Transport Anomalies and Adaptative Pressure-Dependent Topological Constraints in Tetrahedral Liquids: Evidence for a Reversibility Window Analogue

Topological rigid constraints can be computed rather simply with changing composition and temperature but their estimation remains challenging for other thermodynamic variables. Here, the investigation of densified silicate liquids from molecular dynamics simulations combined with an analysis of radial and angular atomic excursions allows defining a pressure dependence of such constraints. Results show, that for a given composition, the dependence is nonmonotonic as it depends on the interplay between constraints broken by thermal activation and additional constraints arising from the increase of network connectivity under pressure. An anomalous behavior for oxygen bending constraints is obtained in the ($P$, $T$) map which connects to reported anomalies in transport properties and is identified as the pressure analogue of the stress-free Boolchand intermediate phase in rigidity driven by composition.

Figure 1

Bridging oxygen (BO) standard deviation distribution in a simulated NS2 liquid at 2000 K for various pressures. Broken lines serve to track the peak positions ${\sigma }_i$ and ${\sigma }_b$ of the bimodal distribution. The behavior of the peak position ${\sigma }_i$ with pressure is represented in the inset for different isotherms.

Figure 2

Fraction $q(T)$ of intact angular (BB) constraints for different isobars in simulated NS2 liquids (symbols). The lines represent least-squares fit using Boltzmann prefactors (see text, for details).

Figure 3

Contour plot of the fraction (in %) of intact constraints $q(T,P)$ of liquid NS2 in the ($T$, $P$) map.

Figure 4

(a) Behavior of the step function $q(P)$ in liquid NS2 for two isotherms ($T=1500\mathrm{K}$ and 2000 K, black curves) compared to the computed activation energy (right axis) for viscous flow ${\mathrm{E}}_{\mathrm{A}}^{\eta }$, (filled red circles) and for oxygen diffusion ${\mathrm{E}}_{\mathrm{A}}^D$ (open red squares). (b) Computed viscosity $\eta (P)$ as a function of pressure for $T=2000\mathrm{K}$. See also Ref. 18. Right axis: Oxygen diffusion constant (blue symbols) $D_O(P)$. The grey area represents an approximate delimitation of the intermediate phase based on the window defined by the trend in $q(P,2000\mathrm{K})$.