mP-BaP₃: a new phase from an old binary system.

A polyphosphide, mP-BaP3, with a unique two-dimensional phosphorus layer has been discovered and characterized. It crystallizes in the monoclinic space group P2₁/c with unit-cell parameters a=6.486(1), b=7.710(1), c=8.172(2) Å; β=104.72(3)°; Z=4. Its phosphorus polyanion can be derived from the strong elongation of 2/3 of the P-P bonds present in the layers of black phosphorus. The unit-cell volume of the mP-BaP3 phase is 1.4% larger than the volume of another polymorph, mS-BaP3, reported more than 40 years ago. The latter phase features the presence of one-dimensional phosphorus chains separated by Ba atoms. The differences in the structures of the phosphorus fragments in both polymorphs of barium triphosphide result in large differences in both the thermal stability of these materials as well as in their properties as evidenced by DSC, (31)P solid-state MAS NMR, UV/Vis, and surface photovoltage spectroscopies, alongside quantum-chemical calculations.