Crystal Growth From Ionic Melts Studied by Molecular Dynamics Simulation

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，单元顶部向真空敞开。对NaCl3晶体在(100)、(110)和(111)面上的生长进行了研究。在6-14层熔化离子后，通过将除底层以外的所有离子保持在给定的温度下开始晶体生长。晶体不仅在(100)晶系中生长，而且在(110)晶系中也沿[100]方向生长。对氧化镁也发现了非常类似的趋势。对于具有扭曲金红石结构的CaCl2，在(001)、(100)和(110)面上进行了晶体生长研究。在这种情况下，晶体在大约1 ns的MD时间内只在(001)面上生长，可能是因为在这个面上保持了电荷中性。但它的生长速度比氯化钠晶体慢100倍。在分子水平上，晶体似乎是在阳离子对等小电荷中性单元中生长的。在NaCl型的情况下，阳离子-阴离子对可以成为这样的单元。另一方面，在CaCl2中，需要一个Ca^lt；2+^t+^C+^C</sup></sup>>两个^C</sup>S来组成这样一个基元，这样很容易产生缺陷。这些缺陷反过来又会阻碍晶体的进一步生长，因此晶体生长缓慢。对于钙钛矿结构的CaSiO_3，两种阳离子竞争一层结晶的位置。此外，由于粘度很高，即使在(001)晶面上生长速度也非常缓慢。对于运行时间)也已从氧化镁熔体结晶。

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.