Structural, electronic properties, and chemical bonding in quaternary layered titanium pnictide-oxides Na_2Ti_2Pn_2O and BaTi_2Pn_2O (Pn - As, Sb) from FLAPW-GGA calculations

摘要

By means of the first-principles FLAPW-GGA calculations, we have investigated the main trends in structural, electronic properties, and chemical bonding for a series of quaternary titanium pnictide-oxides: Na_2Ti_2As_2O, Na_2Ti_2Sb_2O, BaTi_2As_2O, and BaTi_2Sb_2O, which attracted now much attention as parent phases for a novel group of layered Fe-free superconducting materials. Our results cover the optimized lattice parameters and atomic positions, electronic bands, Fermi surface topology, total and partial density of electronic states. Besides, Bader analysis and the charge density maps are used to discuss the chemical bonding for the examined materials. We find that the atomic substitutions lead to anisotropic deformation of the crystal structure; this effect is related to strong anisotropy of inter-atomic bonds, which are of a mixed (covalent-ionic-metallic) type - in blocks [Ti_2Pn_2O], whereas the bonding between blocks [Ti_2Pn_2O] and atomic sheets of Na, Ba ions is of an ionic type. The actual effective atomic charges differ from the formal ionic charges due to covalency in blocks [Ti_2Pn_2O]. The near-Fermi electronic bands, which are responsible for metallic-like behavior of these materials and will be involved in the formation of superconducting state, arise mainly from the Ti 3d_(xy), d_(x2_y2), and d_(z2) states of the blocks [Ti_2Pn_2O], which define also the anisotropic character of conduction happening mainly in these blocks. The differences in the topology of the multi-sheet Fermi surfaces of these materials are discussed.