Theoretical study of the ground-state structures and properties of niobium hydrides under pressure
Theoretical study of the ground-state structures and properties of niobium hydrides under pressure
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加压下氢化铌基态结构与性能的理论研究
DOI：
10.1103/physrevb.88.184104
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发表时间：
2013-11-12
影响因子：
3.7
通讯作者：
Ma, Yanming
中科院分区：
文献类型：
--
作者：
Gao, Guoying;Hoffmann, Roald;Ma, Yanming
作为寻求增强超导电性的一部分，我们从理论上探索了一些Nb氢化物在一定压力范围内的基态结构和性质，特别是那些含有大量氢的氢化物。一个主要的动因是，在正常情况下，Nb是超导转变温度(T-c)最高的元素，而且它的一些化合物又是具有很高T-c的金属。因此，Nb与氢的组合也引起了人们的极大兴趣，因为它具有高的动态能级。进一步加强这一点的建议是，在接近其绝缘体-金属转变的地方，通过添加相对较小浓度的合适的过渡金属，可以诱导氢在某种程度上过早地进入金属状态。在这里，所用的方法正确地再现了在更高的Nb浓度下的一些Nb氢化物的基态结构。有趣的是，当包括质子零点能量时，NbH4和NbH6所代表的特殊化学计量比在相当低的压力下稳定下来。虽然在我们研究的任何氢化物中都没有发现配对的H-2单元，但我们确实在高压NbH6中的Nb周围发现了复杂而有趣的氢网络。在相应的压力下，NbHn中的Nb-Nb分离始终大于Nb金属中的Nb-Nb分离。在高氢化物的基态中发现的结构表明，在NbHn(n=1-4)中，每个Nb原子周围的氢原子的配位数约为4N，而在NbH6中的配位数高达20。NbH4被认为是在高压下成为超导体的一个可能的候选者，在300 GPa时的T-c估计与38K相似。
As part of a search for enhanced superconductivity, we explore theoretically the ground-state structures and properties of some hydrides of niobium over a range of pressures and particularly those with significant hydrogen content. A primary motivation originates with the observation that under normal conditions niobium is the element with the highest superconducting transition temperature (T-c), and moreover some of its compounds are metals again with very high T-c's. Accordingly, combinations of niobium with hydrogen, with its high dynamic energy scale, are also of considerable interest. This is reinforced further by the suggestion that close to its insulator-metal transition, hydrogen may be induced to enter the metallic state somewhat prematurely by the addition of a relatively small concentration of a suitable transition metal. Here, the methods used correctly reproduce some ground-state structures of niobium hydrides at even higher concentrations of niobium. Interestingly, the particular stoichiometries represented by NbH4 and NbH6 are stabilized at fairly low pressures when proton zero-point energies are included. While no paired H-2 units are found in any of the hydrides we have studied up to 400 GPa, we do find complex and interesting networks of hydrogens around the niobiums in high-pressure NbH6. The Nb-Nb separations in NbHn are consistently larger than those found in Nb metal at the respective pressures. The structures found in the ground states of the high hydrides, many of them metallic, suggest that the coordination number of hydrogens around each niobium atom grows approximately as 4n in NbHn (n = 1-4), and is as high as 20 in NbH6. NbH4 is found to be a plausible candidate to become a superconductor at high pressure, with an estimated T-c similar to 38 K at 300 GPa.