• Le 24 juin 2019
    De 10:00 à 11:00
    Campus Lombarderie
    Amphithéâtre de l'IMN

L'Institut des Matériaux Jean Rouxel (IMN | CNRS, Université de Nantes) accueille le Professeur C. Austen Angell (School of Molecular Sciences, Arizona State University, Tempe AZ, 85287, USA) pour un séminaire "Metal-to-semiconductor transitions in liquid chalcogenides and relevance to phase change materials for ultrafast non-volatile digital memory technology" le lundi 24 juin à 10h00.

Metal-to-semiconductor transitions in liquid chalcogenides and relevance to phase change materials for ultrafast non-volatile digital memory technology.

C. Austen Angell, School of Molecular Sciences, Arizona State University, Tempe AZ, 85287

Sometimes strange analogies are found in the behavior of physical systems. One of the strangest is that of supercooled water, (whose divergent behavior below the melting point indicates an approach to a phase transition) and the so-called “phase change materials”(high temperature liquids that become low temperature glasses or metallic crystals), that are currently under intense study for digital memory device applications. The latter are typically formed from mixtures of the heavy elements tellurium, antimony and germanium, so the notion of any analogy with water might seem preposterous. However, the surprising but now well-known observation that water and the liquid element tellurium both exhibit density maxima, and also anomalous heat capacities, that largely superpose when the temperatures are scaled by the respective melting points, makes the phase change material analogy seem more plausible and potentially more important.
In this talk we will follow the easily visualized density maximum phenomenon into the consideration of more complex mixtures of Te, Sb and Ge. We emphasize that the density maximum is a precursor to a metal-to-semiconductor (M-SC) transition that occurs in association with maxima in the heat capacity and the (negative) expansivity which are found a little below the temperature of maximum density (TMD). In some cases, e.g. the binary compound As2Se3, this may occur near the upper limit of the liquid state of the material, and can be seen associated with a closing of the band gap. In others it occurs close to the melting point.
We find a hyperbolic correlation of the T(M-SC) scaled by melting point, with a new parameter, the “metallicity”, MP, defined as the inverse of the atomic fraction-averaged Pauling electronegativity for the liquid alloy. As the metallicity increases, the characteristic temperatures of the (M-SC) transitions, sharpen and decrease in temperature relative to their melting points. At MP = 0.46, they submerge below the melting points as shown in the RH panel below, and become inaccessible to further direct study, except by ultrafast probe techniques.
Interestingly enough, the compositions of all the known “phase change materials” (under active development [Intel chip of 16 GB available from Amazon]) fall within the shaded zone and indicate a characteristic temperature of about 0.85 of the melting point. The fast crystallization of these compositions aided by the M-SC/( fragile-to-strong liquid) transition, is regarded as a key to the success of the PCM technology. We note, without further comment, that the temperature at which the water divergences occur is, by a number of estimates, 228K, or 0.84 of the ice Ih melting point.