The determination of the crystal structure of the σ phase in the iron–chromium and iron–molybdenum systems

The crystal structure of the a phase in the iron-chromium system at 46.5 atomic % chromium was determined by the application of powder and single-crystal X-ray diffraction methods.The crystal structure of this phase in the iron-molybdenum system was also determined by the use of powder and single-crystal methods, and found to be essentially the same as that for the iron-chromium phase.Reliable information regarding the distribution of iron and chromium among the various positions could not be obtained directly for a-FeCr.However, reasonably reliable information regarding the distribution of iron and molybdenum in a-FeMo was obtained, and probably the chromium atoms in a-FeCr occupy substantially the same positions as the molybdenum atoms in a-FeMo.The iron-chromium a-phase structure has space group D~-P4~/mnm, with a 0 = 8.800A and c o = 4.544 A. The iron-molybdenum presumably has the same space group, and has a 0 = 9.188A and co = 4.812 A. There are 30 atoms in the unit cell.The positions of the atoms are at the points of two pseudo-hexagonal nets normal to c at z = 0 and z = ½.Eight of the 30 atoms are displaced parallel to c by approximately the distance ~c 0, so that there are four atoms at z ~ ¼ and four at z ~ ~.The structure is nearly identical with that found independently by Tucker for fl-uranium.