Superconductivity in the intermetallic pnictide compound Ca11Bi10-x

The pnictide phase Ca11Bi10-x shows bulk superconductivity around 2.2 K in temperature-dependent resistivity and magnetic susceptibility data. The compound is a p-type metal with low carrier density of 6.5 x 10(18) cm(-3). Ca11Bi10-x (x = 0.12) was synthesized by the reaction of Ca metal with excess Bi and crystallizes in the tetragonal space group I4/mmm with a = 12.2842(6) angstrom and c = 17.866(4) angstrom. The structure of Ca11Bi10 contains three discrete units: isolated Bi atoms, dumbbells, and square planar rings of Bi surrounded by Ca atoms. Vacancies were found in the isolated Bi(1) atoms and square planar Bi-4 units of the structure. The Ca11Bi10 system is the first member found to exhibit superconductivity among the intermetallic class M11X10 (M = Ca, Sr, Ba; X = Bi, Sb), suggesting that a broader family of Bi or Sb containing superconductors may exist. Electronic structure density functional theory calculations confirm the metallic nature of the compound with several steep Bi p-orbital bands crossing the Fermi level as well as a single flat band reaching the Fermi level upon the introduction of Bi vacancies.