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arXiv 1204.4607cond-mat.mtrl-scicond-mat.stat-mechphysics.comp-ph
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点缺陷赝相变引起的非化学计量比二氧化铀反常

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

Hua Y. Geng, Hong X. Song, Q. Wu

摘要:

点缺陷在其他方面完美的晶体结构中的均匀分布通常描述了非化学计量材料的该状态的唯一伪相。以非化学计量比的二氧化铀为原型,我们发现,类似于传统的相变,这些赝相也将从一种状态转变为另一种状态,通过改变主要的缺陷物种时,压力,温度,或化学成分的外部条件变化。利用第一性原理计算,沿UO 2的冲击Hugoniots曲线和等温压缩曲线数值观测到了这种奇异的转变。在低温下,它会导致热力学性质和电子结构的异常(或准不连续)。特别是,在冲击温度和比热在恒定压力下的异常是明显的。但随着温度的升高,它逐渐转变为平滑的交叉,变得不易辨别。这种类型的转变的潜在的物理机制和特性编码在吉布斯自由能,并阐明清楚地通过分析与作为压力和温度的函数的缺陷布居的变化的相关性。这种相变的可能的实验观察的机会和挑战也进行了讨论。

英文摘要:

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.

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