Probing liquid-liquid phase transitions under dynamic compression: an X-ray diffraction and <i>ab initio</i> MD study of selenium

One of the most interesting phenomena of non-crystalline materials is polyamorphism, or the ability of an amorphous material, with medium range order, to exist in several distinct modifications. In liquids, the transition between these states has been termed the liquid-liquid phase transition (LLPT) and has been observed experimentally under static compression experiments in several materials. In selenium, ab initio calculations have shown significant changes in the liquid structure with increasing pressure, yet experimental data at high-pressure is limited. Even less is known regarding the evolution of the liquid structure under rapid shock compression and whether a LLPT between complex liquid structures can occur on nanosecond timescales.
Here we report on femtosecond X-ray diffraction measurements of the LLPT in selenium following shock compression and release and find that the liquid structure transformations are dramatic and we show that they can occur on nanosecond timescales. The results are supported by molecular dynamic simulations of the liquid state at a range of pressure, temperature, density states covering the LLPT. Experiments were performed at the Matter at Extreme Conditions end-station of the Linac Coherent Light Source (LCLS). This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.