As regards stage 3 in As (As-II), the normal decrease (increase) of the interatomic bond distance (bond charge density) of MMBs in this stage are common behaviors of covalent materials with sp 3 geometry and no secondary bonding, such as zinc-blende Si and Ge . This normal behavior of the bond distances and the charge densities of MMBs is consistent with the experimentally and theoretically observed hardening of all phonons in As-II (Fig. ), as it occurs for Si and Ge . This normal behavior in As-II contrasts with the soft optical phonons of the A7 phase in As and of the R3m phase in GeTe ; i.e. in the pre-MMB scenario. The normal behavior in sc-As and in zinc-blende Si and Ge is the result of the lack of trans influence due to the lack of secondary bonds in these crystalline structures; in other words, there is only a single type of bond in these crystalline structures. Therefore, we tentatively conclude that the soft optical phonons in materials with primary covalent ppσ-bonds and secondary bonds, as confirmed by a recent three-center interaction model , are a clear fingerprint for the previously suggested instability of these primary bonds at HP . The softening of optical phonons in p-type covalent materials can be understood as an instability of the structure due to the transformation of covalent bonds into MMBs at HP. This instability of covalent p-type materials is analogous to the instability of tetrahedrally-coordinated sp 3 ionocovalent materials, such as zinc-blende Si and Ge, whose soft acoustic modes at the Brillouin zone edges are signatures of the instability of the sp 3 σ-bonds against octahedral coordination at HP .