Violation of the Stokes–Einstein relation in Ge₂Sb₂Te₅, GeTe, Ag₄In₃Sb₆₇Te₂₆, and Ge₁₅Sb₈₅, and its connection to fast crystallization

Phase-change materials (PCMs) are already commercialized in optical and non-volatile memory devices. Yet, the dynamics of atomic rearrangement processes and their temperature dependence, which govern their ultrafast switching, are still not fully understood. Here we use quasi-elastic neutron scattering to investigate the liquid-state dynamics of four prevailing PCMs Ge₂Sb₂Te₅, GeTe, Ag₄In₃Sb₆₇Te₂₆(AIST), and Ge₁₅Sb₈₅ above their respective melting points T_m. Self-diffusion coefficients and structural relaxation times on the timescale of picoseconds are extracted from dynamic structure factors. The results indicate an unusual systematic violation of the Stokes-Einstein relation (SER) for each PCM in high-temperature regions above T_m, where the atomic-mobility is high. This is likely related to the formation of locally favored structures in liquid PCMs. Absolute values of diffusivity in the supercooled liquid AIST are derived from crystal-growth velocity, which are almost one order of magnitude higher than that expected from the SER in the technologically relevant temperature range ~20% below T_m. This is relevant to understand the crystallization kinetics of PCMs as crystal growth is controlled by diffusivity. Furthermore, the instantaneous shear modulus is determined ranging from 2 to 3 GPa for liquid PCMs, which permits extracting viscosity from microscopic structural relaxations usually accessible to simulations and scattering techniques.