非化学计量钚化合物结构和性质的理论研究

As the most complex element and a very important radiocativematerial in the field of nuclear industry, plutonium metal and its alloys have been long intensively studied and large number of remarkable scientific breakthroughs have been frequently reported in the literature. Modern-day problems concerning plutonium involve predicting its properties under long-term aging in the storage enviroment. Due to its very high chemical reactivity, plutonium readily corrodes and forms a series of complicated compounds when exposed to air, moisture, and common gases such as oxygen, hydrogen, carbon monoxide and dioxide, and so on. Understanding the structure and the properties of these corrosion products is crucial in probing into the inherent mechanism of plutonium chemical corrosion, and evaluating the validity of its practical applications. However, most investigators are inclined to concentrate their attention on stochiometric plutonium compounds, and to ignore nonstochimetric compounds that may contribute to the overall corrosion behavior.Unfortunately, plutonium is one of the most exotic metals and the experimental characterizations become extremly difficult. So, development of a predictive aging model for plutonium is a major goal of many relevant laboratories. Such predictions of the structure and the properties of nonstochimetric compounds require an electonic-structure level model based on first-principles calculations. In the work,we attempt to develop first-principles calculations method, focusing on the investigations of crystal structure, electronic structure, chemical bonding,thermodynamics, phase transition and chemical reactivity for nonstochimetric plutonium oxides, hydrides and carbides. According to the calculation results, the complicated structure and properties of nonstochimetric plutonium compounds should be well understood, and the formation mechanism of these compounds and their roles played in the plutonium chemical corrosion can be discussed in detail. By combining the theoretical results with the available experimental findings, the influence of nonstochimetric compounds on plutonium chemical corrosion can be reasonably evaluated, and some valuable instructions can be provided for the practical applications of plutonium-based materials.

钚是核工业中非常重要的核材料，化学活性强，极易与环境气氛反应而腐蚀，对这些腐蚀产物结构和性质的准确理解是探讨钚化学腐蚀内在机制及合理评价钚材料在实际使用过程中是否有效的重要依据。已有的研究主要集中在化学计量钚化合物体系，而对于同样广泛存在的非化学计量钚化合物体系则鲜有报道。鉴于钚材料的潜在危险性，直接针对钚材料的结构表征异常困难，第一性原理计算方法有利于从电子结构水平理解非化学计量钚化合物的结构和性质。本项目将建立和完善强关联电子体系第一性原理计算方法，重点研究非化学计量钚氧化物、氢化物和碳化物的晶体结构、电子结构、成键特征、热力学性质、相变和化学反应特征等，理解非化学计量钚化合物体系复杂的结构和性质，深入探讨钚化学腐蚀过程中非化学计量钚化合物的形成机制及所起的作用，结合相关实验研究成果，合理评价非化学计量钚化合物对钚化学腐蚀的影响效应，为钚的实际使用提供若干重要的指导原则。

准确理解非化学计量钚化合物的结构和性质对于钚化学腐蚀研究具有十分重要的作用。采用第一性原理DFT+U为主的方法，并结合杂化密度泛函理论方法，较为系统、深入地开展了非化学计量钚化合物（氧化物、氢化物、碳化物等）、钚氧化物缺陷体系等方面的计算研究，获得了这些非化学计量钚化合物的结构和性质，部分成果具有国际先进水平，对于实验上存有争议的一些问题提出了新见解，并且也为相关实验研究提供了理论依据。重要研究结果及科学意义如下：1) 钚氧化物及钚氧化层的结构和性质一直是钚化学腐蚀研究领域的关键科学问题。超化学计量钚氧化物形成机制的研究以及混合价态钚氧化物研究成果丰富了对钚-氧体系的物理和化学认识，可以应用于Pu-O完整相图的建立，部分理论预测结果对于相关的实验研究具有很强的指导作用。2) 获得了钚氢化物中电子结构、热力学性质、晶格动力学等与氢含量的关系，从体相结构上理解了复杂的钚氢反应过程。随着氢含量增加，钚的5f电子发生局域-离域转变，钚氢化物的成键特征也发生明显的变化，离子键逐渐增强。钚氢化物在不同压力下的晶体结构和电子结构预测结果为后续钚氢化物的结构确定和表征提供了一定的理论依据。3) 获得了PuC0.75、PuC、Pu2C3和PuC2晶体结构、电子结构、晶格动力学以及其它性质的计算结果，探讨了钚碳化物随碳含量增加性质变化的趋势。此外，根据上述几种碳化物热力学计算数据，探讨了碳化物的相对稳定性，从理论上初步预测了钚在含碳气体环境下可能的碳化物形式。Pu-C-O三元体系的计算结果理解了低价态钚氧化物稳定化的内在机制，从理论上澄清了该问题的争议，对于钚的氧化腐蚀及抗腐蚀研究具有很好的理论参考价值。4) 获得了钚氧化物中缺陷和杂质行为的规律性认识，从杂质原子的性质以及钚5f电子离域-局域转变上理解了杂质原子行为差异的本质，理解了Ga在不同钚氧化层中可能的分布状态和行为，理解了H在锕系二氧化物中行为差异的本质，对于锕系金属-氢反应的研究具有重要的理论价值。