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The aim of this work is to investigate the amorphous-to-crystalline phase transformation process in Ge2Sb2Te5 ternary alloys. The experiments were carried out using impedance and X-ray diffraction measurements. The results have shown that the crystallization process in an amorphous Ge2Sb2Te5 alloy starts from nuclei, which appear below Tc and which were identified as the crystalline Ge1Sb4Te7. As the temperature increases above Tc these nuclei were transformed into fcc-Ge2Sb2Te5 crystals. In order to establish the mechanism of crystallization for this system, the Brugerman effective medium approximation was implemented to analyze nucleation and growth of the two stage involved (i.e., metastable Ge1Sb4Te7 and stable Ge2Sb2Te5) through impedance measurements. The results of the simulations demonstrate a close agreement with experimental ones showing that phase transformation in Ge2Sb2Te5 is carried in two steps.