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arXiv 1901.08402cond-mat.mtrl-sci
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密度泛函理论研究锂原子在室温下的结构和振动性质

Structural and vibrational properties of lithium under ambient conditions within density functional theory

Michael Hutcheon, Richard Needs

摘要:

我们采用一般的第一性原理方法,推导出金属锂在0和350 K之间的环境压力下的相图,包括候选相的识别。我们使用从头算随机结构搜索(AIRSS),以确定竞争阶段和补充的振动特性和相关的衍生中子衍射图案的计算结果。强烈的量子核效应存在,促使振动的仔细处理。我们直接映射的玻恩-奥本海默表面的李第一次,允许扩展的正常准谐波治疗振动的“准非谐”的方法,其中包括非谐性的影响。利用各种状态方程推导出FCC、BCC、HCP和9 R相的吉布斯自由能。我们发现,非谐贡献的吉布斯自由能的Li相是相同的顺序相之间的差异。非简谐性对BCC相的0 K声子色散也有显著影响,其中在N点声子处的差异最大。我们发现的相变的顺序与Ackland等人的计算一致。[Science 356,1254-1259,(2017)],即使包括非谐波效应,也表明准谐波处理足以获得正确的相位行为。我们明确地表明,加热时从密排锂到BCC锂的马氏体相变是由声子自由能的熵贡献驱动的。

英文摘要:

We apply a general first-principles approach to derive the phase diagram of metallic Lithium at ambient pressure between 0 and 350 K, including identification of candidate phases. We use ab initio random structure searching (AIRSS) to identify competing phases and supplement the results with calculations of vibrational properties and relevant derived neutron diffraction patterns. Strong quantum nuclear effects are present, prompting a careful treatment of vibrations. We directly map the Born-Oppenheimer surface of Li for the first time, allowing the extension of the normal quasi-harmonic treatment of vibrations to a "quasi-anharmonic" approach, where the effects of anharmonicity are included. The Gibbs free energies of the FCC, BCC, HCP and 9R phases are derived using a variety of equations of state. We find that the anharmonic contribution to the Gibbs free energies of Li phases is of the same order as the differences between phases. Anharmonicity also makes a noticeable difference to the 0 K phonon dispersion of the BCC phase, with the largest difference at the N-point phonon. The ordering of phase transitions that we find agrees with the calculations of Ackland et al. [Science 356, 1254-1259, (2017)], even when anharmonic effects are included, suggesting that a quasi-harmonic treatment is sufficient for correct phase behaviour. We show explicitly that the Martensitic phase transition from close-packed to BCC lithium upon heating is driven by entropic contributions to the phonon free energy.

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