High-Pressure Melt Curve and Phase Diagram of Lithium

We investigate the phase diagram of lithium at temperatures of 200 to 400 K, to pressures over 100 GPa using x-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the $hR1$ and $cI16$ phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40 and 60 GPa, below which the solid is crystalline. Analysis of the stability fields of the $cI16$ and $oC88$ phases suggest the existence of a triple point between these and an undetermined solid phase at 60 GPa between 220 and 255 K.