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.

Magnetic structure of the swedenborgite compound CaBaMn2Fe2O7 derived by powder neutron diffraction and Mössbauer spectroscopy

“…The analysis of the diffraction pattern recorded at RT confirmed the hexagonal crystal structure reported in Ref. [26] and the presence of an impurity phase MnO which was refined to 3.8(2) wt. %.…”

“…The occupation factors of Fe and Mn as well as instrumental parameters were refined at RT and fixed for the refinements at lower temperatures. surplus of Fe in the original work [26], also agreeing with the observation of a minor MnO impurity. The possible slight oxygen excess of x = 0.14 (charge neutrality) may form octahedral defects as reported in Ref.…”

“…The parasitic phase MnO itself reveals a magnetic phase transition at T ≈ 120 K which is manifest in the strong magnetic satellite reflection ( 1 2 1 2 1 2 ) at 2θ = 28.5 • expressed in Miller indices corresponding to the conventional nuclear cell (i.e., with a magnetic unit cell doubled along all three axes). Figure 4(b) exhibits how the magnetic diffuse scattering around 2θ = 32 • transforms into a sharp resolution-limited Bragg reflection <∼205 K, which corresponds very well with the reported Néel temperature [26] derived from specific heat and magnetization measurements on the same sample batch. With respect to the crystal structure, the magnetic ordering goes along with an inflection point in the temperature evolution of the lattice parameter c, ceasing to decrease with decreasing temperature [shown in Fig.…”

“…2) could only be achieved with a nonzero Mn occupation of O Mn,2a = 0.182 (7) on the 2a site, while a more homogeneous mixing is present on the 6c site with O Mn,6c = 0.513 (3). Note that this result was inaccessible with x-ray diffraction employed in the original work [26]. The refined composition is therefore CaBaMn 1.72 Fe 2.28 O 7+x , confirming the expected mc derived at different temperatures.…”

“…In this paper, powder samples were employed originating from the same batch that was used for the experiments reported in Ref. [26]. Powder neutron diffraction experiments were performed on the high-flux diffractometer D1B at the Institut Laue-Langevin (Grenoble, France) using a 350 mg sample within a sample holder made from vanadium, which has a negligible neutron coherent scattering length and therefore only contributes to the incoherent background.…”