Solid State and Structural Chemistry Unit, Indian Institute of Science , Bangalore 560012, India.
J Phys Chem B. 2013 Oct 24;117(42):13154-63. doi: 10.1021/jp4031199. Epub 2013 Jun 12.
In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. Sometimes, the metastable phase might have a lower free energy minimum than the liquid but higher than the stable-solid-phase minimum and have characteristics in between the parent liquid and the globally stable solid phase. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can "wet" the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier because surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work, we discuss a density functional theory (DFT)-based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order-parameter-dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier, and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of the Ostwald step rule and the well-known phenomenon of "disappearing polymorphs" that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics, some of which may be explored via modern nanoscopic synthetic methods.
在许多系统中，稳定固相的成核可能会在其他（通常不止一种）亚稳相的存在下发生。这些可能是多晶型固体，甚至是液相。有时，亚稳相的自由能最小值可能低于液相，但高于稳定固相的最小值，并具有介于母体液相和全球稳定固相之间的特性。在这种情况下，从熔体中析出固相可能会受到亚稳相的促进，因为后者可以“润湿”母相与子相之间的界面，尽管在母体液相和稳定固相的热力学性质中可能没有亚稳相存在的迹象。这里，经典成核理论（CNT）的直接应用存在缺陷，因为它高估了成核势垒，因为表面张力被高估（通过忽略中间阶亚稳相），而子相与母相之间的热力学自由能间隙保持不变。在这项工作中，我们讨论了一种基于密度泛函理论（DFT）的统计力学方法来探索和量化这种促进作用。我们构建了一个简单的依赖于序参量的自由能表面，然后在 DFT 中使用它来计算（i）序参量分布，（ii）整体成核自由能势垒，以及（iii）母体液相和亚稳固相以及母体液相和稳定固相之间的表面张力。该理论确实发现，在润湿的情况下，成核自由能势垒可以显著降低。这种方法可以为奥斯特瓦尔德步规则和众所周知的“消失多晶型”现象提供微观解释，该现象取决于温度和其他热力学条件。理论揭示了相变动力学的多样化情景，其中一些可能通过现代纳米合成方法来探索。
