【作者】 陈长波；

【导师】 崔田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2014， 博士

【摘要】 氢是最轻的元素，其在宇宙中的含量最大，约占宇宙质量的75%。由于氢可用于工业原料也可以作为一种不可替代的清洁能源，因此成为人们一直重点关注的对象。氢的研究主要集中于两个方面，一个是高温超导性质的研究，另一个是如何储氢的研究。早在1935年，E.Winger和H. Huntington提出了高压能够导致绝缘态的氢固体转化为金属态。尽管理论预测347GPa下，氢的超导转变温度大约为107K,但是实验室条件下，直到342GPa仍没有看到金属化现象，所以人们将目光转移到含氢化合物，致力于借助所谓的化学预压在更低压力下实现金属化。同时，储氢手段的探索也需要人们对氢化物高压下的结构和性质进行研究。本文的第一个研究内容为高压下BaH2新相的第一原理预测。在金属-氢体系中，第II主族金属氢化物比较简单，是有应用前景的储氢材料之一。碱土金属氢化物（MgH2, CaH2, SrH2, BaH2）在高压下的结构相变更是引起科学家广泛的关注。在常压条件下，MgH2呈现出金红石（Rutile）结构，接下来的高压相为CaCl2结构，在更高压力下会相变为“cotunnite”结构，其空间群为Pnma。碱土金属氢化物CaH2和SrH2的基态结构被证实具有“cotunnite”结构，并且在高压下相变为Ni2In结构（空间群为P63/mmc）。Smith等人通过室温下的X射线衍射和拉曼光谱实验确定了BaH2具有和CaH2，SrH2相同的基态结构，随着压力增加到1.6GPa，结构变为Ni2In结构,该相变为一级结构相变。K Kinoshita等利用粉末X射线衍射实验方法证实BaH2由“cotunnite”结构到Ni2In结构的一级相变发生在2.5GPa, Ni2In结构一直保持到50GPa，新的高压相变从50GPa开始直到65GPa才完成，实验证实新的高压相具有AlB2结构（空间群为P6/mmm）。在此实验结果基础之上， Li等人从理论上也预测了CaH2在高压下由Ni2In结构相变到AlB2结构。然而，到目前为止，对BaH2的高压相的研究却很少，而且更高压下是否具有新的结构也未见报道。本文利用第一原理方法，研究了BaH2在高压下的结构相变并且进一步探讨了高压结构相变机制。我们的结果显示出由cotunnite→Ni2In的相变发生在2.3GPa,这与实验测量非常吻合，压力达到34GPa时，发生Ni2In→AlB2相变，此相变压力点比实验结果略低。从布居分析可知cotunnite→Ni2In→AlB2相变过程，是由Ba s→d和Ba s→H s电子转移引起的。高压下AlB2结构变得不稳定，相变为YbZn2结构，此相变为二级相变且由L点的横声学支声子振动模式软化导致的，YbZn2结构是动力学和力学稳定的并且具有金属特性。本文第二个研究内容为高压下MH2（M=V, Nb）的结构相变。后期过渡族金属（例如：Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au）的电负性比氢大，其氢化物在常压下很难形成，只有在高压下才能够形成。然而，对于电负性较小的前期过渡族金属（Zr, Ti, V, Hf, Nb）在常压下就能形成氢化物，在常温常压下，前期过渡族金属氢化物为CaF2类型的结构。最近，X射线衍射实验证实，在压力达到0.6GPa时，TiH2立方的Fm-3m结构相变为四方的I4/mmm结构，第一原理方法也预测了在高压63GPa时，I4/mmm结构相变为P4/nmm结构。而在低温下，TiH2、 ZrH2,和HfH2的CaF2Fm-3m结构变得不稳定，并且相变为四方的I4/mmm结构。而VH2,和NbH2不论是在室温还是在低温下，Fm-3m结构都是最稳定的，但是其高压下的结构研究还没有报道，因此其高压结构的探索是非常重要的。研究发现，在常压下立方的Fm-3m MH2是最稳定的，这和实验结果一致。压力达到50GPa和45GPa时，Fm-3m VH2和Fm-3m NbH2分别相变为Pnma VH2和P63mcNbH2，此两种结构相变均为一级结构相变。为了验证高压结构的稳定性，分别计算了声子谱和弹性常数，从而证明了两种结构是稳定的，并且一直持续到100GPa。能带结构和电子态密度结果证实两种结构均具有金属特性， Pnma VH2的超导转变温度最高，能达到4K。本文的第三个研究内容是高压下BHn(n=1-5)结构和超导电性研究。富氢化合物被认为是非常具有潜力的高温超导体。最近科学工作者发现了第三主族化合物AlH3和GaH3在较低的压力下实现金属化，并且具有超导电性。硼，作为第三主族最轻的元素，和氢原子能够形成传统的化合物BH3（硼烷）。在一个大气压下，硼烷具有单斜结构（空间群为P21/c），在这个结构中两个BH3分子形成具有B2H6的环。压力达到4GPa时，硼烷形成了三聚的B3H9,其结构对称性为P-1。压力达到36GPa时，BH3分子形成多聚的化合物（空间群为P21/c），该结构在100GPa以下是能够稳定存在的。Ashcroft等人理论预测BH3在100GPa～350GPa之间是不稳定的，350GPa以上能够再一次形成正交结构（空间群为Pbcn），而且该结构具有较高的超导温度100K。因此探索100GPa～350GPa之间硼氢化合物的结构和性质是非常必要的。通过研究发现，压力在50GPa～104GPa区间，P21/c BH3一直是最稳定的结构，而压力高于104GPa时，BH3会分解成BH和H2, IbamBH在104GPa～170GPa区间是最稳定的，压力高于170GPa时，P6/mmm BH是最稳定的，一直保持到所研究的压力点350GPa， C2/cBH2在压力250GPa以上作为亚稳相存在。在整个压力范围内，BH4和BH5是不能稳定存在的。压力在170GPa时, P6/mmm BH超导温度达到21K，亚稳相C2/c BH2具有更强的电-声子耦合，超导温度在250GPa时达到90K。更多还原

【Abstract】 Hydrogen, as the lightest atom, is the most in the universe,accounting for about75%of the mass of the universe. Hydrogen can beused as the resources in the industry and is the irreplaceable clean energy.Therefore, hydrogen is always being focused on by the people. Thestudies of hydrogen have mainly two aspects, one is the superconductivity,and the other is the storage of hydrogen. As early as1935, E. Winger andH. Huntington promoted that hydrogen molecule decomposed andconverted into the metallic state. The theory have predicted thesuperconducting transition temperature of hydrogen can reach about107K at347GPa, however the experimental works can’t find thesuperconductivity until342GPa, so people shift attentions to thehydrogen-containing compound, trying their best to achieve metallizationat lower pressures. In addition, the storage of hydrogen also requires thepeople understand the structures and physical properties of the hydridesunder high pressure.The first topic of this paper is “new phase of BaH2predicted by thefirst principles”. In metal-hydrogen systems, the main group II hydridesare very simple and have broad application as the hydrogen storage materials. High pressure structural phase transition of the alkaline earthhydrides (MgH2CaH2, SrH2, and BaH2) have been focused on by thescientists. Under the ambient conditions, MgH2has rutile structure. Withthe pressure increasing, two high pressure phases are CaCl2and cotunnitestructures (space group Pnma). The ground states of CaH2and SrH2areconfirmed as cotunnite structure, which transforms to Ni2In structure(space group P63/mmc). Smith et al. confirmed BaH2shared the sameground state structure with CaH2and SrH2,and transformed to Ni2Instructure at1.6GPa, which is the first phase transition through X-raydiffraction and Raman scattering experiments. K. Kinoshita et al.identified the first phase transiton from cotunnite structure to Ni2Instructure occurs at2.5GPa, and Ni2In structure existed until50GPa,meanwhile, a new high pressure phase transition commenced at pressuresaround50GPa and completed at65GPa through powder X-raydiffraction experiment，which has AlB2structure (space group P6/mmm).On the basis of the experiment, YiWei Li et al. predicted the phasetransiton of CaH2from Ni2In structure to AlB2structure under highpressures. To the best of our knowledge, phase transition of BaH2fromNi2In-type to AlB2-type structure hasn’t been identified theoretically.Moreover, there is no experimental and theoretical research report on postAlB2-type structure under higher pressure. In the present work, we havestudied the high pressure structural phase transition of BaH2by using ab initio calculations. The results indicate that the phase transition fromcotunnite to Ni2In occurs at2.5GPa, which is consistent with theexperiment. Ni2In structure transforms to AlB2structure at34GPa, whichis lower than the results of the experiment. The Mulliken populationanalysis indicates Ba s→d and Ba s→H s charge transfers might beresponsible for the phase transformations from cotunnite to InNi2, and toAlB2phases. Under higher pressure, the AlB2phase is unstable andtransforms to a phase predicted to be YbZn2–type structure. The phasetransition is driven by the softening of transverse acoustic phonon modeat the L point, which is considered as a second-order phase transition.YbZn2–type structure is dynamical and mechanical stability and ismetallic.The second topic of this work is “pressure induced phase transitionin MH2(M=V, Nb)”. Late transition metal has a biggerelectronegativity than hydrogen, which can’t form hydrides underambient conditions. However, early transition metal has a smallerelectronegativity than hydrogen, which promotes early transition-metalhydrides can be synthesized under ambient conditions such as TiH2, ZrH2,HfH2, VH2, and NbH2, etc. At ambient conditions, the typical structure ofthe early transition-metal dihydrides is the CaF2(Fm-3m space group)crystal structure. Recent synchrotron X-ray diffraction (XRD)experiments of TiH2at room temperature and high pressure revealed a phase transition from fcc to I4/mmm at0.6GPa and a further transitionfrom I4/mmm to P4/nmm occurs at63GPa predicted by the firstprinciples. At lower temperature, the Fm-3m structures of TiH2, ZrH2,and HfH2aren’t stable, and transform to tetragonal I4/mmm structure, butthe Fm-3m structures of VH2and NbH2is stability at low temperature.Therefore, exploring of high–pressure structures of the earlytransition-metal dihydrides such as MH2(M=V, Nb) is essential. In thestructures predicted, Fm-3m MH2is most stable at ambient pressure,which is consistent with the other works. Fm-3m VH2transforms to Pnmaphase at50GPa and Fm-3m NbH2changes to P63mc phase at45Gpa,respectively. The two phase transitions are identified as the first orderphase transitions by volume reductions. The calculations of the phononcurves and the elastic constants indicate the two structures are stable till100GPa. Calculated DOS and band structure real that the Pnma VH2andP63mc NbH2are metallic. The Tcof Pnma VH2can reach4K by theelectron-phonon calculations.The third tropic of the work is “the structures and properties ofBHn(n=1-5) under high pressures”。 Recently, it has been proposed thatgroup III hydrides (e.g. AlH3and GaH3) also become metallic at lowerpressures and present a high superconducting critical temperature. Boron,the lightest element in group III, with hydrogen can form the traditionalcompound BH3. At1atm, BH3tend to be monoclinic structure with P21/c, in which, two BH3molecules form the ring of B2H6. At4GPa, amolecular crystal is built of trimer B3H9which has the structure with thegroup P-1. At36GPa, BH3becomes the polymeric compounds (spacegroup P21/c), which is the stable till100GPa. Kazutaka Abe and N. W.Ashcroft predicted BH3wasn’t the stable between100GPa and350GPa,and above350GPa, Pbcn BH3formed again, which has a highsuperconducting transition temperature of100K at350GPa. Therefore,exploring the structures and properties of boron hydrides becomesessential between100GPa and350GPa. It is found that BH3compoundis the most stable at50-104GPa. Under higher pressure, BH3becomesunstable and dissociates into the mixtures of BH and H2. Ibam BH is themost stable at104-170GPa, and transforms to P6/mmm BH at170GPa,which keep up to350GPa. C2/c BH2is found to be meta-stable above250GPa. In the range of pressure considered, BH4and BH5can’t form.The superconducting critical temperatures of P6/mmm BH reach21K at170GPa. The meta-stable C2/c BH2has more stronger electron-phononcoupling. The superconducting critical temperature of the C2/c BH2reaches90K at250GPa.更多还原