Crystal structure and atomic vibrations of laser ramp-compressed Pb to 600 GPa

Laser ramp-compression is an increasingly popular means for accessing high pressure states in a solid far out of the range of traditional static-compression experiments, for the purpose of probing the phase diagram and testing first-principles predictions. However, the effects of nanosecond compression rates on the kinetics of high pressure phase transformations and on the temperature are poorly constrained. Using x-ray diffraction at the NIF and Omega laser facilities, we have explored these effects for the Pb system, which has two well-known high pressure phase transitions and a melting curve established by previous static experiments [1,2]. We will present the results of diffraction measurements exploring the effect of dynamic compression on the phase boundaries by measuring in-situ crystal structure and constraining the mean squared atomic displacement, which has a direct correlation with temperature, using the Debye-Waller attenuation of diffraction peak intensities at high angle. [1] Vohra et al., PRB 42, 8651 (1990). [2] Dewaele et al., PRB 76, 144106 (2007).