Flat branches and pressure amorphization

After summarizing the phenomenology of pressure amorphization (PA), we present a theory of PA based on the notion that one or more branches of the phonon spectrum soften and flatten with increasing pressure. The theory expresses the anharmonic dynamics of the flat branches in terms of local modes, represented by lattice Wannier functions, which are in turn used to construct an effective Hamiltonian. When the low-pressure structure becomes metastable with respect to the high-pressure equilibrium phase and the relevant branches are sufficiently flat, transformation into an amorphous phase is shown to be kinetically favored because of the exponentially large number of both amorphous phases and reaction pathways. In effect, the critical-size nucleus for the first-order phase transition is found to be reduced to a single unit cell, or nearly so. Random nucleation into symmetrically equivalent local configurations characteristic of the high-pressure structure is then shown to overwhelm any possible domain growth, and an ``amorphous'' structure results.Comment: 8 pages with 3 postscript figures embedded; Proceedings of the 4th International Discussion Meeting on Relaxations in Complex Systems, Hersonissos, Heraklion, Crete, June 16-23, ed. K. L. Ngai, Special Issues of the Journal of Non-Crystalline Solids, 200

Physics of the liquid-liquid critical point

Within the inherent structure (IS) thermodynamic formalism introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys. Rev. A {\bf 25}, 978 (1982)] we address the basic question of the physics of the liquid-liquid transition and of density maxima observed in some complex liquids such as water by identifying, for the first time, the statistical properties of the potential energy landscape (PEL) responsible for these anomalies. We also provide evidence of the connection between density anomalies and the liquid-liquid critical point. Within the simple (and physically transparent) model discussed, density anomalies do imply the existence of a liquid-liquid transition.Comment: Physical Review Letters, in publicatio

Mechanical versus thermodynamical melting in pressure-induced amorphization: the role of defects

We study numerically an atomistic model which is shown to exhibit a one--step crystal--to--amorphous transition upon decompression. The amorphous phase cannot be distinguished from the one obtained by quenching from the melt. For a perfectly crystalline starting sample, the transition occurs at a pressure at which a shear phonon mode destabilizes, and triggers a cascade process leading to the amorphous state. When defects are present, the nucleation barrier is greatly reduced and the transformation occurs very close to the extrapolation of the melting line to low temperatures. In this last case, the transition is not anticipated by the softening of any phonon mode. Our observations reconcile different claims in the literature about the underlying mechanism of pressure amorphization.Comment: 7 pages, 7 figure

Polymorphism, thermodynamic anomalies and network formation in an atomistic model with two internal states

Using molecular dynamics simulations we study the temperature-density phase diagram of a simple model system of particles in two dimensions. In addition to translational degrees of freedom, each particle has two internal states and interacts with a modified Lennard-Jones potential which depends on relative positions as well as the internal states. We find that, despite its simplicity, the model has a rich phase diagram showing many features of common network-forming liquids such as water and silica, including polymorphism and thermodynamic anomalies. We believe our model may be useful for studies concerning generic features of such complex liquids.Comment: 7 pages, 8 pdf figure

Entropy-driven liquid-liquid separation in supercooled water

Twenty years ago Poole et al. (Nature 360, 324, 1992) suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of liquid-liquid separation of lower-density and higher-density water. Here we present an explicit thermodynamic model based on this hypothesis, which describes all available experimental data for supercooled water with better quality and with fewer adjustable parameters than any other model suggested so far. Liquid water at low temperatures is viewed as an 'athermal solution' of two molecular structures with different entropies and densities. Alternatively to popular models for water, in which the liquid-liquid separation is driven by energy, the phase separation in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the 'reaction' equilibrium constant. In particular, the model predicts the location of density maxima at the locus of a near-constant fraction (about 0.12) of the lower-density structure.Comment: 7 pages, 6 figures. Version 2 contains an additional supplement with tables for the mean-field equatio