Ca₁₄GaAs₁₁: A new compound containing discrete GaAs₄ tetrahedra and a hypervalent As₃ polyatomic unit

In the course of investigating the synthesis and properties of several ternary Ga-As Zintl compounds, a new Zintl compound has been prepared: Ca₁₄GaAs₁₁. This compound has been prepared in high yield from the stoichiometric combination of the elements in a sealed Nb tube, sealed in an evacuated quartz ampule, at 1100 °C for 4 days. It crystallizes in the tetragonal space group I4₁/acd with lattice parameters a = 15.642 (2) Å and c = 21.175 (4) Å (room temperature). Single-crystal X-ray diffraction data were collected at 130 K (I4₁/acd, Z = 8, a = 15.620 (3) Å, and c = 21.138 (4) Å). Its structure is composed of isolated GaAs₄ tetrahedra that are separated by As₃ linear units, As and Ca. The As₃ units are situated between the tetrahedra and alternate by 90° down the c axis. Although the As-As distance in the As₃ unit is rather long (2.956 (2) Å), it is consistent with the long I-I distance observed in the isoelectronic, hypervalent I₃ ^- anion and therefore is considered to be formally an As₃ ^7- anion. Optical data are consistent with a semiconducting compound, indicating an optical band gap of about 1.49 eV, slightly higher in energy than that observed for GaAs. The synthesis, structure, and bonding in this compound will be discussed.