Ma Yanming, Eremets Mikhail, Oganov Artem R, Xie Yu, Trojan Ivan, Medvedev Sergey, Lyakhov Andriy O, Valle Mario, Prakapenka Vitali
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.
Nature. 2009 Mar 12;458(7235):182-5. doi: 10.1038/nature07786.
Under pressure, metals exhibit increasingly shorter interatomic distances. Intuitively, this response is expected to be accompanied by an increase in the widths of the valence and conduction bands and hence a more pronounced free-electron-like behaviour. But at the densities that can now be achieved experimentally, compression can be so substantial that core electrons overlap. This effect dramatically alters electronic properties from those typically associated with simple free-electron metals such as lithium (Li; refs 1-3) and sodium (Na; refs 4, 5), leading in turn to structurally complex phases and superconductivity with a high critical temperature. But the most intriguing prediction-that the seemingly simple metals Li (ref. 1) and Na (ref. 4) will transform under pressure into insulating states, owing to pairing of alkali atoms-has yet to be experimentally confirmed. Here we report experimental observations of a pressure-induced transformation of Na into an optically transparent phase at approximately 200 GPa (corresponding to approximately 5.0-fold compression). Experimental and computational data identify the new phase as a wide bandgap dielectric with a six-coordinated, highly distorted double-hexagonal close-packed structure. We attribute the emergence of this dense insulating state not to atom pairing, but to p-d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices. We expect that such insulating states may also form in other elements and compounds when compression is sufficiently strong that atomic cores start to overlap strongly.
在压力作用下,金属的原子间距离会越来越短。直观地说,这种反应预计会伴随着价带和导带宽度的增加,从而表现出更明显的类似自由电子的行为。但在目前实验所能达到的密度下,压缩程度可能非常大,以至于内层电子会发生重叠。这种效应极大地改变了电子性质,与锂(Li;参考文献1 - 3)和钠(Na;参考文献4, 5)等典型的简单自由电子金属的性质不同,进而导致结构复杂的相和具有高临界温度的超导性。但最引人关注的预测——看似简单的金属锂(参考文献1)和钠(参考文献4)在压力下会由于碱金属原子配对而转变为绝缘态——尚未得到实验证实。在此,我们报告了在约200吉帕(相当于约5.0倍压缩)压力下钠向光学透明相转变的实验观察结果。实验和计算数据确定这个新相是一种宽带隙电介质,具有六配位、高度扭曲的双六方密堆积结构。我们将这种致密绝缘态的出现归因于价电子的p - d杂化以及它们被内层电子排斥到晶格间隙中,而不是原子配对。我们预计,当压缩足够强烈以至于原子核开始强烈重叠时,其他元素和化合物中也可能形成这种绝缘态。