First-principles approach to chemical diffusion of lithium atoms in a graphite intercalation compound
First-principles approach to chemical diffusion of lithium atoms in a graphite intercalation compound
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DOI：
10.1103/physrevb.78.214303
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发表时间：
2008-12
影响因子：
3.7
通讯作者：
K. Toyoura;Y. Koyama;A. Kuwabara;F. Oba;I. Tanaka
中科院分区：
文献类型：
--
作者：
K. Toyoura;Y. Koyama;A. Kuwabara;F. Oba;I. Tanaka
我们基于过渡态理论，从第一性原理出发，以锂在li6中通过间隙和空位机制扩散为模型，评估了晶体中原子跳跃的平均频率。在谐波近似下，从跃迁初始态和鞍点态的势势势垒和声子频率定量计算了平均跃迁频率。在量子统计中对晶格振动进行了处理，而不是使用Vineyard对应经典极限的常规处理，并定量讨论了两种处理之间的差异。与经典近似不同，表观活化能和平均跳频的振动前因子基本上取决于温度。活化能的差异对应于0 K时零点振动能的变化，即使在1000 K时仍有影响。对于振动前因子，经典近似在低温时对前因子的估计过高，而随着温度的升高，差异迅速减小，例如在室温下减少30%，在1000 K时减少5%。通过间隙机制和空位机制计算得到的锂原子化学扩散系数分别为1 (cid:1) 10−11和1 (cid:1) 10−10 cm 2 / s。
We evaluate mean frequencies for atomic jumps in a crystal from first principles based on transition state theory, taking lithium diffusion by the interstitial and vacancy mechanisms in LiC 6 as a model case. The mean jump frequencies are quantitatively evaluated from the potential barriers and the phonon frequencies for both initial and saddle-point states of the jumps under the harmonic approximation. The lattice vibrations are treated within quantum statistics, not using the conventional treatment by Vineyard corresponding to the classical limit, and the discrepancy between the two treatments is quantitatively discussed. The apparent activation energies and the vibrational prefactors of the mean jump frequencies essentially depend on temperature, unlike in the case of the classical approximation. The discrepancies of the activation energies correspond to the changes in zero-point vibrational energy at 0 K, and there remains the effect even at 1000 K. With regard to the vibrational prefactors, the classical approximation extremely overestimates the prefactors at low temperatures while the discrepancies rapidly decrease with increasing temperature, e.g., by 30% at room temperature and by 5% at 1000 K. The calculated chemical diffusion coefficients of lithium atoms by the interstitial and vacancy mechanisms are 1 (cid:1) 10 −11 and 1 (cid:1) 10 −10 cm 2 / s, respectively.