Cycling through the glass transition: Evidence for reversibility windows and dynamic anomalies

Abstract

Molecular dynamics simulations of densified glass-forming liquids, $2{\mathrm{SiO}}_{2}\ensuremath{-}{\mathrm{Na}}_{2}\mathrm{O}$, are presented. We perform a cooling/heating cycle across the glass transition, and important energy variations are obtained when the material relaxes at low temperature, leading to a hysteresis loop. However, for selected system densities, minuscule energy changes are found, revealing glasses which can be viewed as ``thermally reversing,'' in close correspondence with experiments performed in the context of isostatically rigid glasses. The topological constraint count of the atomic network structure shows that such ``reversible'' liquids adapt under the density-driven coordination increase, by experiencing larger bond-angle excursions at the atomic scale, quantified from the evolution with density of the Mauro-Gupta function $q(T,\ensuremath{\rho})$, which exhibits a broad minimum around $2.75\phantom{\rule{0.16em}{0ex}}{\mathrm{g}/\mathrm{cm}}^{3}$. The stiffening of the network structure is also evidenced from an inspection of the vibrational density of states, which shows an important decrease in the low-frequency contributions across the reversibility window. Dynamic anomalies are detected from the evolution of isothermal diffusivity with density, which underscore the possible generality of ``glass reversibility'' in densified tetrahedral glass-forming liquids.