Ba and Sr Binary Phosphides: Synthesis, Crystal Structures, and Bonding Analysis

Synthesis, crystal structures, and chemical bonding are reported for four binary phosphides with different degrees of phosphorus oligomerization, ranging from isolated P atoms to infinite phosphorus chains. Ba<sub>3</sub>P<sub>2</sub> = Ba<sub>4</sub>P<sub>2.67</sub>□<sub>0.33</sub> (□ = vacancy) crystallizes in the anti-Th<sub>3</sub>P<sub>4</sub> structure type with the cubic space group <i>I</i>4̅3<i>d</i> (no. 220), <i>Z</i> = 6, <i>a</i> = 9.7520(7) Å. In the Ba<sub>3</sub>P<sub>2</sub> crystal structure, isolated P<sup>3–</sup> anions form distorted octahedra around the Ba<sup>2+</sup> cations. β-Ba<sub>5</sub>P<sub>4</sub> crystallizes in the Eu<sub>5</sub>As<sub>4</sub> structure type with the orthorhombic space group <i>Cmce</i> (no. 64), <i>Z</i> = 4, <i>a</i> = 16.521(2) Å, <i>b</i> = 8.3422(9) Å, <i>c</i> = 8.4216(9) Å. In the crystal structure of β-Ba<sub>5</sub>P<sub>4</sub>, one-half of the phosphorus atoms are condensed into P<sub>2</sub><sup>4–</sup> dumbbells. SrP<sub>2</sub> and BaP<sub>2</sub> are isostructural and crystallize in the monoclinic space group <i>P</i>2<sub>1</sub><i>/c</i> (no. 14), <i>Z</i> = 6, <i>a</i> = 6.120(2)/6.368(1) Å, <i>b</i> = 11.818(3)/12.133(2) Å, <i>c</i> = 7.441(2)/7.687(2) Å, β = 126.681(4)/126.766(2)° for SrP<sub>2</sub>/BaP<sub>2</sub>. In the crystal structures of SrP<sub>2</sub> and BaP<sub>2</sub>, all phosphorus atoms are condensed into <sub>∞</sub><sup>1</sup><i>P</i><sup>1–</sup> cis–trans helical chains. Electronic structure calculations, chemical bonding analysis via the recently developed solid-state adaptive natural density partitioning (SSAdNDP) method, and UV–vis spectroscopy reveal that SrP<sub>2</sub> and BaP<sub>2</sub> are electron-balanced semiconductors.