X-Ray Diffraction of Ramp-Compressed Silicon to 390 GPa

Silicon exhibits a rich collection of phase transitions under compression at ambient temperature. We report in-situ measurements of the crystal structure of silicon between 41 and 390 GPa achieved through quasi-isentropic compression. Through angle dispersive x ray, we observe hexagonal close-packed structure between 33 and 99 GPa, and face-centered cubic structure above that to 390 GPa, the highest stress at which the crystal structure of silicon has ever been measured. Previously predicted double-hexagonal-close-packed phase between 22 and 55 GPa was not observed. A statistical model is developed to infer sample temperature from optical pyrometry data, previously thought not possible for temperatures below 5000 K, revealing temperature higher than anticipated from hydrodynamic simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and NSF Physics Frontier Center award PHY-2020249.