Crystal Growth From Ionic Melts Studied by Molecular Dynamics Simulation

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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.