• Zykova-Timan, Tania
• Sekkal, Wassila
• Tosatti, Erio
• Jagla, Edoardo
• Tartaglino, Ugo
• Ceresoli, Davide

Abstract

We investigated theoretically the high temperature thermodynamic properties and the (lack of) self-wetting of ionic crystal surfaces, with NaCl(100) as a prototype case. We found by classical molecular dynamics simulations that NaCl(100) is a non-melting crystal surface and can be overheated to a spinodal temperature T<SUB>S</SUB> 150 K above the bulk melting temperature (T_M). While surface non-melting is in itself well known, and is observed on semiconductors as well as on close packed metal surfaces, the basic mechanism for NaCl appears to be different from either of them. Moreover, there appear to be two distinct metastability regimes of solid NaCl(100) above T_M. The first regime T<SUB>M</SUB> &lt; T &lt; T_1, (T<SUB>1</SUB> - T<SUB>M</SUB> = 60 K) the metastable solid surface is strongly protected by a nucleation barrier of large thickness. This thickness decreases with temperature, until it drops to a lattice spacing a at T_1. In the second regime T<SUB>1</SUB> &lt; T &lt; T<SUB>S</SUB> is characterized by a ``fragile'' metastability, with a nucleation barrier one lattice spacing thick. Here, the smallest seed is able to cause melting, i.e., it is enough to melt the first monolayer for the liquid to break through and melt the whole system. A basic explanation of these facts and of the two regimes is given, based on a short-range oscillatory interaction between the solid-liquid and the liquid-vapor interfaces. In addition, we also simulated a liquid NaCl nanodroplet, deposited on a solid NaCl surface in the vicinity of the bulk melting point. The anomalously large contact angle measured long ago by Mutaftschiev [1] is well reproduced by this realistic molecular dynamics droplet simulation. Based on these results, and on independent determinations of the liquid-vapor (γ_LV) and the solid-vapor interface free energy (γ_SV) an estimate of the solid-liquid interface free energy (γ_SL) is extracted. The solid-vapor surface free energy turns out to be anomalously small and similar to the liquid-vapor one, providing a direct thermodynamic explanation of the reduced wetting ability of the ionic melt. <P />G. Grange and B. Mutaftschiev, Surf. Sci. 47, 723 (1975)....

Topics

• impedance spectroscopy
• surface
• simulation
• melt
• semiconductor
• molecular dynamics
• melting temperature