Swedenborgite CaBa(Mn2Fe2)O7 with Spin Ordering on a Geometrically Frustrated, Polar, Non-centrosymmetric S = 5/2 Lattice

Abstract

By solid-state reaction in closed ampoules, almost phase pure Swedenborgite CaBa(Mn2Fe2)O7 [P63mc, a = 6.4100(1) Å, c = 10.3752(2) Å] was obtained. Thermodynamic and melting behavior investigations suggest that this complex oxide melts incongruently and that the true composition contains a minor surplus of Fe as compared to Mn. However, in first approximation, the magnetic transition metal lattice (Mn2Fe2) can be described as a homogeneous S = 5/2 system and consists of kagome layers with an intermediate trigonal site. A long range antiferromagnetic ordering is observed at TN = 205.5 K, as proven by magnetometry and specific heat measurements. The S = 5/2 (high-spin d5) on-site configuration is confirmed by ab initio quantum chemical calculations, which additionally indicate antiferromagnetic Heisenberg interactions with a ratio of 1.56 between the spin coupling constants of the two distinct d5–d5 links (in and out of the kagome plane). In light of recent numerical studies for the extended Swedenborgite spin lattice, this quantum chemical estimate for the Jout/Jin ratio in combination with magneto-electric coupling, as a consequence of the polar, non-centrosymmetric crystal structure, explain the experimentally found antiferromagnetic order.