Physics and Nanofriction of Alkali Halide Solid Surfaces at the Melting Point

TL;DR

This study uses simulations to explore why alkali halide (NaCl) surfaces resist melting at the triple point, revealing factors that stabilize the solid surface and affect wetting behavior.

Contribution

It identifies the combined effects of surface anharmonicities, density jumps, and molecular correlations as reasons for nonmelting behavior of NaCl(100).

Findings

NaCl(100) surface is nonmelting at the triple point.

Surface anharmonicities stabilize the solid surface.

Destabilization of liquid surface reduces entropy.

Abstract

Alkali halide (100) surfaces are anomalously poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within a simple (BMHFT) model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a nonmelting surface, and that the incomplete wetting can be traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface, reducing in particular its entropy much below that of solid NaCl(100). Presently, we are making use of the nonmelting properties of this surface to conduct case study simulations of hard tips sliding on a hot stable crystal surface. Preliminary results reveal novel phenomena whose applicability is likely of greater generality.