Phase coexistence of cluster crystals: beyond the Gibbs phase rule

TL;DR

This study investigates the phase behavior of cluster crystals with multiple occupancy, emphasizing the importance of treating lattice sites as a thermodynamic variable, and provides new phase diagrams and elastic properties through simulation.

Contribution

It introduces a novel simulation approach that accounts for lattice site constraints, enabling accurate phase diagram and elastic constant calculations for cluster crystals.

Findings

Phase diagram of cluster crystals obtained via simulation.

Identification of two types of density fluctuations and sound modes.

Evaluation of elastic constants for high-density cluster crystals.

Abstract

We report a study of the phase behavior of multiple-occupancy crystals through simulation. We argue that in order to reproduce the equilibrium behavior of such crystals it is essential to treat the number of lattice sites as a constraining thermodynamic variable. The resulting free-energy calculations thus differ considerably from schemes used for single-occupancy lattices. Using our approach, we obtain the phase diagram and the bulk modulus for a generalized exponential model that forms cluster crystals at high densities. We compare the simulation results with existing theoretical predictions. We also identify two types of density fluctuations that can lead to two sound modes and evaluate the corresponding elastic constants.