Continuous thermodynamic-equilibrium glass transition in high-energy grain boundaries?

Recent simulations of silicon grain boundaries equilibrated at high temperatures and subsequently cooled to zero temperature have revealed a 'confined amorphous' equilibrium structure of uniform thickness for the high-energy boundaries while low-energy boundaries are crystalline. Here we demonstrate that, above the glass transition temperature T-g, these high-energy boundaries undergo a reversible structural and dynamical transition from a confined amorphous solid to a confined liquid. By contrast with the bulk glass transition, however, this equilibrium transition is continuous and thermally activated, starting at T-g and being complete at the melting point T-m, at which the entire film is liquid. The coexistence of the confined amorphous and liquid phases in this two-phase region of less than 1 nm thickness is shown to have a profound impact on grain-boundary self-diffusion.