Phase-Change Materials (PCMs) are used as active switching elements in novel electronic memory devices. The writing mechanism of this memory is based on atomic kinetics that offer (i) sub-nanosecond, fast crystallization over a wide range of elevated temperatures and (ii) stable glass formation at ambient conditions. It was shown previously that these regimes are characterized by a low and high activation energy of atomic diffusivity, respectively. The transition between these regimes has been associated with a fragile-to-strong crossover[1] and/or with a glass transition[2], but its microscopic origin has remained elusive - mostly due to the experimental limitations imposed by the fast crystallization of this disordered state