钚(Pu)是核工业领域中非常重要的放射性材料，化学活性强，极易与环境气氛发生化学反应而腐蚀，这对钚的长期贮存和核废料的循环利用极为不利。在钚的腐蚀产物中，钚的氧化物已经被大量的文献集中报道，不论是在实验上还是在理论方面，而对在钚腐蚀过程中起同样重要作用的钚氢化物的研究较少，这主要是由于钚氢化物具有剧毒、粉化以及易自燃等特性使得相关实验表征异常困难，至今钚氢化物的结构仍存在许多争议；此外，钚的氢化反应还有一个很重要的用途就是将武器级的钚转化为可直接利用的核燃料。目前仅有的一些相关文献主要是有关钚氢化物的合成、钚的氢化反应动力学和热力学方面的初步研究。所以，开展基于理论模拟的研究，可以从电子和原子层面上给出细致、深入的微观机理认识，对解读钚氢化物的结构和钚的氢化腐蚀机理具有重要的现实意义。 钚元素被公认为是元素周期表中最为神秘、最为复杂的元素；其复杂性至今也是基础科学研究领域的一个重大挑战。主要是由于钚5f壳层的电子同时具有局域性和非局域性的双重特性，再加上5f电子之间的强关联效应，导致了钚具有非常复杂的物理和化学性质。因此，精确地描述5f电子的特性对研究钚氢化物的结构和钚的氢化腐蚀机理的认识至关重要。 本论文主要利用密度泛函理论，结合粒子群优化算法和过渡态搜索法(CI-NEB)详细的研究了钚氢化物在常压和高压下的晶体结构、电子性质、弹性性质、光学性质、晶格动力学性质、热力学性质、氢原子在金属钚表面的吸附等性质。在整个研究中我们同时考虑了钚5f电子的强关联效应(U)和自旋轨道耦合效应(SOC)。 首先，我们从最简单的立方钚氢化物开始研究，利用GGA+ U和GGA+ U + SOC的方法对比研究了五种立方PuH〓(x = 0，0.25，0.5，0.75，1)的晶格结构和形成焓。根据结构参数的计算结果，我发现GGA+U方法计算的晶格参数都比实验值高，而GGA+ U+ SOC方法得到的结果与实验值符合较好；进一步通过形成焓的比较也得到同样的结论：考虑SOC效应的结果与实验值符合较好；这就表明SOC效应对钚氢化物的作用不能忽略。这为后面精确地计算电子结构、化学键、力学性质的研究打下坚实的理论基础。 其次，利用 GGA+ U+ SOC 方法研究了立方 PuH₃(x = 0，0.25，0.5，0.75，1)在常压下的电子结构、差分电荷密度、弹性性质、晶格动力学和热力学性质，得到了非化学计量比PuH〓、PuH〓和PuH〓稳定结构的原子占位。结构稳定性研究表明这五种立方结构从力学和晶格动力学两方面分析都是稳定的，而且随着H原子的增加钚氢化物的晶胞出现了晶格收缩现象。电子结构的研究显示这五种结构都表现出金属性。通过态密度、差分电荷密度和Bader电荷的分析一致表明Pu-H之间主要呈现离子特性，并且随着H原子的增加离子性逐渐减弱。力学性质分析给出的体积模量B和杨氏模量Y表明PuH〓晶体具有较明显的各向异性特性。此外，还计算了钚氢化物的热力学性质，得到的熵与现有的实验结果相吻合，再次证明了我们的理论方法是可靠性。 然后，对PuH₃的基态结构进行研究，一些研究认为PuH₃属于立方LaF₃ (P6₃/mmc)型，还有一些研究认为是正交的YF₃(Pnma) 型，但至今仍然没有一致的结果。我们利用粒子群优化算法结合GGA + U+SOC方法预测PuH〓的最稳定结构。预测找到的六方P6₃/mmc PuH₃结构在能量上要比立方结构低0.2 eV/f.u.。进一步对P6₃/mmc PuH₃结构的动力学分析发现声子谱在高对称点K处出现约6.75 THz的负频，表明结构是不稳定的，通过声子态密度发现虚频主要来自间隙H (2b)原子的贡献；为了得到稳定的结构，我们沿着虚频方向给H (2b)原子一个小的位移后重新优化，最后得到两个稳定的结构：六方P6₃/㎝型和三方P3cl型PuH3，这两个结构非常相似，加压下P3cl型会转变到P6₃/㎝型，声子谱结果表明这两种结构的动力学都是稳定的，能带结构显示它们分别是带隙为0.85 eV和0.87 eV的半导体。Pu-H之间成混合的离子/共价键，离子性略强于共价性。此外，我们还给出了钚氢化物的光谱性质，包括拉曼光谱和光学介电函数。 接下来，我们用CALYPSO结构搜索软件进一步预测了 PuH〓(x= 1，2， 3， 4)在高压(0～200 GPa)下结构相变。经过全局搜索优化得到了钚氢化物在高压下的结构演化规律，这些结构从力学和晶格动力学两方面分析都是稳定的。电子结构计算表明：在高压下，除了 Pnma PuH₃是一个带隙为0.61 eV的半导体外，其余PuH〓(x= 1， 2， 3，4)的高压结构都表现金属性。同时我们还给出了这些结构在高压下的Bader电荷转移分析，结果发现相同配比的化合物，随着压力的增加，Pu 失去电子的能力减弱：PuH₂(Pu： -1.34 |e| → -0.68 |e|)， PuH₃(Pu： -1.70 |e| → -0.76 |e|)，同时H得到电子的能力也在减弱，PuH₂(H：0.66 |e| → 0.34 |e|)， PuH₃(H： 0.57 |e| →0.25 |e|)，这表明Pu-H键的离子性减弱，共价性增强。总结在高压下Pu-H成键的规律为：压力增加，晶胞体积减小，原子的间距减小，Pu-H共价性增强，离子性减弱。 最后，基于GGA+ U+ SOC方法，研究了 H原子在金属Pu和PuH₂ (100)、 (110)和(111)表面吸附，经过自由弛豫后研究了 H原子在不同面的吸附行为。结果发现H原子在金属Pu (100)面的center位最稳定，形成Pu-H键，吸附能为-2.9 eV；同样的在PuH₂(100)面的最稳定吸附位也是center位，吸附能为-2.2 eV。此外，通过CI-NEB方法研究了 H原子从稳定吸附位置向其它位置迁移的能量变化过程。结果表明在PuH₂(100)面上H原子从最稳定的吸附位(center)向周围的高对称点位迁移需要至少克服3.6 eV的势垒；对PuH₂ (110)面，从top位到bridge位迁移过程，则需要克服至少0.3 eV的势垒；对PuH₂(111)面，从稳定的bridge位到top位，则需要克服至少3.4 eV的势垒。这些理论研究可以为实验上深入的研究H在金属Pu表面的存在形态以及PuH₂进一步氢化过程提供参考。 关键词：钚氢化物；强关联效应；自旋轨道耦合；结构预测；电子结构；弹性常数；晶格动力学；热力学；表面吸附

 Plutonium (Pu) is familiar to people because of its promising role in the field of nuclear industry. Due to its very high chemical reactivity, plutonium readily corrodes and forms a series of complicated products when exposed to air, and common gases such as oxygen, hydrogen, and so on. These products would reduce the purity of the plutonium metal and changed its structure and properties. However, plutonium hydrides, surface-corrosion products of metallic Pu, are rarely reported in the literature, only some literatures are mainly related to the synthesis, kinetics, and thermodynamics of plutonium hydride. This is mainly due to plutonium hydrides are extremely reactive and have spontaneous ignition and burning characteristics when exposure to air at room temperature. Recent interest in plutonium hydrides has emerged due to the application of the plutonium hydrogen reaction for pyrochemical processing of excess weapon-grade plutonium. To the best of our knowledge, a number of basic properties associated with their structure are still unknown, including chemical bond and Raman spectroscopy. So, development of a predictive aging model for plutonium is a major goal of many relevant laboratories. Such predictions of the structures and the properties of plutonium hydrides require an electronic and atomic level model that can provide detailed and in-depth understanding of the plutonium surface corrosion properties. As the most complex element and a very important radioactive material in the field of nuclear industry, plutonium metal and its compounds have been long intensively studied. The strong correlation interaction between 5f electrons in the Pu element makes the electronic structure so complex that is difficult to be investigated. Moreover, Pu is located at a special site where the transition of 5f electrons from itinerancy to localization occurs, resulting in the occurrence of numerous bizarre properties. So, understanding the 5f states of plutonium has always been a focal point of the engaging and challenging field of plutonium chemical corrosion. By density functional theory, combined with the particle-swarm optimization and CI-NEB methodology, this dissertation presents a systematic study of the crystal structure, electronic structure, elastic constant, optical properties, lattice dynamics, thermodynamics and surface adsorption of plutonium hydrides. Furthermore, the Hubbard parameter U and spin-orbit coupling (SOC) effects have been added for considering the strong correlation and relativistic effect of 5f electrons in Pu atom. First of all, the structure parameters and enthalpies of formation of face-centered cubic (fcc) plutonium hydrides PuH〓 (x = 0,0.25,0.5,0.75,1) are depicted through GGA + U and GGA + U + SOC (U=4 eV) approaches. Compared with the results of the two method, we find that the structure parameters and enthalpies of formation with the SOC approach for fcc PuH₂ and fcc PuH₃ are in consistency with the experimental values available at present. Our results and other works have consistently shown that the SOC plays a critical role in correctly describing the ground-state properties of plutonium hydride. In the following studies, the calculations of the electronic structure, chemical bond and mechanical properties are performed with the SOC approach. Secondly, the electronic structure, charge density difference, elastic constants, and thermodynamic properties of fcc PuH〓 (x = 0,0.25,0.5, 0.75,1) are been investigated by the GGA + U + SOC method. The calculation results indicate that all structures of PuH〓 are mechanical and dynamically stable. The electronic properties, charge density difference, and Bader charge analysis show that mixtures of covalent and ionic character are present in Pu-H bond, but the ionic character is slightly stronger than the covalent character. We also find that the value of Pu atoms' average charge transfer is increasing with increasing H composition. It is suggested that the introduced H atoms into a Pu lattice enhances the electron-losing ability of Pu atoms, and the total ionicity degree of PuH〓 increases with the increase of x. Thus, it reveals that the Pu-H bonds become stronger with the increase of H composition. Based on the elastic constants and their related parameters, the calculated ratio B/G indicating that fcc plutonium hydrides are the brittle materials. It's amazing that the predicted enthalpy S agree well with the experimental data, which illustrated again that our structural models and the theoretical methods for PuH〓 are reasonable and feasible. And then, no unifying crystal structure has been determined for PuH₃, one argue that a LaF3-type hexagonal structure for PuH₃ with the space group P6₃/mmc based on an X-ray diffraction pattern. However, in another experiment they believed that PuH₃ had an orthorhombic YF₃ (Pnma) structure. This inspired us to explore the crystal structure and basic properties of PuH₃ using the CALYPSO method. The P6₃/mmc PuH₃ is obtained using particle swarm optimization methodology. However, the imaginary vibration modes around K points (6.75i THz) in phonon dispersion curves indicate that P6₃/mmc PuH₃ is dynamically unstable. To determine the more stable phase, the interstitial hydrogen is displaced by about 0.1 Å along the negative phonon frequency direction of the P6₃/mmc supercell. After reoptimization, we find two minimum energy structures: hexagonal P6₃㎝ phase (185, Z = 6) and tripartite P3cl phase (158, Z= 6). We also examine the dynamic stability, the results that the P6₃㎝ and P3c1 phase structure of PuH₃ do not exhibit any unstable modes confirming that the phases are dynamically stable. The obtained band structures for P6₃㎝ and P3c1 PuH₃ exhibit direct band gaps semiconductor at Γ point with a band gap of 0.85 eV and 0.87 eV. The charge density difference and Bader charge analysis suggest that the bonding interactions between H and Pu atom are ionic. In addtion, the Raman spectra and optical properties of PuH₃ are also investigated for further experimental investigation. Next, we searched for thermodynamically stable crystalline structures of stoichiometric PuH〓 (x = 1, 2, 3, 4) under ambient and high pressure conditions using the CALYPSO methodology. The calculated phonon dispersion curves of all predicted structures for plutonium hydrides have no soft mode in the whole Brillouin zone, which indicates that the structures are dynamically stable. The results of electronic properties suggest that Pnma PuH〓 displays indirect band gaps with a value of 0.61 eV at 25 GPa, and all the other structures exhibit the metallic behavior under high pressures. The Bader charge analysis results indicate that the covalent character gradually becomes stronger than the ionic character with further increase in compression. Finally, we construct the δ-Pu and PuH₂ (100), (110), and (111) surface according to the optimized δ-Pu and PuH₂ unit cell. Based on GGA + U + SOC method, we investigated the adsorption energy of FI atom on δ-Pu and PuH₂ (100), (110), and (111) surfaces, respectively. The calculated results show that full relaxation of H atom on the Pu (100) surface yielded preferential adsorption in the center site with adsorption energy of 2.9 eV. We then determined the H atom on PuH₂ surfaces in a similar fashion where the most stable adsorption-site of H atom is center site of the PuH₂ (100) surface with adsorption energy of 2.2 eV. In addtion, the CI-NEB methond is applied to calculate the potential barrier of H atom on PuH₂ surfaces migration from most stable adsorption site to other sites. It is found that the potential barrier of FI atom from most stable center site to top site is above 3.6 eV on PuH₂ (100) surface, while the potential barrier is above 3.4 eV on PuH₂ (111) surface. These theoretical studies can provide a useful reference for the experimental study of the corrosion behavior of H atom on the metal Pu surface. Keywords: plutonium hydride; strongly correlated systems; spin-orbit coupling; structure prediction; electronic structure; elastic constant; lattice dynamics; thermodynamics; surface adsorption