Electronic and structural properties of low-temperature superconductors and ternary pnictides $A{\text{Ni}}_{2}P{n}_{2}$ ($A=\text{Sr},\text{Ba}$ and $Pn=\text{P},\text{As}$)

Electronic and structural properties of low-temperature superconductors and ternary pnictides $A {Ni}_{2} P n_{2}$ ( $A = Sr, Ba$ and $P n = P, As$ )

I. R. Shein and A. L. Ivanovskii

Phys. Rev. B 79, 054510 – Published 11 February 2009

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 ${SrNi}_{2} {As}_{2}$ $(T_{C} \sim 0.6 K)$ , ${BaNi}_{2} {As}_{2}$ $(T_{C} \sim 0.7 K)$ , and ${BaNi}_{2} P_{2}$ $(T_{C} \sim 3 K)$ , as well as ${SrNi}_{2} P_{2}$ . Our results show that the replacement of alkaline-earth metal $(Sr \leftrightarrow Ba)$ and pnictogen $(P \leftrightarrow As)$ 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 $(Ca, Sr, Ba) {Fe}_{2} {As}_{2}$ —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 $λ_{ep} \sim 0.16 - 0.24$ . 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.