Unravelling the Mechanism of Pressure Induced Amorphization of Phase Change Materials

Based on ab initio molecular dynamics simulations, we identify the atomistic mechanism of the pressure induced amorphization of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. The simulations reveal that homopolar $\mathrm{Ge}/\mathrm{Sb}$ bonds appear in cubic ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ under pressure, giving rise to square rings rotated by 45° with respect to the crystalline axis whose formation is induced by the displacement of Te atoms filling the voids of neighboring $\mathrm{Ge}/\mathrm{Sb}$ stoichiometric vacancies. The concentration of these topological defects increases with pressure up to 21 GPa at which the system is destabilized and transforms into an amorphous phase in agreement with experiments.

Figure 1

Evolution with pressure of the intensity of the main peaks of the structure factor $S(Q)$ normalized to the intensities of the $⟨200⟩$ peak at zero pressure. Inset: Distribution of the coordination number for (a) $c$-GST and (b) $a$-GST at the same density which corresponds to a pressure of 21 GPa in $a$-GST. The average coordination numbers are $n_{\mathrm{Ge}}=6.28$, $n_{\mathrm{Sb}}=7.18$, and $n_{\mathrm{Te}}=5.23$ for $c$-GST and $n_{\mathrm{Ge}}=6.67$, $n_{\mathrm{Sb}}=7.72$, and $n_{\mathrm{Te}}=5.14$ for $a$-GST (at 21 GPa).

Figure 2

Equation of state of $c$-GST, hex-GST, pressure amorphized GST (filled dots), and melt-quenched $a$-GST [5]. Energy-volume points are fitted with a Murnaghan function. Inset: Sketch of the displacement of Te atom toward the vacancy giving rise to a rotated square ring with homopolar bonds.

Figure 3

Snapshot of the simulation at 14 GPa showing the appearance of squared rings formed by homopolar $\mathrm{Ge}/\mathrm{Sb}$ bonds rotated by 45° with respect to the crystal axis. Large spheres indicate atoms involved in the formation of rotated rings (blue: Ge; green: Sb; light blue: Te). Small red spheres indicate the position of $\mathrm{Ge}/\mathrm{Sb}$ vacancies.

Figure 4

Evolution with pressure of the fraction (over the total number of bonds) of $\mathrm{Ge}/\mathrm{Sb}$ and $\mathrm{Te}/\mathrm{Te}$ homopolar bonds during the compression and decompression runs. The corresponding values in melt-quenched $a$-GST [5] are given for sake of comparison.