Cycloidal Magnetic Order Promoted by Labile Mixed Anionic Paths in M 2 (SeO 3 )F 2 (M = Mn 2+ , Ni 2+ )

Two M2(SeO3)F2 fluoro-selenites (M = Mn 2+ , Ni 2+) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO2F2] 4chains of edge-sharing octahedra with a rare topology of O-O and F-F alternating µ2 bridges. The inter-chain corner sharing connections are assisted by the mixed O vs. F anionic nature and develop a complex set of M-X-M super-exchanges calculated by LDA+U. Their interplay induce prominent in-chain antiferromagnetic frustration while the interchain exchanges are responsible for the cycloidal magnetic structure observed below TN ~21.5 K in the Ni 2+ case. For comparison the Mn 2+ compound develops a nearly collinear spin (canted) ordering below TN ~26 K with ferromagnetic chain-units.