Melting curve Tm(p) of sodium under pressure p: Transition from a Wigner-like nuclear bcc structure near 30 GPa to lower symmetry states with negative dTm/dp at higher pressures

It is known experimentally that the melting temperature T-m(p) of Na as a function of pressure p increases with p out to similar to 20GPa and then passes through a maximum. Here, we interpret the behavior as passing from classical Wigner-like bcc nuclear structure at low pressures to a structure out to about 60GPa where the melting is mediated by topological defects.