Electronic and structural transitions in dense liquid sodium
Electronic and structural transitions in dense liquid sodium
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DOI：
10.1038/nature06123
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发表时间：
2007-09-27
期刊：
影响因子：
64.8
通讯作者：
Bonev, Stanimir A.
中科院分区：
文献类型：
--
作者：
Raty, Jean-Yves;Schwegler, Eric;Bonev, Stanimir A.
在环境条件下，轻碱金属是具有高度对称结构的自由电子状晶体。然而，最近发现它们在压力下表现出意想不到的复杂性（1-6）。从理论（1，2）中预测-后来由实验（3-5）证实-锂和钠在高压下经历了一系列由Peierls机制驱动的破缺跃迁。钠熔化曲线（6）的测量随后揭示了前所未有的（仍然无法解释的）压力诱导熔化温度从30 GPa下的1,000 K下降到120 GPa下的室温。在这里，我们报告从头计算的结果，解释了不寻常的熔化行为在致密的钠。我们表明，熔融钠经历了一系列的压力诱导的结构和电子的转变，类似于在固体钠中观察到的，但在液体无序的存在下，在低得多的压力开始。随着压力的增加，液态钠最初通过呈现更紧凑的局部结构而演变。然而，在大约65 GPa的压力下，会发生向低配位液体的转变，伴随着电导率的三倍下降。这种转变是由电子态密度在费米能级上的赝能隙的打开所驱动的，这种效应迄今为止还没有在液态金属中观察到。较低配位的液体在高温下出现，并且高于紧密堆积的自由电子类金属的稳定区域。我们预测，类似的奇异行为在其他材料中也是可能的。
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.