High Pressure Phase Diagram of Silicon

Advised by Professors Gilbert Collins and Ryan Rygg

Hopeman 224

The experiment described in this thesis explores the phase diagram of silicon near its isentrope from 40 to 400 GPa, by ramp compressing silicon by a laser drive. Thermodynamic states of silicon at these states are measured by velocimetry, and the crystal structure is determined by nanosecond in-situ x-ray diffraction. The experiment shows a significant increase of the stability range of the Si hcp phase compared to theoretical predictions. The hcp phase is observed at the pressure and temperature range where dhcp phase was predicted, and no evidence of the dhcp phase is observed. Furthermore, the hcp-fcc phase transition pressure is at least 93 GPa, much higher than the 55 GPa predicted by computation. This observation is consistent with previous shock compression experiments. The fcc phase is confirmed to remain stable to at least 400 GPa.

Currently, no temperature data exist from nanosecond pyrometric measurements on ramp compression experiments. Such measurements are difficult due to the low number of photons emitted from low temperature (lower than 4000 K) targets. In this work, we present the foundational framework for analyzing low signal-to-noise ratio data. This method yields identical results as traditional techniques at high temperatures, but is more robust at low temperatures. This sets the stage for analyzing future low temperature pyrometry data.