Orientational interaction and ordering of Cd_4 tetrahedra in a quasicrystal approximant

TL;DR

This paper models the orientational ordering of tetrahedra in the CaCd6 quasicrystal approximant, revealing complex ground states and first-order thermal transitions below 100 K through Monte Carlo simulations.

Contribution

It introduces a novel effective Hamiltonian approach for inter-tetrahedron orientations in quasicrystal structures, combining all-atom relaxations with Monte Carlo methods.

Findings

Identification of complex ground state ordering patterns.

Observation of two first-order thermal transitions below 100 K.

Development of a modeling framework for orientational interactions in quasicrystals.

Abstract

We model the quasicrystal-related structure CaCd$_6$, a bcc packing of icosahedral clusters containing tetrahedra which undergo orientational orderings at T<100 K. We use general schemes to evaluate an effective Hamltonian for inter-tetrahedron orientations, based on all-atom relaxations, either in terms of discrete cluster orientations, or of continuous rotation angles. The effective Hamiltonian is used in Monte Carlo simulations to find the (complex) ground state ordering pattern as a function of pressure. A preliminary investigation of thermal transitions found (in part of the pressure range) two different first-order transitions occurring below 100 K.