Atomic and Electronic Properties of a 155 HS Cluster under Pressure.

This is an all-electron density functional study of a cluster with 155 H2S molecules subjected to pressures between 0.2 and 681.2 GPa. For modeling pressure, the cluster was in a container made of 500 He atoms. As the pressure increased, the bond length between the atoms decreased. This decrease changed the atomic distribution of the cluster. Initially, the HS molecules interacted weakly through hydrogen bonds. Then, the pressure moved the H atoms along the axis connecting two sulfur atoms, with S-H bond lengths between 1.4 and 1.6 Å. At high pressures, the atomic distribution consisted of interleaved layers of H and S atoms. The energy density of states of the valence band had two sub-bands with an energy gap between them. The overlapping of the 2a1 molecular orbitals of the HS molecules determined the molecular orbitals in the low-energy sub-band. In this sub-band, the molecular orbital with the lowest energy has no nodes; at high pressures, it has non-zero values for all the internuclear regions of the cluster. The overlapping of the molecular orbitals 1b2, 3a1, and 1b1 of the HS molecules determined the orbitals in the high-energy sub-band. The energy band gap (lowest unoccupied molecular orbital-highest occupied molecular orbital) decreased with the pressure, from 5.3906 eV for 0.2 GPa to 0.4980 eV for 681.2 GPa, whereas the gap between the sub-bands decreased from 4.7729 eV for 0.2 GPa to 0.03 eV for pressures higher than 125.5 GPa. The present study provides, from first principles, an idea on the role of hydrogen atoms in the evolution of solid phases of HS with pressure, which is difficult to obtain from experiments.