Synthesis and structural characterization of the ternary Zintl phases AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As)

Ten new ternary phosphides and arsenides with empirical formulae AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) have been synthesized using molten Ga, Al, and Pb fluxes. They have been structurally characterized by single-crystal and powder X-ray diffraction to form with two different structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4 crystallize with the Ca3Al2As4 structure type (space group C2/c, Z=4); Ba3Al2P4 and Ba3Al2As4 adopt the Na3Fe2S4 structure type (space group Pnma, Z=4). The polyanions in both structures are made up of TrPn4 tetrahedra, which share common corners and edges to form [TrPn2]3–∞2 layers in the phases with the Ca3Al2As4 structure, and [TrPn2]3–∞1 chains in Ba3Al2P4 and Ba3Al2As4 with the Na3Fe2S4 structure type. The valence electron count for all of these compounds follows the Zintl–Klemm rules. Electronic band structure calculations confirm them to be semiconductors.

Graphical abstractAE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) crystallize in two different structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4, are isotypic with the previously reported Ca3Al2As4 (space group C2/c (No. 15)), while Ba3Al2P4 and Ba3Al2As4 adopt a different structure known for Na3Fe2S4 (space group Pnma (No. 62). The polyanions in both structures are made up of TrPn4 tetrahedra, which by sharing common corners and edges, form [TrPn2]3–∞2layers in the former and [TrPn2]3–∞1 chains in Ba3Al2P4 and Ba3Al2As4.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) are new ternary pnictides. ► Ba3Al2P4 and Ba3Al2As4 adopt the Na3Fe2S4 structure type. ► The Sr- and Ca-compounds crystallize with the Ca3Al2As4 structure type. ► The valence electron count for all title compounds follows the Zintl–Klemm rules.