液态碳的局域结构控制金刚石成核
Local structure of liquid carbon controls diamond nucleation
摘要：
钻石在4000 K以上的温度下熔化。目前还没有测量逆过程的稳态速率：熔体中的金刚石成核，因为在这些极端温度和压力下进行实验很困难。使用数值模拟，我们估计金刚石成核率，并发现它增加了许多数量级时，在恒定的过饱和度的压力增加。原因是压力的增加将碳原子的局部配位从三倍改变为四倍。事实证明，在四重配位液体中使金刚石成核比在三重配位液体中容易得多，因为在后一种情况下，产生金刚石-液体界面的自由能成本更高。我们推测这种成核控制机制与许多网络形成液体中的结晶有关。根据我们的计算，我们得出结论，均匀的金刚石成核可能在富碳恒星和气态行星不太可能。
英文摘要：
Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process: diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that an increase in pressure changes the local coordination of carbon atoms from three-fold to four-fold. It turns out to be much easier to nucleate diamond in a four-fold coordinated liquid than in a liquid with three-fold coordination, because in the latter case the free-energy cost to create a diamond-liquid interface is higher. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. On the basis of our calculations, we conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.
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