| Summary: | A class of Fe-Mn-Si–based alloys exhibit a reversible martensitic transformation between the γ phase with a face-centered cubic (fcc) structure and an ε phase with a hexagonal close-packed (hcp) structure. During the deformation-induced γ-ε transformation, we identified a phase that is different from the ε phase. In this phase, the electron diffraction spots are located at the 1/3 positions that correspond to the {0002} plane of the ε (hcp) phase with 2H structure, which suggests long-period stacking order (LPSO). To understand the stacking pattern and explore the possible existence of an LPSO phase as an intermediate between the γ and ε phases, the phase stability of various structural polytypes of iron was examined using first-principles calculations with a spin-polarized form of the generalized gradient approximation in density functional theory. We found that an antiferromagnetic ordered 6H_{2} structure is the most stable among the candidate LPSO structures and is energetically closest to the ε phase, which suggests that the observed LPSO-like phase adopts the 6H_{2} structure. Furthermore, we determined that the phase stability can be attributed to the valley depth in the density of states, close to the Fermi level.
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