  We report a joint experimental and theoretical study of the structural,
vibrational, and electronic properties of layered monoclinic arsenic sulfide
(alpha-As2S3), aka mineral orpiment, under compression. X-ray diffraction and
Raman scattering measurements performed in orpiment samples at high pressure
and combined with ab initio calculations have allowed us to determine the
equation of state and the tentative assignment of the symmetry of many
Raman-active modes of orpiment. From our results, we conclude that no
first-order phase transition occurs up to 25 GPa at room temperature; however,
compression leads to an isostructural phase transition above 20 GPa. In fact,
As coordination increases from threefold at room pressure to more than fivefold
above 20 GPa. This increase in coordination can be understood as the formation
of metavalent bonding at high pressure, which results in a progressive decrease
of the electronic and optical bandgap, an increase of the dielectric tensor and
Born effective charges, and a considerable softening of many high-frequency
optical modes with pressure. The formation of metavalent bonding may also
explain the behavior of other group-15 sesquichalcogenides under compression.
Moreover, our results suggest that group-15 sesquichalcogenides either show
metavalent bonding at room pressure or undergo a transition from p-type
covalent bonding at room pressure towards metavalent bonding at high pressure,
as a preceding phase towards metallic bonding at very high pressure.
