Novel superconducting skutterudite-type phosphorus nitride at high pressure from first-principles calculations

State of the art variable composition structure prediction based on density functional theory demonstrates that two new stoichiometries of PN, PN(3) and PN(2), become viable at high pressure. PN(3) has a skutterudite-like Immm structure and is metastable with positive phonon frequencies at pressures between 10 and 100 GPa. PN(3) is metallic and is the first reported nitrogen-based skutterudite. Its metallicity arises from nitrogen p-states which delocalise across N(4) rings characteristic of skutterudites, and it becomes a good electron-phonon superconductor at 10 GPa, with a T(c) of around 18 K. The superconductivity arises from strongly enhanced electron-phonon coupling at lower pressures, originating primarily from soft collective P-N phonon modes. The PN(2) phase is an insulator with P2/m symmetry and is stable at pressures in excess of 200 GPa.