Structural Uniqueness of the [Nb(As)] Cluster in the Zintl Phase CsNbAs.

The structure of the novel Zintl phase, CsNbAs, is reported for the first time. This compound crystallizes in the monoclinic 2/ space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence 32. The most important structural element is the unprecedented [Nb(As)] cluster anion, formed by a Nb atom enclosed between two As rings. These nonaromatic cyclic species, formally [As], adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As] ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As)] cluster and the [As] rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on CsNbAs suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.