[en] Theoretical or Mathematical, Experimental/ ab initio calculations ; electrical conductivity transitions ; electronic density of states ; Fermi level ; high-pressure effects ; liquid metals ; liquid structure ; melting ; Peierls instability ; sodium/ electronic transitions ; pressure-induced structural transitions ; liquid sodium ; light alkali metals ; free-electron-like crystals ; symmetry-breaking transitions ; Peierls mechanism ; melting ; ab initio calculations ; electrical conductivity ; Fermi level ; electronic density of states ; temperature 1000 K to 298 K ; pressure 30 GPa ; pressure 120 GPa ; Na/ A7125L Electronic structure of liquid metals and semiconductors and their alloys A7215C Electrical and thermal conduction in amorphous and liquid metals and alloys A6125M Structure of liquid metals and liquid alloys A6470D Solid-liquid transitions A6250 High-pressure and shock-wave effects in solids and liquids A7115A Ab initio calculations (condensed matter electronic structure) A7260 Mixed conductivity and conductivity transitions/ temperature 1.0E+03 to 2.98E+02 K ; pressure 3.0E+10 Pa ; pressure 1.2E+11 Pa/ Na/el
[en] At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1-6. It was predicted from theory 1.2 - and later confirmed by experiment 3-5 - that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states - an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.