Structure of the commensurate phase p″ of wiistite Fe₀.₉₀₂O studied by high resolution electron microscopy

Atomic level observations have been made of the commensurate phase P” of Fe₀.₉₀₂O. In the ideal state, the crystal structure is monoclinic with lattice parameters a=b=5/2√2a₀, c=5a₀ and γ=90°, and belongs to the space group P2_x/m. Here, a₀ is the lattice parameter of the fundamental cell taking the NaCl type of structure. The unit cell contains four iron vacancy clusters, each of which is made of two vacancy tetrahedra with a common edge and contains no tetrahedral iron atoms. This ideal structure is not usually realized in a real crystal, where stacking disorders in the cluster layers are successively introduced and in some cases the cluster itself is heavily deformed.