetic chain units.