Figure 6: Bond distributions around Ge in crystalline, liquid and relaxed amorphous GeTe.

Plots have been obtained with vdW-DF2 functionals. The panels represent the angular-limited three-body correlation (ALTBC, normalized per atom), which expresses the probability of having a bond of length r₁ almost aligned with a bond of length r₂ (angular deviations smaller than 25°). r₁ and r₂ define the axes of the plots, which are symmetrical by nature. Top panels: (a) liquid phase at T=1,100 K, (b) relaxed glass obtained by substituting Sn with Ge in a-SnTe and (c) crystal. All systems are at the amorphous density. The white circles correspond to (r₁, r₂) in crystalline GeTe at the crystalline density. The plots show that the Peierls distortion ratio (r₂/r₁) in relaxed amorphous GeTe is larger than in the crystalline phase. Bottom panels: ALTBC plots for Ge^H (d) and Ge^III (e). The plot for Ge^H (a high-energy defect, which disappears on aging) exhibits a broad distribution with three peaks corresponding to short–long (S-L) bonds and ‘medium-length’ bonds (M). On the contrary, the plot for the Ge^III motifs, which are predominant in the relaxed glass, displays two sharp, well-defined peaks corresponding to a genuine 3+3 Peierls distortion.