Formation and Decomposition of Iron Nitrides Observed by in situ Powder Neutron Diffraction and Thermal Analysis

In order to gain more information on the formation and decomposition behavior of various iron nitrides from different starting materials in situ neutron diffraction and thermal analysis under application of different gas atmospheres and heating rates were carried out. The following phases were observed during these investigations: crystalline alpha-Fe, gamma-FeNz, gamma'-Fe4Ny, epsilon-Fe3N1+x, zeta-Fe2N, FeCl2, [Fe(NH3)(6)]Cl-2, Fe(NH3)(2)Cl-2, and amorphous Fe(NH3)Cl-2. In situ neutron diffraction data were collected in high quality, due to an optimized experimental setup with a time resolution of two minutes on D20 (Institut Laue-Langevin) allowing for detailed Rietveld analyses. For all phase transitions, decomposition and formation temperatures a strong dependency from the heating rate, thermal history of the sample, gas flow conditions, and particle size exists. The nitrogen contents observed during thermal decomposition of zeta-Fe2N were related to the binary phase diagram Fe-N. At low temperatures (<400 degrees C) epsilon-and zeta-phase are non-equilibrated. However, through annealing equilibrium state is reached. For gamma'-Fe4Ny formed in situ at higher temperatures an expansion of the homogeneity range towards lower nitrogen content is observed above 600 degrees C. For the formation of epsilon-Fe3N1+x from FeCl2 and NH3 a previously proposed reaction sequence involving different ammoniates was confirmed. This reaction occurs via formation of amorphous Fe(NH3)Cl-2. The therein observed in situ formed ammoniates were additionally characterized by IR spectroscopy.