Vanishing-Harmonicity and Phase-Change Materials

Phase-change material (PCMs) store data using the contrast (electrical or optical) between two phases: a conductive crystalline phase and a weakly conductive amorphous phase. Most PCMs have a distorted octahedral structure. The contrast comes mainly from the electronic structure. In PCMs, a spontaneous symmetry breaking mechanism, the Peierls distortion, transforms the metallic crystalline structure into a lower-density semiconducting structure. In a simple tight-binding model of the covalent bond, the parameters that control this dis- tortion, characterized by a parameter η, are analyzed. The effective interatomic potential E(η) is developed in a Landau-type series in η : E(η) 1⁄4 E0 þ E2η2 þ E4η4. The PCMs with the largest contrast are those for which the effective potential E(η) of the crystalline phase has a disappearing harmonic contribution (E2 1⁄4 0) and a vanishing electronic gap. This is called as an “incipient Peierls distortion.” It coincides with the so-called “incipient metal”. The hardness of the repulsive potential and the number of electrons per atom play an important role. The vibrational properties and the anomalous Grüneisen parameter, specific to PCMs, are also studied.