Vibrational and elastic properties of As4O6 and As4O6·2He at high pressures: Study of dynamical and mechanical stability.

The formation of a new compound with stoichiometry As₄O₆·2He at relatively low pressure (3 GPa) has been recently reported when arsenolite (As₄O₆) powder is compressed with helium as a pressure-transmitting medium. In this work, we study the lattice dynamics of As₄O₆ and As₄O₆·2He at high pressures from an experimental and theoretical perspective by means of Raman scattering measurements and ab initio calculations and report the theoretical elastic properties of both compounds at high pressure. Raman scattering measurements show a completely different behaviour of As₄O₆ and As₄O₆·2He at high pressures. Furthermore, the theoretical calculation of phonon dispersion curves and elastic stiffness coefficients at high pressure in both compounds allow us to discuss their dynamical and mechanical stability under hydrostatic compression. Both compounds are dynamically stable even above 35 GPa, but As₄O₆ becomes mechanically unstable at pressures beyond 19.7 GPa. These results allow explaining the pressure-induced amorphization of As₄O₆ found experimentally above 15-20 GPa and the lack of observation of any instability in As₄O₆·2He up to the highest studied pressure (30 GPa). [ABSTRACT FROM AUTHOR]