Ultrafast phase transitions in polyamorphic materials triggered by swift heavy ion impacts

We report unexpectedly fast phase transitions inside the track as pressure and temperature develop after a swift heavy ion impact. By means of molecular dynamics simulations we reveal the origin of the core-shell fine structure observed in a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ and a-${\mathrm{Si}}_3{\mathrm{N}}_4$, as well as the dense core in a-Si. The fine structure of the track is composed of two distinct amorphous phases that result from the different pressure levels in the core and in the shell during solidification. We link this behavior to the tetrahedral network present in these materials.

Figure 1

Time evolution of the radial density after the impact of Au 185 MeV ion in three materials: a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ (left), a-${\mathrm{Si}}_3{\mathrm{N}}_4$ (center), and a-Si (right). The color shows spatiotemporal radial distribution of the relative density change away from the ion path (at the beginning of the horizontal axis). The figures from the bottom to the top (the vertical axis) show the dynamic of radial distribution of the density change from the instant right after the impact and until the track has cooled down. The density change is averaged along the $z$ axis within the concentric cylindrical shells from the ion path. The range of density changes is given by the color scale bar at the top of the figure. The solid yellow lines illustrate the propagation of the melting/solidification front and the black lines are the isobaric contours for some reference pressure values.

Figure 2

Density evolution during a heating/cooling cycle at different pressures in a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ (WS) with Watanabe-Samela, a-${\mathrm{Si}}_3{\mathrm{N}}_4$ and a-Si (GAP). We show the heating data (also referred as ${\rho }^{q\mathrm{EOS}}(T,P)$ in the text) with points, and the cooling data for 0.002 K/fs and 0.02 K/fs with dotted and dashed lines, respectively.

Figure 3

Density relaxation of amorphous structures with time after a change in pressure. The $y$ axis shows the excess density in the structure calculated according to Eq. (1). The top and bottom subfigures show the density relaxation for a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ (WS) and a-${\mathrm{Si}}_3{\mathrm{N}}_4$ structures, respectively. The black lines indicate the fit of the data with an exponential decay function from Eq. (2).

Figure 4

Time evolution of the density and pressure inside the track in a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ with Watanabe-Samela potential (top), a-${\mathrm{Si}}_3{\mathrm{N}}_4$ and a-Si with GAP potential. The solid lines denote the track shell and the dashed ones the core; we show the actual simulation density, as well as the qEOS ${\rho }^{\mathrm{q}\mathrm{EOS}}(T^{*},P^{*})$ prediction. We also include the density ${\rho }^{\mathrm{hist}}(T^{*},P^{*})$ for an amorphous structure cooled at the corresponding solidification pressure of each material according to Eq. (3). The original density in the cell is indicated with a black horizontal dashed line.

Figure 5

Analysis of the a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ structure in the core and shell regions of the track. We include as reference the analogous analysis for amorphous structures quenched at 0 and 2 GPa pressures. The subfigures include the following analysis: (a) Distribution of the Si-O-Si bond angles. (b) Ring distribution of the amorphous structure.

Figure 6

Illustration of mechanisms leading to formation of the fine structure inside the track. The subfigures correspond to different amorphous materials: (a) for a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ and a-${\mathrm{Si}}_3{\mathrm{N}}_4$, (b) for a-Si.