Bonding properties of Ca₁₄GaAs₁₁: A compound containing discrete GaAs₄ tetrahedra and a hypervalent As₃ polyatomic unit

We have theoretically studied the bonding properties of Ca₁₄GaAs₁₁ using the ab initio, self-consistent pseudopotential method within the local density approximation of density functional theory. Ca₁₄GaAs₁₁ is an example of a new series of recently prepared compounds which have the stoichiometry A₁₄MPn₁₁ (A = Ca, Sr, Ba; M = Al, Ga, Mn; Pn = As, Sb, Bi). We have found that the electronic structure and bonding of the Ca₁₄GaAs₁₁ compound are consistent with the Zintl concept. In addition, the GaAs₄ tetrahedra and the linear As₃ groups contained in this compound are isolated units which are not arranged into an extended bonding network. The bonding within the GaAs₄ group is slightly more ionic than the bonding in a GaAs crystal. The linear As₃ unit can be accurately described as a hypervalent three-center, four-electron bonded structure. The A₁₄MPn₁₁ structure type contains the first example of a discrete, hypervalent group 15 linear anion.