Order-disorder structure of the δ1k phase in the Fe-Zn system determined by scanning transmission electron microscopy

The crystal structure of the delta(1k) phase in the Fe-Zn system was investigated by scanning transmission electron microscopy. The delta(1k) phase has a superlattice structure based on the delta(1p) phase having a tripled periodicity along the a-axis direction of the delta(1p) phase, accompanied by one-dimensional stacking disorder of structural blocks (called order disorder (OD) packets) along the c-axis direction. The crystal structure can be described crystallographically in terms of the OD theory so as to belong to the category IV OD structures composed of two types of non-polar OD layers (L-2n and M2n+1). The tripled periodicity along the a-axis direction is due to chemical ordering of the constituent Fe and Zn atoms in the OD layer M2n+1. Because of the tripled periodicity of the OD layer M2n+1, three different equivalent stacking positions are generated in stacking an OD layer M2n+1 on top of the OD layer L-2n. Depending on the stacking order, the crystal structure of the delta(1k) phase can be ordered with various periodicities along the stacking direction or completely disordered. Based on the OD theory, two maximum degree of order (MDO) polytypes belonging to the space groups of P6(3)/mcm (MDO1) and R3c (MDO2) are deduced for the delta(1k) phase. The most stable MDO polytype in the OD family of the delta(1k) phase is determined experimentally to be the MDO2 polytype. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.