Journal Article

Electronic and structural transitions in dense liquid sodium

Schwegler, Eric, Raty, Jean-Yves, Bonev, Stanimir A.

2007

Overview

Sodium under pressure It has recently been shown that, when high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions — including (for sodium) a striking and as yet unexplained pressure-induced drop in the melting temperature. Jean-Yves Raty et al . have now identified the cause of this unusual melting behaviour: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. Intriguingly, one of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states, the first time such an effect has been seen in a liquid metal. When high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions, including a striking pressure-induced drop in the melting temperature. The cause of the unusual melting behaviour has now been identified: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. One of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states. 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 , 2 , 3 , 4 , 5 , 6 . It was predicted from theory 1 , 2 —and later confirmed by experiment 3 , 4 , 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.

• Publisher: Nature Publishing Group UK,Nature Publishing,Nature Publishing Group
• Subject: ab initio calculations / Alkali metals / Atomic properties / Condensed matter: electronic structure, electrical, magnetic, and optical properties / Condensed matter: structure, mechanical and thermal properties / Crystals / Density / Electric properties / electrical conductivity / electrical conductivity transitions / Electron states / electronic density of states / Electronic structure of liquid metals and semiconductors and their alloys / Electronics / Exact sciences and technology / Fermi level / Fermi surfaces / free-electron-like crystals / High temperature / high-pressure effects / Humanities and Social Sciences / letter / light alkali metals / liquid metals / Liquid sodium / liquid structure / Liquids / Lithium / Melting / multidisciplinary / 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 / Peierls instability / Peierls mechanism / Physical, chemical, mathematical & earth Sciences / Physics / Physique / Physique, chimie, mathématiques & sciences de la terre / pressure 1.2E+11 Pa/ Na/el / pressure 120 GPa / pressure 3.0E+10 Pa / pressure 30 GPa / pressure-induced structural transitions / Science / Science (multidisciplinary) / Sodium / sodium/ electronic transitions / Structure / Structure of liquids / Structure of simple liquids / Structure of solids and liquids; crystallography / symmetry-breaking transitions / temperature 1000 K to 298 K / Temperature effects / Theoretical or Mathematical, Experimental/ ab initio calculations