Theoretical description of phase coexistence in model C60

Area of Science:

• Computational physics and chemistry
• Materials science
• Thermodynamics

Background:

• Accurate modeling of C60 fullerene phase behavior is crucial for understanding its properties and applications.
• Previous theoretical models required validation against experimental or simulation data.

Purpose of the Study:

• To investigate the phase diagram of a C60 fullerene pair interaction model using integral equation theories and perturbation theory.
• To assess the accuracy of modified hypernetted chain (MHNC), self-consistent Ornstein-Zernike approximation (SCOZA), and perturbation theory (PT) against Monte Carlo simulations.

Main Methods:

• Employed MHNC with global thermodynamic consistency and SCOZA for liquid state theories.
• Utilized PT with varying refinement for solid phase free energy calculations.
• Compared theoretical predictions for free energy, pressure, and internal energy with Monte Carlo simulation results.

Main Results:

• All theories reproduced the overall phase portrait well, with high accuracy for melting and solid-vapor equilibrium lines.
• Discrepancies were observed in fluid-solid coexistence pressure above the triple point for all theories.
• MHNC and SCOZA showed high accuracy for liquid-vapor coexistence and critical points; SCOZA underestimated freezing line density.

Conclusions:

• MHNC for the fluid phase and first-order PT for the solid phase were identified as the most accurate theoretical tools.
• Combining different theoretical approaches for fluid and solid phases provides semiquantitative reproduction of C60 fullerene thermodynamic properties.
• The findings establish a robust benchmark for future theoretical studies on C(n>60) fullerenes and related materials.