Oriented Nanomagnets Originating from Topotactic and Pseudomorphic Reactions

The topotactical transformation of goethite to hexaferrite was investigated using 2D synchrotron powder diffraction. The relation between structure, size, and texture in the synthesis was investigated using combined quantitative texture analysis and Rietveld refinements. Based on the refinements, the topotaxial relation from the precursor (goethite) through the intermediate (hematite) and to the product (hexaferrite) could be understood. In all three cases, the orientation of texture and smallest crystallite dimension is along the closest packed oxygen lattice, the c-axis. In the intermediary step where goethite is converted to hematite, a pseudomorphic transition is observed where particle shape is maintained along the a- and b-axis. Furthermore, the structural relation from the intermediate to the product was demonstrated to aid the understanding of the final step in the topotactical transition. Based on this study, we conclude that synthesis can be designed to promote texture in materials beyond magnets by appropriate choice and understanding of precursor structure and morphology.