Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen

A new class of strongly infrared (IR) active structures is identified for phase III of compressed molecular H-2 by constant-pressure ab initio molecular dynamics and density-functional perturbation calculations. These are planar quadrupolar structures obtained as a distortion of low-pressure quadrupolar phases, after they become unstable at about 150 GPa due to a zone boundary soft phonon. The nature of the II-III transition and the origin of the IR activity are rationalized by means of simple electrostatics, as the onset of a stabilizing dipole-quadrapole interaction.