Specific Heat of (GeTe)_x(Sb₂Te₃)_1–x Phase-Change Materials: The Impact of Disorder and Anharmonicity

Phase-change materials (PCM) are bad glass formers, and their rapid crystallization is accompanied by a drastic change in optical and electrical properties, which opens opportunities for novel nonvolatile data storage devices. Many of these materials are located on the pseudobinary line between GeTe and Sb 2 Te 3 and form a metastable rock-salt-like atomic arrangement in which Te atoms occupy one of the two sublattices and the other is randomly filled with Ge and Sb atoms as well as vacancies. The resulting disorder has profound impact on, for example, transport properties, causing disorder-induced localization of charge carriers. Here we discuss the impact of disorder on thermal properties. We have investigated several PCMs from the pseudobinary line between GeTe and Sb 2 Te 3 . A significant enhancement of the specific heat is found for the disordered rock-salt-like phase compared with the ordered trigonal phase, in which Ge and Sb atoms occupy separate layers. The magnitude of this enhancement is correlated with the fraction of stoichiometric vacancies in the Ge/Sb sublattice. The additional contribution to the specific heat is shown to consist of a reversible fraction and an irreversible fraction, which are attributed to anharmonic lattice dynamics and irreversible vacancy ordering, respectively. These findings underline the prominent role of vacancy ordering in electrical and thermal transport.