Quadrupole Arrangements and the Ground State of Solid Hydrogen

Abstract

The electric quadrupole-quadrupole (${\mathcal{E}}_{qq}$) interaction is believed to play an important role in the broken symmetry transition from Phase I to II in solid hydrogen. To evaluate this, we study structures adopted by purely classical quadrupoles using Markov Chain Monte Carlo simulations of fcc and hcp quadrupolar lattices. Both undergo first-order phase transitions from rotationally ordered to disordered structures, as indicated by a discontinuity in both quadrupole interaction energy (${\mathcal{E}}_{qq}$) and its heat capacity. Cooling fcc reliably induced a transition to the P$a3$ structure, whereas cooling hcp gave inconsistent, frustrated and $c/a$-ratio-dependent broken symmetry states. Analysing the lowest-energy hcp states using simulated annealing, we found P$6_3/m$ and P$ca{2}_1$ structures found previously as minimum-energy structures in full electronic structure calculations. The candidate structures for hydrogen Phases III-V were not observed. This demonstrates that ${\mathcal{E}}_{qq}$ is the dominant interaction determining the symmetry breaking in Phase II. The disorder transition occurs at significantly lower temperature in hcp than fcc, showing that the ${\mathcal{E}}_{qq}$ cannot be responsible for hydrogen Phase II being based on hcp.

References in corpus (6)

• Density functional theory study of phase IV of solid hydrogen
• Solid molecular hydrogen: The Broken Symmetry Phase
• Room-Temperature Structures of Solid Hydrogen at High Pressures
• High-pressure behavior of dense hydrogen up to 3.5 TPa from density functional theory calculations
• A simple thermodynamic model for the hydrogen phase diagram
• Stacking Characteristics of Close Packed Materials

Cited by in corpus (5)

• Understanding high pressure hydrogen with a hierarchical machine-learned potential
• Structures of solid hydrogen at 300K
• Nuclear Spin Crossover in Dense Molecular Hydrogen
• Two state model for critical points and the negative slope of the melting-curve
• Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens