Solid-solid phase equilibria in the NaCl-KCl system

Area of Science:

• Materials Science
• Computational Chemistry
• Thermodynamics

Background:

• Solid solutions are mixtures with ordered structures but disordered compositions, found in salts, metals, and organic compounds.
• The sodium chloride-potassium chloride (NaCl-KCl) system forms a solid solution above 505 °C but exhibits a miscibility gap below this critical temperature.
• Understanding solid-solid equilibria is crucial for materials design and prediction.

Purpose of the Study:

• To calculate the phase diagram of the NaCl-KCl system in the region exhibiting a miscibility gap.
• To apply and validate the semi-grand canonical Widom method for computing solid-solid equilibria.
• To determine the critical point of the NaCl-KCl system.

Main Methods:

• Utilizing the semi-grand canonical Widom method, which involves averaging over virtual particle transmutations.
• Calculating free energies through thermodynamic integration for verification.
• Extrapolating results to determine the critical point location.

Main Results:

• The calculated phase diagram closely matches experimental data for the NaCl-KCl system.
• The study successfully reproduces the miscibility gap and coexisting Na-rich and K-rich phases.
• The critical temperature of 505 °C was accurately reproduced.

Conclusions:

• The semi-grand canonical Widom method is effective for computing phase diagrams of solid solutions.
• This approach allows for the calculation of solid-solid equilibria and chemical potentials, even under metastable conditions.
• The findings provide a computational framework for studying materials that form solid solutions.