  Chalcogenide phase-change materials (PCMs) show a significant contrast in
optical reflectivity and electrical resistivity upon crystallization from the
amorphous phase and are leading candidates for non-volatile photonic and
electronic applications. In addition to the flagship Ge2Sb2Te5 phase-change
alloy, doped Sb2Te alloys, in particular AgInSbTe used in rewritable optical
discs, have been widely investigated for decades, and nevertheless the
theoretical insights on the optical properties of this important family of PCMs
are scarce. Here, we carry out thorough ab initio simulations to gain an
atomistic understanding of the optical properties of Sb2Te and AgInSbTe. We
show that the large optical contrast between the amorphous and crystalline
phase stems from the change in bond type in the parent compound Sb2Te. Ag and
In impurities serve mostly the purpose of stabilization of the amorphous phase,
and have marginal impact on the large variation in the dielectric function upon
the phase transitions.
