固态氢的高压相以及金属化的演化路径理论研究

Locating the first position in the periodical table, hydrogen is regarded as the simplest element and can be found easily in the universe. However, it has great potential application because of the various and complex state of matter. Controlled nuclear fusion, production of metallic hydrogen, and high temperature superconductivity have been listed as the top three key problems of physics by Ginzburg in the Nobel Lecture. Solid metallic hydrogen was predicted by Wigner and Huntington in 1935. The phase transition from solid molecular hydrogen to solid atomic hydrogen through the strategy of pressure-induced metallization, hydrogen gives a serial of complex and confusing evolution of phases, until the observation of solid metallic hydrogen was announced by the group leader of Silvera in SCIENCE in 2017. But doubts have been raised since then..Although solid molecular crystalline phases I, II, III, and IV/IV’of hydrogen have been extensively studied experimentally and the experimental phase diagram becomes more and more clear, only the structure of the low temperature and pressure phase I is known with precision and the controversy has never stopped. This project will investigate the structures of the solid hydrogen under high pressure, and the configurations of the isotope compounds composed of hydrogen and hydrogen isotopes (e.g. HxDy, HxTy, DxTy). This project will focus on the important basic physics and chemistry characteristics in solid hydrogen and their isotope compounds, such as chemical bond and bond dissociation, pressure-induced metallization, electronic transport properties and superconductivity, which will be explored mainly by ab initio DFT method including the considerations of van der Waals interactions and anharmonic effects etc., and checked by experimental data reported. The variation trends of pressure point of phase transition and superconducting critical temperature in metallic hydrogen could be investigated in this project, which are controlled by the various stoichiometric ratio of hydrogen isotopes. And it will benefit to the discovery of the evolutionary route to metallic hydrogen in the isotope compounds which pressure point of phase transition has been reduced by special stoichiometric ratio of hydrogen isotope.

氢被认为是最简单的元素，却具有丰富的物质形态和潜在巨大的应用价值。Ginzburger在诺贝尔奖演讲稿中提到的物理学最关键的十个问题中，如控核聚变、金属氢、高温超导电性等都与氢有密切关系。早在1935年，Wigner和Huntington就预言了金属氢的存在，然而通过压致金属化方法，从固态分子氢到原子金属氢，经历了非常复杂的相变。直到2017年Silvera 团队宣称获得了金属氢, 但质疑不断。虽然实验给出的相图越来越清晰，但除了相I外，针对相图中每个相的具体空间群结构的争议从未完全停止。本项目将主要利用ab initio DFT的方法并对比实验数据，探讨高压下氢及其同位素化合物（如HxDy等）的结构及其稳定性、成键及解离、金属化、电子输运及超导电性等基本的物理化学特征。揭示同位素浓度对相变压强点、金属氢的超导电性的影响规律，探索能降低相变压强的同位素化合物的金属化的演化路径。

该项目涉及到当前高压凝聚态物理学中三个重要的问题：第一，高压下固态金属氢的相图。近些年来，固态氢的高压相图及其金属化一直是凝聚态物理领域中的一个研究热点，金属氢被喻为高压物理学中的圣杯。近期，《科学》杂志上报道，Silvera课题组通过实验在495 GPa的压强下获得了金属氢。这一结果在国际上引起了广泛的关注和争论。我们采用第一性原理，并基于包含零点振动能、范德瓦尔斯力等修正的高精度的计算方案，对固体态氢进行了系统研究，理论预测了固态氢会在485 GPa下发生金属化，其金属化演化路径是从绝缘分子相到金属分子相的转变，在600 GPa下相变到金属原子相。该结果为高能密度固态氢以及室温超导的金属原子氢的制备与获得提供重要的理论支撑。.第二，高压诱导的金属化、相变机制及其超导电性。固体理论预言在足够高的压强下，晶体中原子间距将会减小，原子间相互作用将会增强，电子能带将会展宽而交叠，使得金属性增强和发生从绝缘体到金属的转变，即威尔森转变。在研究高压下晶体的金属化过程中，为了定量地去描述金属材料中自由电子的演化过程，我们定义了费米面填充参数这个物理量，并通过对固体氢的费米面填充参数的计算，定量地研究了高压下固态氢的金属化过程，探究了压强诱导的绝缘体-金属相变机制。为了探究高温超导的物理机制，我们对比研究了几种立方晶体氢化物的超导机制，发现化合物中Tc差异巨大主要是由于光学支声子散射电子强度不同所致，并且光学支声子振动频率与波矢的强依赖关系有可能是高温超导体的一个重要特征。该结论对固态氢及其化合物的超导电性物理机制的认识，及高温超导材料的设计具有积极意义。.第三，我们知道，晶体结构直接决定材料的基本物理和化学性质，是我们能够认识、理解和设计材料的重要基础。为此，我们独立自主地开发编写了晶体结构预测和分析软件包ELocR，该软件包具有完全的知识产权，为相关问题的研究和知识创新打下了良好的基础。在该项目的运行过程中，我们拓展了ELocR的功能，使得其具有了能够预测no-stoichiometry化合物和固溶体的功能，并成功地应用在对高压下铼基固溶体中氢溶解的演化过程及其超导性的研究上。该研究成果对固态氢及其化合物相关的高压实验中的普遍问题——垫片与样品的化学反应问题，给与了重要参考信息。