Novel ternary Zintl phosphide halides Ba₃P₅X (X = Cl, Br) with 1D helical phosphorus chains: synthesis, crystal and electronic structure

Black single crystals of two novel ternary phosphide halides, Ba₃P₅Cl and Ba₃P₅Br, were grown using molten metal Pb-flux high-temperature reactions. These compounds were structurally characterized with the aid of the single-crystal X-ray diffraction (SCXRD) method at 100(2) K. The SCXRD shows that both compounds are isostructural and adopt a new structure type (space group R3̄c, No. 167, Z = 6) with unit cell parameters a = 14.9481(16) Å, c = 7.3954(11) Å and a = 15.045(4) Å, c = 7.537(3) Å for Ba₃P₅Cl and Ba₃P₅Br, respectively. Cl^- and Br^- anions are octahedrally coordinated by Ba²⁺ cations, thus composing a face-sharing 1D infinite chain 1∞[XBa₃]⁵⁺ running along the [001] direction. Moreover, the crystal structures feature peculiar one-dimensional disordered infinite helical chains of 1∞P^-, composed of partially occupied phosphorous atoms, each being a superposition of three symmetrical copies of the ordered phosphorus chain, with continuity along the c-axis. Ba₃P₅X (X = Cl, Br) compounds are charge-balanced heteroanionic Zintl phases according to the charge-partitioning scheme (Ba²⁺)₃[P^-]₅X^-. The presumed semiconducting behavior of both compounds corroborates well with the results of the electronic structure calculations performed with the aid of the TB-LMTO-ASA code.