Observations of Sub-Unit-Cell Stacking Elements in Mixed-Layer Structures by Electron Microscopy

Abstract

The ``hexagonal ferrites'' comprise a quasipolytypic series of ferrimagnetic oxides containing Ba, Fe3+, and a divalent metal cation (Me2+). These encompass a family of hexagonal and rhombohedral mixed-layer structures derived by the c-axis stacking of two complex invariant building blocks, mainly ``M'' and ``Y''. The M stacking element is a 5-anion-layer block, 11.6 Å in height with the composition BaFe12O19, and the Y stacking element is a 6-anion-layer block, 14.5 Å high with the composition Ba2Me22+Fe12O22. The largest series, M2Yn(n=0 to 21), involves the stacking of two M blocks and a variable number of Y blocks. Permutation of the M and Y elements allows mixed-layer polytypism within a given stoichiometry, i.e., M to Y ratio. More than twenty (20) mixed-layer structures and their polytypes have been examined by electron microscopy after direct replication of the basal face. Basal faces which were etched with dilute hydrochloric acid exhibit etch steps of sub-unit-cell heights. The specific M and Y stacking sequence of a mixed-layer structure may be obtained by measuring the step-height ratio, MYx/MYn−x, of two adjacent steps, provided either the shadow angle or the x-ray c-axis dimension is known. In addition it has been found that the c-axis dimension and the specific stacking sequence may be uniquely determined by counting the individual M and Y building blocks which are replicated as separate and distinct etch-planes on a crystal surface that has been process-etched.