Phase transitions, including melting, during static and shock compression conditions*

Extreme conditions of high pressure and high temperature can be achieved through static compression and the diamond anvil cell, or through dynamic shock/ramp techniques using high power lasers to reach P > 1 TPa. Studies of phase transitions under static compression have traditionally been carried out using a combination of DAC and synchrotron X-ray diffraction techniques. With the high energy and high flux of X-rays that a synchrotron can deliver, one can determine even complex crystal structures such as incommensurate host-guest structures through observations of diffraction peaks with very low relative intensities. Until recently, this has not been viable for shock compression studies even at moderate pressures. Powder XRD with image plates have been performed during ramp compression into the TPa pressure range, but only a few peaks are observed above the background noise. Now, with the advent of 4^th generation light sources (X-ray FELs) ultrafast XRD experiments can be carried out with fs exposure times and sufficient flux to identify complex crystal structures and even diffuse liquid scattering during shock compression; these new results will be presented and discussed.