Synthesis, Crystal and Electronic Structures of the New Zintl phases Ba3Al3Pn5 (Pn = P, As) and Ba3Ga3P5

The new Zintl compounds Ba3Al3P5, Ba3Al3As5, and Ba3Ga3P5 have been synthesized using molten metal fluxes. They are isoelectronic and isotypic, crystallizing with a novel rhombohedral structure type in the space group R3̅c with unit cell constants a = 14.5886(9) Å, c = 28.990(3) Å for Ba3Al3P5, a = 14.613(3) Å, c = 28.884(8) Å for Ba3Ga3P5, and a = 14.9727(13) Å, c = 29.689(4) Å for Ba3Al3As5, respectively. The structures are based on TrPn4 (Tr = Al, Ga; Pn = P, As) tetrahedra that share both edges and corners, leading to intricate arrangements embodied in the [Tr4Pn9]15– and [Tr3Pn6]9– strands, interconnected by dimeric [Tr2Pn6]12– units. The Ba2+ 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 [Ba2+]3[Tr3+]3[Pn3–]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.