标题： 非化学计量铀 dioxide 中由点缺陷的伪相变引起的异常 显示英文标题

标题： Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase transition of point defects

摘要： 在原本完美的晶体结构中，点缺陷的均匀分布通常描述了一种非化学计量材料该状态的独特伪相。 以非化学计量的二氧化铀作为原型，我们表明，类似于常规的相变，这些伪相也会在外部条件如压力、温度或化学组成变化时，通过改变主要缺陷种类而从一种状态转变为另一种状态。 这种奇特的转变在二氧化铀的冲击胡克尼奥曲线和等温压缩曲线中通过第一性原理计算被数值观察到。 在低温下，它会导致热力学性质和电子结构中的异常（或准不连续性）。 特别是，在冲击温度和恒压比热中都表现出明显的异常。 然而，随着温度的升高，它会逐渐转变为平滑的交叉，变得不那么明显。 这种类型转变的潜在物理机制和特征编码在吉布斯自由能中，并通过分析缺陷种群随压力和温度变化的相关性得到了清晰的阐明。 也讨论了观察这种相变的可能实验机会和挑战。 显示英文摘要

摘要： A uniform distribution of point defects in an otherwise perfect crystallographic structure usually describes a unique pseudo phase of that state of a non-stoichiometric material. With off-stoichiometric uranium dioxide as a prototype, we show that analogous to a conventional phase transition, these pseudo phases also will transform from one state into another via changing the predominant defect species when external conditions of pressure, temperature, or chemical composition are varied. This exotic transition is numerically observed along shock Hugoniots and isothermal compression curves in UO2 with first-principles calculations. At low temperatures, it leads to anomalies (or quasi-discontinuities) in thermodynamic properties and electronic structures. In particular, the anomaly is pronounced in both shock temperature and the specific heat at constant pressure. With increasing of the temperature, however, it transforms gradually to a smooth cross-over, and becomes less discernible. The underlying physical mechanism and characteristics of this type of transition are encoded in the Gibbs free energy, and are elucidated clearly by analyzing the correlation with the variation of defect populations as a function of pressure and temperature. The opportunities and challenges for a possible experimental observation of this phase change are also discussed.