X-Ray Diffraction of Potassium in the High-Pressure Regime*

Due to the high compressibility of alkali metals, they provide a unique window into the high-density behavior of matter at accessible pressures.  We are working to quasi-isentropically compress potassium into the terapascal regime to explore the evolution of its ionic and electronic structural complexity with pressure. Theoretical predicts potassium transforms to a double hexagonal-close-packed (dhcp) structure at ~250 GPa.[1] Moreover, the melting curve is seen to drop to a minimum at ~20 GPa[2] then rising precipitously with pressure and without experimental bounds beyond 25 GPa. The high compressibility and low sound speed of potassium make it very difficult to explore these properties at high pressure. Hydrodynamic simulations are used to guide experimental designs to map these physical properties of potassium to pressures approaching 1 TPa. We show preliminary powder diffraction[3] and optical reflectivity measurements for potassium, ramp compressed to 500 GPa. 

[1] P. Emma et al., Nat. Phys. 4, 641 (2010).

[2] O. Narygina et al., Phys. Rev. B 84, 054111 (2011).

[3] J. R. Rygg et al., Rev. Sci. Instrum. 83, 113904 (2012).