Phase transition and in-plane anisotropy of GeAs under high pressure

GeAs exhibits excellent chemical stability and high in-plane anisotropy under ambient conditions. Pressure is one effective approach to regulate structures or properties of two-dimensional materials. In this work, the high-pressure effect on the phase transition and in-plane anisotropic properties is investigated for GeAs using Raman spectroscopy, infrared spectroscopy, high-pressure resistance measurement, and density functional theory calculations. The results show that GeAs undergoes an irreversible monoclinic-to-cubic transition around 18.4 GPa during compression. IR and resistivity measurements indicate that this cubic rock-salt structure exhibits a metallic state. Upon decompression, high-pressure cubic GeAs converts to a tetragonal structure rather than the initial monoclinic structure. Moreover, tetragonal GeAs also exhibits a metallic state. This transition is reversible, as tetragonal GeAs will convert back to cubic GeAs around 18.2 GPa. Angle-resolved polarized Raman spectroscopy reveals that for both A_g and B_g modes of monoclinic GeAs, pressure will induce a 10°-15° deflection in their polarization direction up to 10 GPa, but has no effect on their anisotropy periods. Tetragonal GeAs also exhibits excellent in-plane optical anisotropy with a period of 180°. But different from monoclinic GeAs, both the polarization direction and periods remain stable in its Raman mode under pressure. The result is conducive to understanding the structural stability and anisotropic properties of GeAs.