Electron microscopy of the barium ferrite layer structures

A very large number of crystallographically distinct compounds, with the generalized formula MnYm has been reported in the barium ferrite system, formed by regular coherent intergrowth of BaFe12O19 (‘phase M’) with Ba2Me2Fe12O22 (‘phase Y '; Me = Fe2+, Ni, etc.) and with c-axes reported from 10 to over 150 nm. Lattice imaging electron microscopy shows that although the crystallographic repeat units may be extremely long, extensive crystallographic domains of complex stacking sequences can be built up with regularity; occasional mistakes in stacking sequence are found. Within a single microscopic crystal, however, the stacking pattern and chemical composition may vary at different points along the c-axis, i. e. at different stages in crystal growth. The crystals therefore have a domain structure and electron microscopy reveals domains with compositions and stacking sequences not hitherto found by X-ray methods. The high-resolution lattice images present two-dimensional projections of the structures, which can be correlated with the existence of tunnels, 0.25 nm x 0.45 nm, running through the crystal structure along [112̄0] and differing in their orientation within M units and Y units.