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As active phases in low-temperature Fischer–Tropsch synthesis for liquid fuel production, epsilon iron carbides are critically important industrial materials. However, the precise atomic structure of epsilon iron carbides remains unclear, leading to a half-century of debate on the phase assignment of the ε-Fe₂C and ε′-Fe_2.2C. Here, we resolve this decades-long question by a combined theoretical and experimental investigation to assign the phases unambiguously. First, we have investigated the equilibrium structures and thermal stabilities of ε-Fe_xC (x = 1, 2, 2.2, 3, 4, 6, 8) by first-principles calculations. We have also acquired X-ray diffraction patterns and Mössbauer spectra for these epsilon iron carbides and compared them with the simulated results. These analyses indicate that the unit cell of ε-Fe₂C contains only one type of chemical environment for Fe atoms, while ε′-Fe_2.2C has six sets of chemically …