Using　electron　microscopy　and　diffraction　techniques,　as　well　as　first-principles　calculations,　we　demonstrate　that　as　much　as　35%　of　the　total　Ge　atoms　in　the　cubic　phase　of　Ge2Sb2Te5　locate　in　tetrahedral　environments.　The　Ge-vacancy　interactions　play　a　crucial　stabilizing　role,　leading　to　Ge-vacancy　pairs　and　the　sharing　of　vacancies　that　clusters　tetrahedral　Ge　into　domains.　The　Ge2Sb2Te5　structure　with　coexisting　octahedral　and　tetrahedral　Ge　produces　optical　and　structural　properties　in　good　agreement　with　experimental　data　and　explains　the　property　contrast　as　well　as　the　rapid　transformation　in　this　phase-change　alloy.