The crystal structure of the δ1kphase in the Fe-Zn system was investigated by scanning transmission electron microscopy. The δ1kphase has a superlattice structure based on the δ1pphase having a tripled periodicity along the a-axis direction of the δ1pphase, 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 (L2nand 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+1on top of the OD layer L2n. Depending on the stacking order, the crystal structure of the δ1kphase 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 P63/mcm (MDO1) and R3c (MDO2) are deduced for the δ1kphase. The most stable MDO polytype in the OD family of the δ1kphase is determined experimentally to be the MDO2 polytype.
- Electron diffraction
- Intermetallic compounds (IMCs)
- Scanning transmission electron microscopy (STEM)
- Single crystal