Decoupling of self-diffusion and structural relaxation during a fragile-to-strong crossover in a kinetically constrained lattice gas.

The viscosity of glass formers increases by several orders of magnitude upon cooling near the glass transition temperature. For strong glass formers, this increase is Arrhenius-like, that is, the viscosity increases exponentially with inverse temperature, whereas, for fragile glass formers, this increase is super-Arrhenius-like. Recent experimental and theoretical evidence indicates that fragile glasses can undergo a crossover to strong behavior at low enough temperatures. Here, we show that a fragile-to-strong crossover can be modeled by a union of two kinetically constrained lattice gas models, and that for this model, the variation of the product of the self-diffusion constant and the structural relaxation time is non-monotonic with temperature as a result of the crossover. The viscosity of salol (phenyl salicylate) seems to exhibit a fragile-to-strong crossover with lowering temperature, and a non-monotonic decoupling behavior similar to what we find in our model has been observed for the shear viscosity and dielectric relaxation in salol. Our interpolated model is based upon the kinetically constrained triangular lattice gas (TLG) models introduced by J!ckle and Krçnig. These two-dimensional models are var-