Melting of LiF and NaCl to 1 Mbar: Systematics of Ionic Solids at Extreme Conditions

We report the first diamond cell data for the melting curves of LiF and NaCl up to 1 Mbar (100 GPa). The melting curve of NaCl is in agreement with shock data and shows a break in slope at 290 kbar due to the B1(NaCl)-B2(CsCl) transition. LiF remains $B1$ to the highest pressure, and molecular dynamic simulations predict that molten LiF retains a simple-cubic $B1$-like structure. A plot of the recent Mainz diamond anvil melting curves for MgO, LiF, KBr, KCl, CsI, and NaCl shows that at very high pressure all the melting curves have very similar slopes approaching a value of order 1--2 K/kbar.