Solid-solid phase transition in hard ellipsoids

TL;DR

This study uses computer simulations to analyze the stability of crystalline phases in hard ellipsoids, revealing a solid-solid phase transition at specific aspect ratios and densities.

Contribution

It provides the first detailed free energy calculations comparing the stability of different crystalline phases of hard ellipsoids across various aspect ratios.

Findings

SM2 phase is more stable than stretched-fcc for aspect ratios ≥ 2.0.

A solid-solid phase transition occurs between aspect ratios 1.55 and 2.0.

The stability of phases depends on density and aspect ratio.

Abstract

We present a computer simulation study of the crystalline phases of hard ellipsoids of revolution. A previous study [Phys. Rev. E, \textbf{75}, 020402 (2007)] showed that for aspect ratios $a/b\ge 3$ the previously suggested stretched-fcc phase [Mol. Phys., \textbf{55}, 1171 (1985)] is unstable with respect to a simple monoclinic phase with two ellipsoids of different orientations per unit cell (SM2). In order to study the stability of these crystalline phases at different aspect ratios and as a function of density we have calculated their free energies by thermodynamic integration. The integration path was sampled by an expanded ensemble method in which the weights were adjusted by the Wang-Landau algorithm. We show that for aspect ratios $a/b\ge 2.0$ the SM2 structure is more stable than the stretched-fcc structure for all densities above solid-nematic coexistence. Between $a/b=1.55$ and $a/b=2.0$ our calculations reveal a solid-solid phase transition.