2120.     LIPSON, H., AND PETCH, N. J.  Crystal Structure of Cementite (Fe3C).  Jour. Iron Steel Inst. (London), vol. 142, 1940, pp. 95-106; Chem. Abs., vol. 34, 1940, p. 6501.

        Specimen of fine powder (through 380-mesh) was prepared by passing CO over Fe₂O₃ at 550°.  Annealing at 600° resulted in partial decomposition (into a structure, probably Fe₂C) and sharpening of the cementite lines.  Intensities were measured from the unannealed specimens, and spacings from one annealed at 600° for 17 hr.  The structure is an almost close-packing of Fe atoms with C atoms in the interstices.  The environment of the C atoms consists of 6 Fe atoms arranged at the corners of a triangular prism.  The average Fe-C distance is 2.03 Å.  2 other Fe atoms are only 2.31 Å from each C.  The relation of the structure to either the ferrite or austenite structures is not very obvious.

                    LISTER, F.  See abs. 2824.