Thermodynamic description of glasses: The case of amorphous ice

Glasses are out-of-equilibrium amorphous solids that can be formed by different thermodynamic paths, including isobaric cooling. The thermodynamic description of these materials is challenging since their properties depend on the corresponding preparation process. Here, we use the potential energy landscape (PEL) formalism and the harmonic approximation of the PEL to obtain a simple equation of state (EOS) for non-annealing (classical) glasses. The PEL-EOS applies to the case of crystalline solids as well. We argue that non-annealing glasses can be treated as equilibrium systems, and hence, the PEL-EOS depends only on the glass form itself and does not require knowledge of its preparation process. The predictions from the PEL formalism are validated using classical molecular dynamics (MD) simulations of low-density and high-density amorphous ice (LDA and HDA) and hexagonal ice. Importantly, the PEL-EOS for amorphous ice holds up to temperatures relevant to experiments, approximately T ≤ 100 K and T ≤ 50 K for LDA and recovered HDA, respectively (depending on the density). The PEL-EOS for amorphous ice is applied to model the pressure-induced LDA-HDA transformations at low temperatures. Our results strongly suggest that the LDA-to-HDA transformation is a true first-order phase transition that can be described by equilibrium thermodynamics and that this transformation occurs when the system reaches the (LDA-to-HDA) spinodal limit. Our MD simulations, while not conclusive, support a similar picture for the HDA-to-LDA transformation.