A comparative thermodynamic and structural investigation of Ge

Thermodynamic properties (density and sound velocity) of liquid GeSb2Te4 and Ge2Sb2Te5 alloys have been measured and compared with previously published data on Ge0.075Te0.925 and Ge0.15Te0.85 alloys that display a negative thermal expansion (NTE) in the liquid state. This comparison is extended to the analysis of their structural and dynamical properties investigated by neutron scattering measurements performed on the D4 and IN6 spectrometers at the ILL (Grenoble). These experimental results, complemented by First Principles Molecular Dynamics simulations of the liquids, lead us to propose a model for the density anomaly observed in some of these tellurium based systems, when they are not too dense: it corresponds to a structural change between a low temperature liquid, characterized by a low density and an octahedral local order distorted by a Peierls-like distortion mechanism, and a high temperature liquid in which the vibrational entropy gain favors a more symmetric (less distorted) octahedral local order