Several mechanisms have been suggested to explain the superionic phase transitions in various superionic solid electrolytes. These include order-disorder transitions , competition between local atomic preferences (often described as frustration) , and overpopulation of existing lattice sites (sometimes referred to as jamming) . Moreover, in some materials the superionic transition is accompanied by significant changes in vibrational spectra. For instance, in CuCrSe 2 , γ-LiAlO 2 , and AgCrSe 2 , the superionic transition is linked to a breakdown of specific phonon modes .