Ca₁₄FeAs₁₁ – A structure comprising structural motifs of iron-based superconductors and Ca-As Zintl phases

Ca₁₄FeAs₁₁ is the first iron-containing representative of the Ca₁₄AlSb₁₁ structure type and the first compound with isolated FeAs₄ tetrahedra. It can be regarded as a salt-like compound containing isolated [FeAs₄]⁹⁻ tetrahedra, linear [As₃]⁷⁻ units, isolated As³⁻ anions and Ca²⁺ cations and thus is a borderline compound between binary Fe−As intermetallics and salt-like calcium arsenides. Ca₁₄FeAs₁₁ is obtained by a two-step high-temperature reaction, in which preformed FeAs alloy is heated with Ca in the presence of additional As in a sealed silica tube. Structure determination from single crystals at 150 K reveal the Ca₁₄AlSb₁₁ structure type with space group I4₁/acd (Z=8, a=15.6268(2) Å, c=20.9781(5) Å, and V=5122.79(15) ų). Band structure calculations predict metallic properties with strong contribution of Fe orbitals to the density of states at the Fermi level.