Fixed crystalline seed

The essential feature of PC materials is the ultrafast phase transition between amorphous and crystalline structures that occurs on a nanosecond time scale. In the previous sections, we have discussed extensively the amorphous and crystalline structures of GST and their properties. These correspond to the starting and end points for the actual phase transition, which are crucial to understand the function of PC materials. We now present results for the nucleation-driven crystallization process of GST using DF calculations combined with MD [31], A sample of fl-GST with 460 atoms was studied at 500, 600, and 700 K, and a second sample of 648 atoms was simulated at 600 K. In all cases we used a fixed crystalline seed (58 atoms, 6 vacancies) in order to speed up the crystallization process. More recent experience has shown that the time scale for the crystallization is of the order of several nanoseconds for these system sizes in the absence of a fixed seed, while those here are of the order of 0.3-0.6ns. This means that we cannot discuss the onset of nucleation, but this is also true in the case of smaller systems (<200 atoms) discussed by other groups. In very small systems, periodic boundary conditions bias the process severely. Our larger samples reduce finite-size effects, and we show the effect of choosing different annealing temperatures. Simulations of this scale (up to 648 atoms over 1 ns) are near the limit of present day DF/MD calculations. 

Fig. 17.15 PDF of atom pairs in 600 K simulation. Each plot is the average over 20 ps of atrajectory. Red 80-100 ps, green 180-200 ps, blue 280-300 ps, magenta 380-400 ps, black 480-500ps, orange 580-600 ps. Successive plots are shifted by 0.5. The vertical lines correspond to the fixed crystalline seed with lattice consteint of 3.0 A...

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... 

The simulations follow the pattern described in previous sections. The CPMD program is used with Born-Oppenheimer MD and a time step of 3.0236 fs (125 a.u.). We employ an NVT ensemble with cubic simulation cell, periodic boundary conditions, and a single point (k = 0) in the Brillouin zone. Simulations have been performed on two samples of a-GST (Fig. 17.1), with 460 and 648 atoms, respectively. We embedded in both a crystalline seed of 4 x 4 x 4 sites (13 Ge, 13 Sb, and 32 Te atoms, 6 vacancies) in a rock salt structure with lattice constant of 3.0 A. The structure of the crystallite adopted the model of Yamada [7], which assumed that one sublattice of the rock salt structure comprises Te atoms, the other a random arrangement of Ge, Sb, and vacancies. We removed the atoms of the amorphous structure inside this volume, fixed the coordinates of the seed, and optimized the resulting structure. 

The seeds contain some volatile oil, resin and a large amount of fixed oil (Meisner, 1818). The fruit (without the seeds) contains volatile oil, resin, fat, tannin, pectin and mucilage. The volatile oil (oil of star-anise) amounts to about 4-5% and is almost identical with oil of anise (from P. anisum, LinnS). Star-anise oil from Chinese fruit has a specific gravity at 15°C (59°F) of 0.980-0.990 and its known constituents are anethol, phel-landrene, safrol and hydro-quinone-ethyl-ether (Fliickiger, 1879). Poisonous sikimin has been detected in the fruit (Eykmann, 1881), while Schlegel (1885) found a crystalline principle of a pronounced odour of musk. He also found saponin in the watery extract. 

For the 460-atom model, the initial geometry used was the structure of a-GST that gave excellent agreement between DF simulations and experimental x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS) measurements [26]. With fixed coordinates of the seed, we performed DF/MD simulations at 500, 600, and 700 K over 600 ps (500/600 K) and 350 ps (700 K). The path to crystallization at 600 K is shown in Fig. 17.13(a-d). The amorphous and crystalline densities of GST differ, and the size of the cubic simulation cell was changed from 24.629 A (amorphous density of 0.0308 atoms/A ) to 24.060 A (crystalline density of 0.0330 atoms/A ) in five steps of 0.114 A. 

---