  We report a discovery, through first-principles calculations, that
crystalline Ge-Sb-Te (GST) phase-change materials exhibit the topological
insulating property. Our calculations show that the materials become
topological insulator or develop conducting surface-like interface states
depending on the layer stacking sequence. It is shown that the conducting
interface states originate from topological insulating Sb2Te3 layers in GSTs
and can be crucial to the electronic property of the compounds. These interface
states are found to be quite resilient to atomic disorders but sensitive to the
uniaxial strains. We presented the mechanisms that destroy the topological
insulating order in GSTs and investigated the role of Ge migration that is
believed to be responsible for the amorphorization of GSTs.
