Superconductivity in the Zintl intermetallic compound Ca$_{11}$Bi$_{10-x}$

The recent discovery of the iron-based superconductors with unconventional superconductivity as a new class of superconductors has attracted great attention and triggered extensive research for new compounds. We report the new superconductor Ca$_{11}$Bi$_{10-x}$, which is in fact a Zintl phase. The structure of Ca$_{11}$Bi$_{10}$ contains three discrete anionic fragments: isolated Bi$^{3-}$ ions, dumbbells of Bi$_{2}^{4-}$ and square planar rings of Bi$_{4}^{4-}$ surrounded by Ca$^{2+}$cations. The Bi$_{4}^{4-}$ squares and the Bi$_{2}^{4-}$ dumbbells interact with one another through Bi---Bi bonding to form an extended 3D framework. The extended three-dimensional Bi-Bi interactions are responsible for the metallic behavior observed above T$_{c}$. Electronic band structure calculations at the density functional theory (DFT) level confirm the metallic character of the material. Defects in the form of vacancies on the Bi-sites were also found using single crystal X-ray analysis. The unexpected finding is that unlike most superconductors Ca$_{11}$Bi$_{10-x}$ has very low carrier density. The Ca$_{11}$Bi$_{10-x}$ system is the first member of the intermetallic class M$_{11}$X$_{10}$ (M$=$Ca, Sr, Ba; X$=$Bi, Sb) that exhibits superconductivity suggesting that a broader family of Bi or Sb-containing superconductors may exist.