Anomalous thermal expansion and chiral phonons in BiB${}_3$O${}_6$

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.