Phase Transition and Metallization of Orpiment by Raman Spectroscopy, Electrical Conductivity and Theoretical Calculation under High Pressure.

The structural, vibrational, and electronic characteristics in orpiment were performed in the diamond anvil cell (DAC), combined with a series of experimental and theoretical research, including Raman spectroscopy, impedance spectroscopy, atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and first-principles theoretical calculations. The isostructural phase transition at ~25.0 GPa was manifested as noticeable changes in the compressibility, bond lengths, and slope of the conductivity, as well as in a continuous change in the pressure dependence of the unit cell volume. Furthermore, a pressure-induced metallization occurred at ~42.0 GPa, accompanied by reversible electrical conductivity. We also determined the metallicity of orpiment at 45.0 GPa by first-principles theoretical calculations, and the results were in good agreement with the results of the temperature-dependent conductivity measurements. The HRTEM and AFM images of the recovered sample confirmed that orpiment remains in the crystalline phase with an intact layered structure and available crystal-shaped clusters. These high-pressure behaviors of orpiment present some crucial information on the structural phase transition, metallization, amorphization and superconductivity for the A₂B₃-type of engineering materials at high pressure.