Alchemical screening of ionic crystals

Area of Science:

• Computational Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Accurate prediction of structural and energetic properties of ionic crystals is crucial for materials design.
• Traditional methods like density functional theory (DFT) can be computationally expensive for extensive material screening.
• Alchemical methods offer a potential route to accelerate these predictions.

Purpose of the Study:

• To introduce and validate alchemical perturbations as a rapid and accurate computational tool for ionic crystals.
• To estimate formation energies, lattice constants, and bulk moduli for alkali halides.
• To assess the accuracy of alchemical predictions against DFT benchmarks.

Main Methods:

• Employed alchemical perturbations and Hellmann-Feynman derivatives to calculate material properties.
• Investigated sixteen iso-valence-electron alkali halides (Me ∈ {Na, K, Rb, Cs}, X ∈ {F, Cl, Br, I}).
• Performed lattice scans on sixteen reference crystals for various symmetries (rock salt, zinc-blende, cesium chloride).

Main Results:

• Alchemical predictions achieved mean absolute errors (MAEs) comparable to DFT for energies and bulk moduli.
• Lattice constant predictions showed qualitative accuracy, with NaF as the best reference salt (MAE < 0.5 Å).
• Identified optimal reference salts for relative energies (NaCl), lattice constants (NaF), and bulk moduli (CsCl).

Conclusions:

• Alchemical perturbations provide a computationally efficient and accurate method for predicting key properties of ionic crystals.
• The method successfully differentiates competing crystal phases (rock salt, cesium chloride) in binary and ternary mixtures.
• Alchemical predictions accurately reproduced stability trend reversals in mixed alkali halide systems.