The study of long range order in hexagonal barium ferrite layer structures

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Abstract
Problems associated with long-range ordering phenomena in polytypic systems, such as SiC, are discussed and applied to the barium ferrite system, in which a very large number of crystallographically distinct compounds of general formula M,Y, have been reported. They are formed by regular coherent intergrowth of Ba Fe12O19 (‘phsee M’) with Ba2Me2Fe12O22 (‘phase Y’, with Me = Fe2+, Ni2+, etc.), and have c axes from below 10 nm to over 150 nm. Electron microscopy has been applied to these materials end conditions are described under which hges can be interpreted on an intuitive basis; ultra-high resolution lattice images of correctly oriented thin crystals show contrast which is a projection of the charge density. The positions of tunnels through the crystals, parallel to [1120], are clearly revealed in these images and allow one to distinguish between M and Y units. It is thus shown that elthougb the crystallographic repeat units may be extremely long, extensive crystallographic domains of complex stacking sequences are built up with regularity, although occasional mistekes are found. Within a single, macroscopic crystal. however, the staoking pattern and ohemical composition may very at different points along the c axis, i.e. at different stages in crystal growth. The crystals have a domain structure and electron microscopy reveals domains with compositions and stacking sequences not hitherto found by X-ray methods.