Nitrogen atom shift and the structural change in chromium nitride

The atomic configurations and structural evolution of disordered Cr2N have been investigated by CS-corrected high-resolution transmission electron microscopy (HRTEM) imaging, electron diffraction analysis, electron energy loss spectroscopy (EELS) and ab initio calculations. The atomic structure of the disordered Cr2N is experimentally confirmed by CS-corrected atomic resolution imaging. Both diffraction and HRTEM image analysis revealed that the disordered Cr2N structure possesses a hexagonal (hcp) arrangement of Cr atoms with N atom random distributions, particularly, N atoms shift away from octahedral interstices at (0 0 1) plane. The nitrogen atom shifts result in a clear reduction of the relative intensity ratio of (0 3 0) to (0 0 2) reflection. It has also been found that the disordered Cr2N can transform into a face-centered cubic (fcc) phase at the specimen surface while chemically retaining a Cr/N atomic ratio close to 2:1. Structurally, such cubic phase is similar to sub-stoichiometric fcc CrN. The analysis corroborated that the structural transformation could be achieved by shearing motion of Cr atoms in (0 0 1) planes. Based on the experimental observations and theoretical calculations, the effects of nitrogen atom redistribution on the structure and the stability of various Cr2N phases are addressed in detail.