Subnitride chemistry: A first-principles study of the NaBa3N, Na5Ba3N, and Na16Ba6N phases

摘要

An ab initio study on the electronic structure of the subnitrides NaBa3N, Na5Ba3N, and Na16Ba6N is performed for the first time. The NaBa3N and Na5Ba3N phases consist of infinite (1)(infinity)[NBa6/2] strands composed of face-sharing NBa6 octahedra surrounded by a "sea" of sodium atoms. The Na16Ba6N phase consist of discrete [NBa6] octahedra arranged in a body-cubic fashion, surrounded by a "sea" of sodium atoms. Our calculations suggest that the title subnitrides are metals. Analysis of the electronic structure shows partial interaction of N(2s) with Ba(5p) electrons in the lower energy region for NaBa3N and Na5Ba3N. However, no dispersion is observed for the N(2s) and Ba(5p) bands in the cubic phase Na16Ba6N. The metallic band below the Fermi level shows a strong mixing of N(2p), Ba(6s), Ba(5d), Ba(6p), Na(3s) and Na(3p) orbitals. The metallic character in these nitrides sterns from delocalized electrons corresponding to hybridized 5d ' 6s(m)6p(n) barium orbitals which interact with hybridized 3s(n)3p(m) sodium orbitals. Analysis of the electron density and electronic structure in these nitrides shows two different regions: a metallic matrix corresponding to the sodium atoms and the regions around them and heteropolar bonding between nitrogen and barium within the infinite (1)(infinity)[NBa6/2] strands of the NaBa3N and Na5Ba3N phases, and within the isolated [NBa6] octahedra of the Na16Ba6N phase. The nitrogen atoms inside the strands and octahedra are negatively charged, the anionic character of nitrogens being larger in the isolated octahedra of the cubic phase Na16Ba6N, due to the lack of electron delocalization along one direction as opposed to the other phases. The sodium and barium atoms appear to be slightly negatively and positively charged, the latter to a larger extent. From the computed Ba-N overlap populations as well as the analysis of the contour maps of differences between total density and superposition of atomic densities, we suggest partial covalent bonding between nitrogen and barium atoms along the infinite (1)(infinity)[NBa6/2] strands and within isolated [NBa6] octahedra. (c) 2005 Elsevier Inc. All rights reserved.