The NaCl(100) Surface: Why Does it Not Melt?

TL;DR

This study uses molecular dynamics simulations to explain why the NaCl(100) surface remains stable and does not melt, even above the bulk melting temperature, due to unusual interface free energies and attractive interactions.

Contribution

It provides a detailed explanation of the surface stability of NaCl(100) through interface free energy calculations and introduces a novel understanding of the attractive interactions preventing melting.

Findings

NaCl(100) remains stable above melting point

Solid-vapor interface free energy is unusually low

Liquid-vapor interface free energy is unusually high

Abstract

The high temperature surface properties of alkali halide crystals are very unusual. Through molecular dynamics simulations based on Tosi-Fumi potentials, we predict that crystalline NaCl(100) should remain stable without any precursor signals of melting up to and even above the bulk melting point $T_m$. In a metastable state, it should even be possible to overheat NaCl (100) by at least 50 K. The reasons leading to this lack of surface self-wetting are investigated. We will briefly discuss the results of calculations of the solid-vapor and liquid-vapor interface free energies, showing that the former is unusually low and the latter unusually high, and explaining why. Due to that the mutual interaction among solid-liquid and liquid-vapor interfaces, otherwise unknown, must be strongly attractive at short distance, leading to the collapse of any liquid film attempting to nucleate at the solid surface. This scenario naturally explains the large incomplete wetting angle of a drop of melt on NaCl(100).