A neutron scattering technique was developed to measure the density of heavy water confined in a nanoporous silica matrix in a temperature-pressure range, from 300 to 130 K and from 1 to 2,900 bars, where bulk water will crystalize. We observed a prominent hysteresis phenomenon in the measured density profiles between warming and cooling scans above 1,000 bars. We interpret this hysteresis phenomenon as support (although not a proof) of the hypothetical existence of a first-order liquid-liquid phase transition of water that would exist in the macroscopic system if crystallization could be avoided in the relevant phase region. Moreover, the density data we obtained for the confined heavy water under these conditions are valuable to large communities in biology and earth and planetary sciences interested in phenomena in which nanometer-sized water layers are involved.confined water | equation of state | liquid-liquid critical phenomenon I n many biological and geological systems, water resides in pores of nanoscopic dimensions, or close to hydrophilic or hydrophobic surfaces, comprising a layer of water, one or two molecules thick, with properties often different from the bulk. Such "confined" or "interfacial" water has attracted considerable attention, due to its fundamental importance in many processes, such as protein folding, concrete curing, corrosion, molecular and ionic transport, etc. (1-3). However, our understanding of the numerous physicochemical anomalies of confined water, and indeed of bulk water, is still incomplete. Basic gaps persist, among which the most interesting one is the origin of the unusual behavior of water in the supercooled region where water remains in the liquid state below the melting point (4-7). Recent studies have aimed at explaining anomalies such as the density maximum and minimum (8-10), and the apparent divergence of the thermodynamic response functions at 228 K at ambient pressure (11). The three major hypothesized scenarios currently under scrutiny are the "singularity-free (SF) scenario" (12, 13), the "liquidliquid critical point (LLCP) scenario" (14, 15), and the "critical point-free (CPF) scenario" (16). It is hypothesized, by all these three scenarios, that in the low temperature range bulk water is composed of a mixture of two structurally distinct liquids: the low-density liquid (LDL) and the high-density liquid (HDL). They are respectively the thermodynamic continuation of the low-density amorphous ice (LDA) and high-density amorphous ice (HDA) into the liquid state. Evidence of a first-order phase transition between LDA and HDA has been reported since 1985 (17-20). Subsequently, several experimental findings have been interpreted as support of the hypothetical existence of two different structural motifs of liquid water (21-27). However, some of the interpretations have been questioned (28,29). So far, direct evidence of a first-order liquid-liquid phase transition between LDL and HDL, as a thermodynamic extension of the first-order transition established in the am...

We present an overview of recent experimental investigations into the properties of strongly-confined water below the bulk freezing temperature. Under strong confinement, the crystallization of water is completely suppressed and the behavior of the confined liquid state can be measured at temperatures and pressures that are inaccessible to the bulk liquid. We focus on two phenomena that have recently been discovered in strongly confined water: the density minimum and the fragile-to-strong dynamic crossover. All experimental results seem to indicate that confined water undergoes a unique kind of transition below the bulk homogeneous nucleation limit. Much of the recent work on deeply-cooled water under strong confinement has been motivated by the liquid-liquid critical point (LLCP) hypothesis. We discuss this hypothesis in the context of the various experimental findings.

The phase behavior of multi-component metallic liquids is exceedingly complex because of the convoluted many-body and many-elemental interactions. Herein, we present systematic studies of the dynamical aspects of a model ternary metallic liquid Cu40Zr51Al9 using molecular dynamics simulations with embedded atom method. We observed a dynamical crossover from Arrhenius to super-Arrhenius behavior in the transport properties (self diffusion coefficient, self relaxation time, and shear viscosity) bordered at Tx ∼ 1300 K. Unlike in many molecular and macromolecular liquids, this crossover phenomenon occurs well above the melting point of the system (Tm ∼ 900 K) in the equilibrium liquid state; and the crossover temperature Tx is roughly twice of the glass-transition temperature of the system (Tg). Below Tx, we found the elemental dynamics decoupled and the Stokes-Einstein relation broke down, indicating the onset of heterogeneous spatially correlated dynamics in the system mediated by dynamic communications among local configurational excitations. To directly characterize and visualize the correlated dynamics, we employed a non-parametric, unsupervised machine learning technique and identified dynamical clusters of atoms with similar atomic mobility. The revealed average dynamical cluster size shows an accelerated increase below Tx and mimics the trend observed in other ensemble averaged quantities that are commonly used to quantify the spatially heterogeneous dynamics such as the non-Gaussian parameter α2 and the four-point correlation function χ4.

Despite surging interest in molten salt reactors and thermal storage systems, knowledge of the physicochemical properties of molten salts are still inadequate due to demanding experiments that require high temperature, impurity control, and corrosion mitigation. Therefore, the ability to predict these properties for molten salts from first-principles computations is urgently needed. Herein, we developed and compared a machine-learned neural network force field (NNFF) and a reparametrized rigid ion model (RIM) for a prototypical molten salt LiF–NaF–KF (FLiNaK). We found that NNFF was able to reproduce both the structural and transport properties of the molten salt with first-principles accuracy and classical-MD computational efficiency. Furthermore, the correlation between the local atomic structures and the dynamics was identified by comparing with RIMs, suggesting the significance of polarization of anions implicitly embedded in the NNFF. This work demonstrated a computational framework that can facilitate the screening of molten salts with different chemical compositions, impurities, and additives, and at different thermodynamic conditions suitable for the next-generation nuclear reactors and thermal energy storage facilities.