摘要
单质元素金属是物质世界的基本构成之一,其高压研究倍受关注。近期研究发现单质元素金属及其金属间化合物在高压下发生了复杂的结构相变,并呈现出一系列新奇的物理现象(如超导电性,超硬、电阻上升、熔点下降等等)。对这些体系的研究不仅能为天文和地球物理的研究提供知识储备,还可以揭示新的物理现象和物理规律。本文利用基于密度泛函理论的第一性原理方法系统的研究了Ca-Li(CaLi2)和Li-H(LiH2、LiH6和LiH8)合金化合物,以及单质钠在高压下的结构相变和高压新相的奇异物理行为,获得了以下创新性结果:
1.预言CaLi2在高压下先后相变为具有C2/c和P21/c空间群的单斜结构,以CaLi2为例首次在二元化合物中发现了压致分解--聚合--再分解的奇异高压行为。电子-声子相互作用的计算表明,CaLi2在高压下转变为超导体,其超导电性产生的物理根源在于Ca原子的低频振动模式与电子之间的强烈耦合。
2.发现LiH2只有在高压下由于杂化能级的出现才转变为金属,但由于费米能级态密度非常小,没有超导电性。H原子在LiH6和LiH8中以“H2”单元的形式存在,由于Li原子与“H2”单元之间的电荷转移,LiH6和LiH8在常压下就已经是金属,在高压下转变为超导体。LiH6的超导温度随压力的升高而增强,在300万大气压(GPa)下达到了81.6开尔文(K),超导温度升高主要是由于H原子费米能级附近态密度的增强导致。
3.发现在200 GPa下金属钠转变为透明绝缘体。理论预言金属钠在超高压下形成c轴极度压缩的双六角密堆hP4结构,该结构首次在单质中发现,是宽带隙绝缘体,其带隙随压力升高而增加,在600 GPa下带隙增大为6.5电子伏特(eV)。超高压下钠的绝缘特性来自于价电子在晶格空隙中的强烈束缚,是钠原子的核内电子强烈交叠的结果。
High pressure, which has been considered as an extreme condition, can change the properties of materials dramatically. Under high pressure, the volume of materials will be compressed and the distance between atoms will be reduced, so that the crystal structure of the materials will be rearranged and the phase transition will be occurred. As a result of the reduced atomic distance, the orbitals of two nearest neighbor atoms will be overlapped and the bonding properties will be changed then the electronic structure transformation of the materials will be induced. Newly formed crystal structure and electronic structure will bring new physical phenomena and behaviors. Thus, high pressure not only can cause various phase transitions, but also can lead us to the new functional materials. High pressure physics, as one of the emerging discipline of condensed matter physics, has attracted many attentions from both experimentalists and theorists.
Simple elemental metal is one of the hottest topics among high pressure physics. At high pressure, simple elemental metals exhibit many exotic properties, such as low symmetric low coordination structures, enhanced supercondictivity, ultra-low melting point, increased electric resistivity, and narrowed valance band width. These behaviors are quite counterintuitive, because at sufficiently high pressure all materials must become free-electron metals; the expected behavior is an increasing tendency to the free-electron limit under pressure. The reasons why simple element metals and their intermetallics have such nontrivial properties are still open questions.
Most recently, with the help of improved theory and computational capability, the first principle method base on the density functional theory has been used widely in the computational condensed matter physics, quantum chemistry, and material science. And it has been successfully solved many applications. The newly developed crystal structure prediction method extended the reseach field for theorist once again. From that on, theory studies are more independent to experiment than before.
Within this thesis, we studied the high pressure properties of three typical materlias, Na, CaLi2 and Li-H alloys, by using first principle method. We first determined their high pressure crystal structures based on the crystal structure prediction method; then we studied their physical properties by analyzing their electronic properties and lattice dynamics behaviors.
First of all, we found two novel high pressure structures for CaLi2 with space groups C2/c and P21/c, stable at pressure ranges of 35–54 GPa and 54–105 GPa, respectively. It is found that decomposition into pure elements is energetically favorable at pressures of 20–35 GPa and above 105 GPa. Such oscillatory decomposition-recombination-decomposition behavior is known, for instance, for MgAl2O4, but to our knowledge, this is the first report of such phenomenon for a binary compound. In C2/c structure, Ca atoms form topologically the same (save for a symmetry- breaking distortion) arrangement as the Si-sublattice in ?-ThSi2, a known superconductor. The two inequivalent Li atoms form Li2 pairs, stuffing the Ca framework. Contrary to the three-dimensional C2/c structure, the P21/c structure has a flavor of 2D geometry. The structure can be viewed as a slight distortion of a trigonal P-3m1 structure or a strong distortion of a hexagonal P6/mmm structure. The Ca and one of the inequivalent Li atoms form alternating graphene layers stacked on top of each other; the another Li atoms adopt linear chains running through centers of hexagonal channels. By comparing the theory and experiment X-ray diffraction spectra, our predict structures are not excluded. After analyzing the details of the crystal structures and related phonon dispersion, we found that the decomposition could happen. However, the solid-state reactions, such as decomposition and crystallization, usually have high activation energies, and thus a high temperature experiment might be necessary to conquer the kinetic barrier of the decomposition/ crystallization of CaLi2. The valence band width and the density of states on Fermi level of CaLi2 decrease as pressure increased. By calculation the electron localization function, it is shown that under high pressure, the wavefunctions of valence electrons and core electrons are overlapped, resulting in a localization of valence electrons in the interstices. The localized valence electrons lost their mobility and induced an increase of the electric resistivity, which is consistent with the observation of experiment. We also found both structures are superconductors; the calculated superconducting temperature is ~ 15 K at 50 GPa, which is in good agreement with experiment data. The electron phonon coupling calculations have shown that the low energy Ca vibrations have the most contributions to superconductivity. And the enhanced superconductivity of CaLi2 under high pressure is strongly related to the phonon softening. Comparing to elemental Li, the superconductivity of CaLi2 at high pressure has the similar behaviors, and the smaller logarithmic average phonon frequency of CaLi2 maybe the reason why CaLi2 has a lower superconducting temperature than Li.
Later on, we studied the high pressure superconductivity of LiH2, LiH6, and LiH8 by calculating their electron phonon coupling strength. It is shown that the insulating LiH2 became a metal under high pressure by band overlapping. But LiH2 is not a superconductor because the rather small density of states near Fermi level. In LiH6, and LiH8, H atoms are forming the H2 units. Both of LiH6 and LiH8 have odd number of valence electrons, and the electron charge transfer between Li and H2 unit partially filled the H orbital, thus they are metals even at ambient pressure. Under high pressure, both of them are superconductors; the superconducting temperatures are 38.34 and 31.04 K at 150 and 100 GPa, respectively. The superconducting temperature of LiH6 increased rapidly under pressure and reached 81.04 K at 300 GPa. In contrary, the superconducting temperature of LiH8 is not changed much, at 200 GPa, the superconducting temperature is 36.54 K. From the Eliashberg phonon spectral function, we found that the vibrations between H2 units are the biggest contributor for the superconductivity. The increased density of states near Fermi level in LiH6 is the reason of the enhanced superconductivity of LiH6. Our results are helpful for the searching of potential high temperature superconductor among hydrogen dense materials. It will be a great success if we can find a hydrogen dense material with odd number of valence electrons and can survive at ultra-low pressure. Finally, we studied the crystal structures of Na under high pressure. We found two new structures with space group Pnma (oP8) and P63/mmc (hP4) stable at pressure ranges of 152-260 GPa and above 260 GPa, respectively. Especially, the hP4 can be viewed as a double-hexagonal close-packed (d.h.c.p.) structure squeezed along the c-axis. By calculating the electronic properties, it is clear that Na becomes an insulator under high pressure; the band gap is 1.3 eV at 200 GPa. The bad gap becomes even bigger at higher pressure. Thus, Na is a wide gap insulator at high pressure. Our calculations reveal that an insulating electronic state emerges because compression causes the 3d bands to rapidly drop in energy relative to the 3p bands and increasingly hybridize with them. This hybridization is the key to strong electron localization: a marked charge accumulation occurs only in the open interstitial regions. Under high pressure the distance between two atoms in Na is reduced dramatically. At 300 GPa, the shortest Na–Na distance is 1.89 ? at 300GPa, which implies strong core-valence overlap (the 3s and 2p orbital radii in Na are 1.71 and 0.28 ?, respectively) and even significant core-core overlap between neighboring Na atoms (the ionic radius of Na+ is 1.02 ?). The afterwards high pressure optic experiment has proved that Na becomes transparent at 200 GPa, thus it is an insulator under high pressure. The X-ray diffraction and Raman spectrum experiments have revealed that the crystal structure of the insulating Na is indeed the hP4 structure. We also noticed that the mechanism we for the insulation is much different from what proposed by Neaton and Ashcroft. Furthermore, if we consider the Na ions as cations and the interstitial electron density maxima as anions, then the Na-hP4 is an analogue of electrides in Ni2In-type structures. Our finding has brought some fresh air for the high pressure physics; it will be very helpful for us to study the internal of Earth and other planets.
引文
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