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首页> 外文期刊>Physical review >Stability of oxygen point defects in UO_2 by first-principles DFT+U calculations: Occupation matrix control and Jahn-Teller distortion
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Stability of oxygen point defects in UO_2 by first-principles DFT+U calculations: Occupation matrix control and Jahn-Teller distortion

机译：通过第一性原理DFT + U计算得出UO_2中氧点缺陷的稳定性：职业矩阵控制和Jahn-Teller失真

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摘要

Point-defect formation energies in uranium dioxide UO_2 are still a matter of debate due to the significant discrepancies between the various studies published in the literature. These discrepancies stem from the density functional theory (DFT) + U approximation that creates multiple energy minima and complexifies the search for the ground state. We report here DFT+ U values of the formation energies for the single oxygen interstitial and vacancy in UO_2, both in the fluorite and the Jahn-Teller distorted structures, using a scheme developed on bulk UO_2 [B. Dorado, B. Amadon, M. Freyss, and M. Bertolus, Phys. Rev. B 79, 235125 (2009)] and based on occupation matrix control. We first investigate the Jahn-Teller distortion in UO_2 in the noncollinear antiferro-magnetic order and we show that the distortion stabilizes the system by 50 meV/UO_2 compared to the fluorite structure. Moreover, it is found that the oxygen atoms are displaced in the <111> directions, in agreement with experiments. For the bulk fluorite structure, we show that the use of the Dudarev approach of the DFT+U without occupation matrix control systematically yields the first metastable state, located 45 meV/UO_2 above the ground state. As a result, all previously published point-defect formation energies are largely underestimated. We then use the occupation matrix control scheme to calculate the formation energies of the single oxygen interstitial and vacancy in UO_2. We confirm that this scheme always allows one to reach the lowest energy states and therefore yields reliable formation energies. Finally, we compare our values with those obtained in previous studies and show that the discrepancies observed stem from the calculations of defective supercells which have reached different metastable states.
机译：由于文献中发表的各种研究之间存在显着差异，因此二氧化铀UO_2中的点缺陷形成能仍然是一个有争议的问题。这些差异源于密度泛函理论（DFT）+ U近似，它创建了多个能量最小值，并使对基态的搜索复杂化。我们在这里报告萤石和Jahn-Teller畸变结构中UO_2中单个氧的间隙和空位的形成能的DFT + U值，使用在整体UO_2上开发的方案[B. Dorado，B.Amadon，M.Freyss和M.Bertolus，物理学。修订版B 79，235125（2009）]并基于职业矩阵控制。我们首先以非共线反铁磁顺序研究了UO_2中的Jahn-Teller畸变，并且我们发现与萤石结构相比，该畸变使系统稳定了50 meV / UO_2。此外，发现与实验一致，氧原子在<111>方向上位移。对于块状萤石结构，我们表明在没有占领矩阵控制的情况下使用DFT + U的Dudarev方法系统地产生了第一个亚稳态，位于基态上方45 meV / UO_2。结果，所有先前公布的点缺陷形成能都被大大低估了。然后，我们使用职业矩阵控制方案来计算UO_2中单个氧间隙和空位的形成能。我们确认，该方案始终允许人们达到最低能量状态，因此可产生可靠的地层能量。最后，我们将我们的值与先前研究中获得的值进行比较，并表明观察到的差异源于已达到不同亚稳态的有缺陷的超级电池的计算。

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