Electronic And Structural Properties Of Low-Temperature Superconductors And Ternary Pnictides Ani(2)Pn(2) (A=Sr,Ba And Pn=P,As)

Based on first-principles full-potential linearized augmented plane wave method (FLAPW)-generalized gradient approximation calculations, we have investigated structural and electronic properties of low-temperature superconductors SrNi2As2 (T-C similar to 0.6 K), BaNi2As2 (T-C similar to 0.7 K), and BaNi2P2 (T-C similar to 3 K), as well as SrNi2P2. Our results show that the replacement of alkaline-earth metal (Sr <-> Ba) and pnictogen (P <-> As) types leads to anisotropic deformations of crystal structure caused by strong anisotropy of interatomic bonds. The band structure, density of states, and Fermi-surface features for (Sr,Ba)Ni-2(P,As)(2) are evaluated and discussed. As distinct from (Ca,Sr,Ba)Fe2As2-the parent phases for "122" FeAs superconductors-the Fermi level in (Sr,Ba)Ni-2(P,As)(2) phases is shifted to the bands with higher dispersion E(k) but lower density of states as a result of increased electron concentration. Therefore the Fermi surfaces for (Sr,Ba)Ni-2(P,As)(2) phases differ essentially from those of the FeAs-based materials and adopt a multisheet three-dimensional type. Our estimations show that (Sr,Ba)Ni-2(P,As)(2) are within the weak-coupling limit with a small average electron-phonon coupling constant lambda(ep)similar to 0.16-0.24. The bonding in (Sr,Ba)Ni-2(P,As)(2) is of a complex anisotropic character. Namely, the bonding in [NiP(As)] layers may be described as a mixture of metallic, ionic, and covalent contributions. In turn, between adjacent [NiP(As)] layers and (Sr,Ba) atomic sheets, ionic bonds emerge, whereas between adjacent [NiP(As)]/[NiP(As)] layers covalent bonds occur owing to hybridization of p states of pnictogen atoms.