Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Departments of Aerospace Engineering, Mechanical Engineering, and Material Science and Engineering, Iowa State University, Ames, Iowa 50011, USA.
J Chem Phys. 2019 Feb 14;150(6):064705. doi: 10.1063/1.5080010.
The solid, secondary explosive nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7 or HMX has four different stable polymorphs which have different molecular conformations, crystalline structures, and densities, making structural phase transitions between these nontrivial. Previous studies of the kinetics of the β-δ HMX structural transition found this to happen by a nucleation and growth mechanism, where growth was governed by the heat of fusion, or melting, even though the phase transition temperature is more than 100 K below the melting point. A theory known as virtual melting could easily justify this since the large volume difference in the two phases creates a strain at their interface that can lower the melting point to the phase transition temperature through a relaxation of the elastic energy. To learn more about structural phase transitions in organic crystalline solids and virtual melting, here we use time-resolved X-ray diffraction to study another structural phase transition in HMX, γ-δ. Again, second order kinetics are observed which fit to the same nucleation and growth model associated with growth by melting even though the volume change in this transition is too small to lower the melting point by interfacial strain. To account for this, we present a more general model illustrating that melting over a very thin layer at the interface between the two phases reduces the total interfacial energy and is therefore thermodynamically favorable and can drive the structural phase transition in the absence of large volume changes. Our work supports the idea that virtual melting may be a more generally applicable mechanism for structural phase transitions in organic crystalline solids.
固体二级爆炸物硝胺-八氢-1,3,5,7-四硝基-1,3,5,7 或 HMX 有四种不同的稳定多晶型物，它们具有不同的分子构象、晶体结构和密度，使得这些结构之间的相变变得复杂。以前对 HMX 的β-δ结构相变动力学的研究发现，这种相变是通过成核和生长机制发生的，其中生长由熔融热或熔化控制，尽管相变温度比熔点低 100 K 以上。一种称为虚拟熔化的理论可以很容易地证明这一点，因为两相之间的大体积差异在其界面处产生应变，通过弹性能量的松弛可以将熔点降低到相变温度。为了更多地了解有机晶体固体中的结构相变和虚拟熔化，我们在这里使用时间分辨 X 射线衍射研究 HMX 中的另一个结构相变γ-δ。再次观察到二级动力学，其符合与熔化相关的相同成核和生长模型，尽管在这种转变中体积变化太小，无法通过界面应变降低熔点。为了解决这个问题，我们提出了一个更通用的模型，表明在两相之间的界面上非常薄的一层熔化可以降低总界面能，因此从热力学上是有利的，可以在没有大体积变化的情况下驱动结构相变。我们的工作支持了这样一种观点，即虚拟熔化可能是有机晶体固体中结构相变的一种更普遍适用的机制。
