A comparative thermodynamic and structural investigation of Ge_xTe_(1−x) and Ge_xSb_yTe_(1−x−y) liquid alloys

¹ Physique de la Matière Condensée, B5, Université de Liège, 4000 Sart-Tilman, Belgium
² Department of Physics, Faculty of Science, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan
³ Centre Interdisciplinaire de Nanoscience de Marseille, CINaM CNRS and Aix-Marseille Universities, Campus de Luminy, Case 913, 13288 Marseille, France

Abstract

Thermodynamic properties (density and sound velocity) of liquid GeSb₂Te₄ and Ge₂Sb₂Te₅ alloys have been measured and compared with previously published data on Ge_0.075Te_0.925 and Ge_0.15Te_0.85 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.