  Evolutionary algorithms (EA) coupled with Density Functional Theory (DFT)
calculations have been used to predict the most stable hydrides of phosphorous
(PHn, n = 1-6) at 100, 150 and 200 GPa. At these pressures phosphine is
unstable with respect to decomposition into the elemental phases, as well as
PH2 and H2. Three metallic PH2 phases were found to be dynamically stable and
superconducting between 100-200 GPa. One of these contains five formula units
in the primitive cell and has C2/m symmetry (5FU-C2/m). It is comprised of 1D
periodic PH3-PH-PH2-PH-PH3 oligomers. Two structurally related phases
consisting of phosphorous atoms that are octahedrally coordinated by four
phosphorous atoms in the equatorial positions and two hydrogen atoms in the
axial positions (I4/mmm and 2FU-C2/m) were the most stable phases between
~160-200 GPa. Their superconducting critical temperatures (Tc) were computed as
being 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula
and using a value of 0.1 for the Coulomb pseudopotential, u*. Our results
suggest that the superconductivity recently observed by Drozdov, Eremets and
Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil
cell may result from these, and other, decomposition products of phosphine.
