Relaxation, crystal nucleation, kinetic spinodal and Kauzmann temperature in supercooled zinc selenide

Abstract

Structural relaxation and crystallization are crucial phenomena in physics, chemistry, and materials science. A thorough study of their relationship could clarify some critical open questions. In this article, we focus on thermodynamic and kinetic properties: the Kauzmann temperature, TK (where the excess entropy tends to zero), the kinetic spinodal temperature, Tks (where the relaxation and crystal nucleation curves cross), and the glass transition temperature, Tg. We used zinc selenide (ZnSe) as a model system for which a reliable potential is available and obtained the self-diffusion coefficient, viscosity, critical nucleus birth times, relaxation times, entropy, Tg, Tks and TK by molecular dynamics (MD) simulations. We confirmed that the Stokes-Einstein equation breaks down in the moderate supercooled regime, impacting the relationships between the dynamic and thermodynamic properties. Two relaxation times were determined in the supercooled liquid (SCL) state: i) using the shear viscosity and the Maxwell equation, τη, and ii) from the self-intermediate scattering function and the Kohlrausch equation, τR. We found that in the whole supercooling regime, τη ≪ τR confirming two recent experimental studies for other substances. The nucleus birth times, τN, were also obtained for two system sizes. The τRT and τNT curves indeedcrossover,confirming the existence of a kinetic spinodal temperature for this system. Hence, for temperatures somewhat above and below the Tks, crystallization of the SCL could be affected by structural relaxation. Finally, our results demonstrate that if the Kauzmann temperature existed, it would be well below the Tks. Hence, crystal nucleation intervenes on the cooling path, and SCL ZnSe cannot reach this temperature, thus averting the paradoxical entropic situation. These findings shed light on some central problems related to supercooled liquids.