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Thin iron films formed by evaporation were carburized in CO gas stream. The carbides ε, χ and θ were found to be formed in the temperature ranges (i) below 250°C, (ii) from 250° to 350°C and (iii) above 350°C, respectively. The ε-carbide is hexagonal with lattice constants a = 4.767Å and c = 4.354Å, and its structure belongs to the space group P 6 3 22, iron atoms occupying 6 g positions with the parameter x = 1/3 and carbon atoms 2 d positions. Irreversible phase transitions from ε to χ and from χ to θ take place at 380°∼400°C and at about 550°C, respectively. By the transitions changes in the chemical compositions may be very small. Observation on the crystal growth suggests that a mechanism of the ε-carbide formation is different from that of the χ- and θ- carbide formations.
Sigemaro Nagakura (Sun,) studied this question.