Figure 4.

Phonon density of states and dispersion relations. Theoretical calculation of partial density of states for a) GeTe and b) Ge_0.92Sb_0.08Te crystals. The red trace in (b) indicates the contribution from Sb. c) The VCA method based on first‐principles calculations for examining of the effects of partially doping Sb into GeTe on phonon dispersion and ‐PDOS. Two Sb doping levels 0.02 (blue traces) and 0.08 (orange traces) are shown for comparing to that of pristine GeTe (green traces). The right panel shows the supercell used in the DFT calculation of GeTe. Similar DFT calculations were performed for GST with different Sb‐concentrations, and the overall phonon frequency softens due to the Sb doping. In pristine GeTe, no phonon modes are present between ≈0 and 10 meV at the Γ point and 12–13 at W point.