TOWARDS UNDERSTANDING THE LOCAL-STRUCTURE OF LIQUIDS

In this article we discuss the problem of well-defined crystalline pat terns of local atomic arrangements in equilibrium liquids, and their s tatistical mechanics modelling. We present arguments in favor of the e xistence of local crystalline structures in liquids (local crystal ord er hypothesis) and discuss a generalized energy landscape picture in t he theory of the liquid state. This picture allows a quantification of the hypothesis of local order and offers basic concepts for the stati stical mechanics modelling of the melting phase transition. We review recent results of probabilistic-based searches for local structures in various two- and three-dimensional computer-simulated liquids. Next, some statistical-mechanics models of melting and amorphization in term s of structural states of small clusters are proposed. The models, whi ch have only two characteristic energies, that of the orientationally disordered locally crystalline stale, and that of completely amorphous state, are studied in a mean-probability approximation. If the amorph ization energy is high, the material retains local crystallinity even in the melt; at higher temperatures a crossover to the locally amorpho us state occurs. A material that has a low energy non-crystalline loca l packing exhibits an amorphization melting; the phase transition is f rom orientationally ordered crystal state to a locally amorphous melt.