Rate of Homogeneous Crystal Nucleation in molten NaCl

TL;DR

This study uses numerical simulations to estimate the rate of NaCl crystal nucleation from its melt at moderate supercooling, revealing faceted nuclei and a significant discrepancy with experimental data.

Contribution

It introduces two independent computational approaches to estimate NaCl nucleation rates and compares their results, highlighting a large gap with experimental observations.

Findings

Nucleation occurs via faceted, cubical critical nuclei.

Two different simulation methods produce consistent nucleation rate estimates.

Simulated nucleation rates differ by nearly five orders of magnitude from experimental data.

Abstract

We report a numerical simulation of the rate of crystal nucleation of sodium chloride from its melt at moderate supercooling. In this regime nucleation is too slow to be studied with "brute-force" Molecular Dynamics simulations. The melting temperature of ("Tosi-Fumi") NaCl is $\sim 1060$K. We studied crystal nucleation at $T$=800K and 825K. We observe that the critical nucleus formed during the nucleation process has the crystal structure of bulk NaCl. Interestingly, the critical nucleus is clearly faceted: the nuclei have a cubical shape. We have computed the crystal-nucleation rate using two completely different approaches, one based on an estimate of the rate of diffusive crossing of the nucleation barrier, the other based on the Forward Flux Sampling and Transition Interface Sampling (FFS-TIS) methods. We find that the two methods yield the same result to within an order of magnitude. However, when we compare the extrapolated simulation data with the only available experimental results for NaCl nucleation, we observe a discrepancy of nearly 5 orders of magnitude. We discuss the possible causes for this discrepancy.