Binary cluster crystals formed by ultrasoft particles: Classical density functional theory

Area of Science:

• Soft Matter Physics
• Computational Materials Science
• Statistical Mechanics

Background:

• Binary mixtures of ultrasoft particles exhibit complex phase behavior.
• Understanding cluster crystal formation is crucial for designing novel materials.
• Previous studies focused on single-component systems or simpler interactions.

Purpose of the Study:

• To investigate the existence and properties of cluster crystals in a binary mixture of ultrasoft, cluster-forming particles.
• To explore the role of inter-particle interaction anisotropy (parameterized by ζ) on crystal structure.
• To map the phase diagram and identify stable ordered phases.

Main Methods:

• Classical density functional theory (DFT) with a mean-field approximation.
• Unbiased optimization of density profiles and lattice structures.
• Investigation of a size-symmetric, equimolar binary mixture with a generalized exponential interaction potential.

Main Results:

• Identification of two stable cluster crystal phases: Body-Centered Cubic (BCC) and tetragonal.
• The tetragonal lattice recovers the Face-Centered Cubic (FCC) structure as the interaction anisotropy parameter ζ approaches 1.
• The BCC phase is stable within a specific region of the temperature-density phase diagram, limited by a threshold value of ζ (ζth).

Conclusions:

• Binary mixtures of ultrasoft particles can form stable cluster crystals with distinct lattice structures (BCC, tetragonal, FCC).
• The interaction anisotropy parameter ζ significantly influences the lattice type and degree of elongation in the tetragonal phase.
• DFT provides a robust framework for predicting cluster crystal phases in complex particle systems, validated by ongoing simulations.