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First-principles calculations of carbon clathrates: Comparison to silicon and germanium clathrates

机译：碳包合物的第一性原理计算：与硅和锗包合物的比较

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We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I (X-46) and type-II (X-34) carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive, and electronic properties. In particular, we present different results about electronic properties of carbon clathrates. From the symmetry analysis of electronic states around the band gap, we deduce their optical properties, and we forecast the effects of hypothetical-doped elements on their electronic band gap. We then report first-principles calculations of vibrational, thermodynamieal, and elastic properties. Whereas vibrational properties of Si and Ge systems can be linked through their atomic weight ratio, we show that the vibrational properties of carbon structures differ strongly. Raman and infrared spectra of all clathrates are also calculated and compared. The effects of pressure and temperature on thermodynamieal properties (heat capacity, entropy, thermal expansion, etc.) within static and quasiharmonie approximations are investigated. It is shown that thermodynamieal properties of carbon clathrates and diamond present a similar evolution up to high pressures (100 GPa) and over a large range of temperatures ([0. 1500] K). Then we deduce the equilibrium phase diagram (P,T) of C-2/C-34/C-46. We conclude the paper with a presentation of elastic properties computed from acoustic slopes.
机译：我们采用基于密度泛函理论和密度泛函摄动理论的最先进的第一性原理计算方法，研究了I型自由客体（X-46）和II型自由客体（ X-34）碳包合物。它们的性能以及硅和锗钻石的性能以及包合物已经在同一方法中进行了计算和比较。我们简要介绍并讨论它们的结构，内聚和电子特性。特别是，我们提出了关于碳包合物的电子性质的不同结果。通过带隙周围电子态的对称分析，我们推导出了它们的光学性质，并预测了假设掺杂元素对其电子带隙的影响。然后，我们报告振动，热动力和弹性特性的第一性原理计算。尽管Si和Ge体系的振动特性可以通过它们的原子量比联系起来，但我们表明碳结构的振动特性差异很大。还计算并比较了所有包合物的拉曼光谱和红外光谱。研究了压力和温度对静态和拟谐近似中热力学性质（热容量，熵，热膨胀等）的影响。结果表明，碳包合物和金刚石的热动力学特性在高压（100 GPa）和大范围温度（[0. 1500] K）下均表现出相似的演变。然后我们推导了C-2 / C-34 / C-46的平衡相图（P，T）。在本文的结尾，我们介绍了根据声学斜率计算出的弹性特性。

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