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

Two M(SeO)F fluoro-selenites (M = Mn, Ni) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MOF]chains of edge-sharing octahedra with a rare topology of alternating O-O and F-F μ 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+. Their interplay induces prominent in-chain antiferromagnetic frustration, while the interchain exchanges are responsible for the cycloidal magnetic structure observed below ≈ 21.5 K in the Ni case. For comparison the Mn compound develops a nearly collinear spin (canted) ordering below ≈ 26 K with ferromagnetic chain units.