Synthesis, Crystal and Electronic Structures of the New Zintl phases Ba₃Al₃<i>Pn</i>₅ (<i>Pn</i> = P, As) and Ba₃Ga₃P₅

Abstract

The new Zintl compounds Ba(3)Al(3)P(5), Ba(3)Al(3)As(5,) and Ba(3)Ga(3)P(5) have been synthesized using molten metal fluxes. They are isoelectronic and isotypic, crystallizing with a novel rhombohedral structure type in the space group R3c with unit cell constants a = 14.5886(9) Å, c = 28.990(3) Å for Ba(3)Al(3)P(5), a = 14.613(3) Å, c = 28.884(8) Å for Ba(3)Ga(3)P(5), and a = 14.9727(13) Å, c = 29.689(4) Å for Ba(3)Al(3)As(5), respectively. The structures are based on TrPn(4) (Tr = Al, Ga; Pn = P, As) tetrahedra that share both edges and corners, leading to intricate arrangements embodied in the [Tr(4)Pn(9)](15-) and [Tr(3)Pn(6)](9-) strands, interconnected by dimeric [Tr(2)Pn(6)](12-) units. The Ba(2+) cations reside within cylindrical channels within the polyanionic framework and provide the valence electrons needed for Tr-Pn covalent bonding. In spite of the large and complex structure, there are no homoatomic Tr-Tr or Pn-Pn interactions, hence, the structures can be readily rationalized in the context of the Zintl-Klemm formalism as follows [Ba(2+)](3)[Tr(3+)](3)[Pn(3-)](5); calculations on their electronic band-structures confirm this reasoning and reveal about 1.4-1.9 eV energy band gaps, that is, semiconducting behavior. Structural parallels with other known Zintl compounds are also presented.