Electronic and structural properties of low-temperature superconductors and ternary pnictides ANi2Pn2 (A=Sr,Ba and Pn=P,As)

Abstract

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 (TC∼0.6 K), BaNi2As2 (TC∼0.7 K), and BaNi2P2 (TC∼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)Ni2(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)Ni2(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)Ni2(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)Ni2(P,As)2 are within the weak-coupling limit with a small average electron-phonon coupling constant λep∼0.16–0.24. The bonding in (Sr,Ba)Ni2(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.

Source-reported materials — not catalogue approval

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