Results for Nucleation-Driven Crystallization

We observed complete crystallization for the 460-atom simulations at 600 K and 700 K within the given time scale, and the former simulation is shown in Fig. 17.13a-d. Crystal growth around the seed was evident in the simulation at 500 K and in the 648-atom simulation at 600 K Fig. 17.13e-f, but crystallization was not complete within 600 ps. It should be noted that 500 K is already within 100K of the experimental crystallization temperature (onset), whereas the process occurs in practice at higher temperatures (laser heating, current). 

The gradual transition between amorphous and crystalline structures is shown in the evolution of the partial PDF at 600 K in Fig. 17.15. The crystalline seed gives rise to Bragg peaks at 3.00,4.24, 5.20, 6.00, 6.71 and 7.35 A, which are enhanced by crystallization. The increase is most rapid between 280-300 and 480-500 ps at 600 K (Fig. 17.15), and the rapid increase at 700K occurs between 80-100 ps and 180-200 ps. The total coordination numbers were calculated from the PDF for Ge, Sb, and Te atoms at the beginning and end of the 460-atom simulations (500 K, 600 K, 700 K), and they show the expected change from amorphous to crystalline values for the samples that order and smaller changes otherwise. 

Ge-Ge bonds near the end, but the first peak shows an unusually short bond less than 2.5 A. 

The evolution of crystallization is also apparent in the mean-square displacement (MSD) of the atomic coordinates from the starting stmcture (Fig. 17.17). There should be no atomic diffusion in a crystalline material, and changes in MSD slow down and eventually stop as crystallization occurs. The order of Ge and Sb varies Sb is slightly more mobile than Ge at 500 K until 500 ps, where the MSD of Ge increases rapidly. At 600 K, Ge has a higher MSD than Sb, while the reverse is tme at 700 K. Furthermore, we have observed that Sb has a higher mobility in the liquid 

It is interesting follow the motion of vacancies/cavities during crystallization. Recent simulations of crystallization in GST suggested that there is cavity diffusion to the crystal/glass interface. This was followed by Ge/Sb diffusion to these sites, aiding the formation of cubic, cavity-free crystallites [37]. Here, the fixed crystalline seed comprises, by definition, 6 vacancies as in c-GST, and we have followed how the other vacancies rearrange in different shells of the growing crystallite. The radial 

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