Crystal Growth From Ionic Melts Studied by Molecular Dynamics Simulation

采用分子动力学模拟方法研究了离子熔体中晶体的生长。在x、y方向上施加周期性边界条件，而在z方向上，电池的顶部对真空开放。对于NaCl，研究了（100）、（110）和（111）晶面上的晶体生长。在熔化6-14层的离子之后，通过将除底层之外的所有离子保持在给定温度下来开始晶体生长。晶体不仅在（100）系统中而且在（110）系统中沿[100]方向生长。对于MgO也发现了相当类似的趋势。对于具有畸变金红石结构的CaCl_2晶体，研究了（001）、（100）和（110）晶面上的晶体生长。在这种情况下，在约1 ns的MD时间内，晶体仅在（001）面上生长，这可能是因为在该面上保持电荷中性。但其生长速率比NaCl型慢100倍。从分子水平上看，晶体似乎是在一个小的电荷中性单元中生长的，如阳离子-阴离子对。在NaCl型的情况下，阳离子-阴离子对可以成为这样的单元。而在CaCl_2中，则需要一个Ca^2++两个Cl ^-来形成这样的单元，在这样的单元中容易产生缺陷。这些缺陷又将阻碍进一步的晶体生长，因此晶体生长缓慢。对于具有钙钛矿结构的CaSiO_3，两种阳离子竞争一层的位置以结晶。此外，由于粘度高，即使在（001）面中生长速率也非常慢。52分钟的运行时间）也已从MgO熔体结晶。

Molecular dynamics simulation has been perfonned to study crystal growth from ionic melts. The periodic boundary condition was imposed in the x-, y- directions while in the z-direction the top of the cell was open to a vacuum. For NaC1, crystal growth on (100), (110) and (111) planes has been studied. After melting the ions at 6-14 layers, crystal growth was started by keeping all the ions at a given temperature except for the bottom layer. The crystal grew in the [100] direction not only in the (100) system but also in the (110) system. Quite a similar tendency has been found also for MgO. The growth rate as a function of temperature is found to have a maximum.For CaC1_2 having a distorted rutile structure crystal growth has been studied on (001), (100) and (110) planes. In this case, crystal grew only on the (001) plane within an MD time of about 1 ns, presumably because charge neutrality is maintained on this plane. However, its growth rate was slower than that of NaCl-type by a factor of 100.On a molecular level, it seems that crystal grows in a small charge neutral unit such as a cationanion pair. In the case of NaC1-type, cation-anion pair can become such unit. On the other hand, in CaCl_2, one Ca^<2+> plus two C1^<-'>s are needed to fonn such a unit, in which a defect can be easily generated. The defects will, in turn, retard further crystal growth, and therefore crystal growth is slow. For CaSiO_3 having the perovskite structure, two kinds of cations compete the position of one layer for crystallization. Further, since the viscosity is high the growth rate is very slow even in the (001) plane.A video film (ca. 52 min. for the running time) has also been made on crystallization from MgO melt.