Synthesis, Structure, and Properties of Ca16Sb11, a Complex Zintl Phase. Twelve Other Isotypic Compounds Formed by Divalent Metals and Pnictogens

The systems A = Ca, Sr, Ba, Eu, Yb and Pn = As, Sb, Bi form isostructural A 16 Pn 11 phases for all but Ba−As and Yb−As when the elements are reacted in Ta containers at about 1100 °C and then annealed at 825−900 °C or slowly cooled. It is important to run particularly the Ae 16 Sb 11 reactions for Ae = Ca, Sr, Ba either under dynamic vacuum or to previously remove the pervasive hydrogen impurity from these metals. The tetragonal structure type was defined for Ca 16 Sb 11 ( P 4̄2 1 m, Z = 2, a = 12.253(3) Å, c = 11.313(4) Å). The complex structure is built of pairs of confacial W 5 Si 3 -like slabs intergrown with single square-prismatic slabs along c⃗ in which Ca atoms in both define the polyhedra centered by Sb. This structure is the missing n = 2 member of the homologous series A 5 n +6 B 3 n +5 (Parthé) with n:1 ratios of intergrown slabs. Four of five Ae 16 (Sb,Bi) 11 examples studied are diamagnetic (Ba 16 Sb 11 excepted), and Ca 16 Sb 11 is a poor semiconductor. Accommodation of these Zintl-phase characteristics in the Ca 16 Sb 11 structure requires that 50% of the Sb atom pairs centering the confacial square antiprisms be bonded ( d = 3.40 Å) on a random basis. Ellipsoids on these atoms are elongated along the proposed bonds.