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

TL;DR

This study identifies a new class of infrared-active structures in phase III of compressed hydrogen, explaining the phase transition and IR activity through dipole-quadrupole interactions using ab initio simulations.

Contribution

It introduces a novel class of planar quadrupolar structures and explains the phase III transition mechanism via electrostatic interactions.

Findings

Identification of strongly IR-active quadrupolar structures

Transition from low-pressure to high-pressure phases explained

Dipole-quadrupole interactions stabilize new structures

Abstract

A new class of strongly infrared active structures is identified for phase III of compressed molecular H2 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-quadrupole interaction.