Re-determination of the crystal structure of χ-Fe5C2 Hägg carbide

The crystal structure of χ-Fe5C2 Hägg carbide was re-determined using the simulated annealing capability of Topas Academic (Coelho, 2000). The unit cell is triclinic (space group P-1) with a = 11.5695(5) Å, b = 4.5713(2) Å, c = 5.0588(2) Å, α = 89.884(4)°, β = 97.765(3)° and γ = 89.964(5)°. The crystal structure was fitted by Rietveld refinement, resulting in R wp = 8.19% and R Bragg = 2.54%. The triclinic space group of χ-Fe5C2 Hägg carbide was determined using selected area electron diffraction (SAED). A TEM study was done to determine exposed crystallographic faces on coarse χ-Fe5C2 Hägg carbide particles. Energy calculations using Castep (Cambridge Sequential Total Energy Package, Payne, et al., 1992 and Milman et al., 2000) indicated the triclinic unit cell of χ-Fe5C2 Hägg carbide to have the same total energy as the monoclinic unit cell. Thermogravimetric analysis was used to characterize the carbon species in the sample.