The race between relaxation and nucleation in supercooled liquid and glassy BaS – A molecular dynamics study

Details of the microscopic mechanisms and dynamics of most processes in supercooled liquids (SCL) and glasses remain blurred due to their complexity and the extraordinarily long or inaccessibly short timescales, depending on the supercooling degree. In this paper, we determined the structural relaxation times in an intermediate supercooling window (0.67–0.78 Tm, Tm = melting point) of barium sulfide liquid, used as a model material, through molecular dynamics (MD) simulations based on a reliable two-body interatomic potential. We also inferred these dynamics in the glassy state using extrapolated data and well-accepted models. The average structural α-relaxation times, , were determined via the intermediate scattering function. Relaxation kinetics were also estimated by the Maxwell relation via the equilibrium shear viscosity and shear modulus, obtained by MD. We found that the viscosity derived (Maxwell) relaxation times are significantly shorter than the intrinsic α -relaxation times, corroborating two recent experimental studies of other substances. For the simulated system, with 36,000 particles and average volume of 1.4x103 nm3, the structural relaxation and nucleus birth time curves cross at the kinetic spinodal temperature, TKS (=0.33 Tm), which is significantly below the glass transition temperature, Tg (=0.44 Tm). However, for larger sample sizes, the TKS occurs at higher temperatures. As for temperatures above TKS, crystal nucleation starts after structural relaxation, and vice-versa, to understand and describe crystal nucleation one must necessarily take the relaxation process into account, however, this is not included in nucleation theories. These discoveries shed light on some obscure aspects of supercooled liquids, i.e., they challenge a critical assumption of nucleation theories (that crystal nucleation always occurs in a fully relaxed SCL), and show that crystallization is BaS ultimate fate, corroborating studies with other substances.