Rhombohedral to simple-cubic phase transition in arsenic under pressure

We have measured powder x-ray diffraction data and Raman spectra of arsenic at pressures up to 33 GPa. X-ray data show a rhombohedral (A7) to simple-cubic transition at 25±1 GPa (relative volume V/${\mathit{V}}_0$=0.772). An upper limit for the volume change ΔV/${\mathit{V}}_0$ at the transition is given by the experimental uncertainty of 0.5%. We propose an empirical relation between the structural u parameter and rhombohedral angle α of the A7 structure, which is used to estimate the overall variation of first- and second-neighbor bond distances under pressure. The two Raman-active fundamentals of the rhombohedral phase shift to lower frequency for pressures up to 24 GPa, but remain at finite values. Near this pressure, the Raman intensity decreases drastically and the two modes are no longer observed at higher pressures. The Raman data are consistent with a phase transition to a monatomic phase near 25 GPa. Results are discussed in light of several recent first-principles calculations of the phase stability and mode softening of arsenic under pressure.