# Anomalous thermal expansion and chiral phonons in BiB$_{3}$O$_{6}$

## TL;DR

This study investigates the causes of unusual thermal expansion in chiral BiB3O6, revealing that librations, anisotropy, and chiral phonons involving bismuth atoms drive the phenomenon, linking symmetry to physical behavior.

## Contribution

It uncovers the role of chiral acoustic phonons and lattice symmetry in anomalous thermal expansion of BiB3O6 using ab initio calculations.

## Key findings

- Chiral phonons involve elliptical bismuth atom revolutions.
- Positive and negative thermal expansion are linked to librations and anisotropy.
- Chirality enables transverse acoustic modes with opposite circular polarizations.

## Abstract

The origins of anomalous thermal expansion in the chiral monoclinic solid $\alpha$-BiB$_{3}$O$_{6}$ have been studied through ab initio calculations. Positive and negative axial thermal expansion are shown to be driven by librations of borate units, elastic anisotropy, and most notably by chiral acoustic phonons involving elliptical revolutions of bismuth atoms. The chirality of the lattice gives rise to these modes by allowing the transverse acoustic branches to have opposite circular polarizations, only one of which couples strongly to the lattice strains. These results further understanding of relationships between crystallographic symmetry and physical properties.