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

International audienceState 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

Quantum versus classical protons in pure and salty ice under pressure

International audienceIt is generally accepted that nuclear quantum effects (NQEs) trigger the transition to the nonmolecular form of ice under increasing pressure. This picture is challenged in salty ice, where Raman scattering measurements up to 130 GPa of molecular ice VII containing NaCl or LiCl impurities show that the transition pressure to the symmetric phase ice X is shifted up by about 30 GPa, even at small salt concentrations. We address the question of how the inclusion of salt induces the drastic reduction of NQEs by selectively including NQEs in ab initio calculations of ice in the presence of distinct ionic impurities. We quantitatively show that this is mainly a consequence of the electric field generated by the ions. We propose a simple model that is able to capture the essence of this phenomenon, generalizing this picture to other charged defects and for any concentration. This result is potentially generalizable to most "dirty" ices in which the electric field due to the doping is much more significant than local lattice distortions