Insights into the atomic and electronic structure triggered by ordered nitrogen vacancies in CrN

We report on the atomic and electronic structure of ordered nitrogen vacancies in CrN by using spherical aberration-corrected high-resolution transmission electron microscopy, electron energy-loss spectra, and ab initio calculations. The ordered nitrogen vacancies are identified to be distributed on 111 atomic planes. The vacancy concentrations were evaluated by quantitative high-resolution transmission electron microscopy. A direct consequence of the ordered nitrogen vacancies is a lattice shrinking leading to a reduced lattice constant, displaying a distorted CrN. The experimental measured lattice constant is quantitatively compared to the ab initio calculations. A relationship between the lattice constant and nitrogen vacancy concentration is theoretically and experimentally established, and quantitatively compared. The presence of the ordered N vacancies further induces the electronic changes as reflected in a very small core-level shift as well as a shift of the volume plasmon energy. Moreover, the change of the ionicity in CrN with nitrogen vacancy concentration is revealed. A direct relation between the covalent-ionic level of the bonding and the nitrogen vacancy concentration is shown.