Ferromagnetic Mn³⁺ Sawtooth Chains in A₂(Mn₂O)(SeO₃)₃ (A = K, Rb) Quaternary Selenites

Abstract

Two quaternary manganese selenites, A 2 (Mn 2 O)-(SeO 3 ) 3 (A = K, Rb), have been synthesized by hydrothermal reactions. They both crystallize in a complex triclinic (P-1) structure built of Jahn−Teller (JT) distorted Mn 3+ O 4+2 octahedra, connected into nearly isosceles [Mn 3 O 14 ] triangles, themselves arranged into socalled "sawtooth (ST) chains". The K and Rb compounds show subtle variations in the orientations of the MnO 4 planes inside the elementary triangles. The ST chains are structurally and magnetically isolated by SeO 3 groups and alkali cations. In the ST chains, predominant ferromagnetic interactions were calculated and verified experimentally, which finally order antiferromagnetically between the chains around T N ≈ 22 K. The spin exchanges calculated by DFT + U and fitted by Monte Carlo simulations allow for the quantification of an effective "overall" model. The specific role of the μ 3 -O bridge on the ferromagnetic (FM) exchanges is discussed, together with spin reorientations observed in the ordered state. Magnetocrystalline anisotropy along the [110] direction stabilized by ∼50 meV per Mn by spin−orbit coupling (SOC) was found by DFT + U + SOC.