Orpiment under compression: metavalent bonding at high pressure

TL;DR

This study combines experimental and theoretical methods to investigate how orpiment's structure, bonding, and electronic properties evolve under high pressure, revealing the formation of metavalent bonding and its implications.

Contribution

It provides the first detailed analysis of metavalent bonding in orpiment under high pressure using combined X-ray, Raman, and ab initio techniques.

Findings

No first-order phase transition up to 25 GPa

Formation of metavalent bonding above 20 GPa

Decrease in electronic and optical bandgap with pressure

Abstract

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.