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. Ba₃P₂ = Ba₄P_2.67□_0.33 (□ = vacancy) crystallizes in the anti-Th₃P₄ structure type with the cubic space group I4̅3d (no. 220), Z = 6, a = 9.7520(7) Å. In the Ba₃P₂ crystal structure, isolated P^3– anions form distorted octahedra around the Ba²⁺ cations. β-Ba₅P₄ crystallizes in the Eu₅As₄ structure type with the orthorhombic space group Cmce (no. 64), Z = 4, a = 16.521(2) Å, b = 8.3422(9) Å, c = 8.4216(9) Å. In the crystal structure of β-Ba₅P₄, one-half of the phosphorus atoms are condensed into P₂^4– dumbbells. SrP₂ and BaP₂ are isostructural and crystallize in the monoclinic space group P2₁/c (no. 14), Z = 6, a = 6.120(2)/6.368(1) Å, b = 11.818(3)/12.133(2) Å, c = 7.441(2)/7.687(2) Å, β = 126.681(4)/126.766(2)° for SrP₂/BaP₂. In the crystal structures of SrP₂ and BaP₂, all phosphorus atoms are condensed into _∞¹P^1– 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 SrP₂ and BaP₂ are electron-balanced semiconductors.