Direct Atomic Observation
in Powdered 4H-Ba<sub>0.8</sub>Sr<sub>0.2</sub>Mn<sub>0.4</sub>Fe<sub>0.6</sub>O<sub>2.7</sub>
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Abstract
A new hexagonal polytype in the BaMn<sub>1‑<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3‑δ</sub> system has been stabilized. Powdered Ba<sub>0.8</sub>Sr<sub>0.2</sub>Mn<sup>IV</sup><sub>0.4</sub>Fe<sup>III</sup><sub>0.6</sub>O<sub>2.70</sub> crystallizes in the 4H hexagonal polytype (space group <i>P</i>6<sub>3</sub>/<i>mmc</i>) according to X-ray
diffraction. HAADF images and chemical maps with atomic resolution
have been obtained by combining Cs-corrected electron microscopy and
EELS spectroscopy. The structure is formed by dimers of face-sharing
octahedra linked by corners. EELS data show a random distribution
of the transition metals ions identified by Fe and Mn-L2,3 chemical
maps. A systematic difference in contrast observed in the O–K
signal mapping suggests that anion deficiency is randomly located
along the hexagonal layers in agreement with ND data. The magnetic
structure consists of ferromagnetic sheets with the magnetic moments
aligned along the <i>x</i>-axis and coupled antiferromagnetically
along the <i>c</i>-axis
