Structural chirality and natural optical activity across the α to β phase transition in SiO2 and AlPO4 from first principles

Natural optical activity (NOA), the ability of a material to rotate the plane of polarized light, has traditionally been associated with structural chirality. However, this relationship has often been oversimplified, leading to conceptual misunderstandings, particularly when attempts are made to correlate structural handedness directly with optical rotatory power. In reality, the relationship between chirality and NOA is more nuanced: optical activity can arise in both chiral and achiral crystal structures, and the sign of the rotation cannot necessarily be inferred from the handedness of the space group. In this work, we conduct a first-principles investigation of NOA in SiO2 and AlPO4 crystals, focusing on their enantiomorphic structural phase transition from high-symmetry hexagonal (P6422 or P6222) to low-symmetry trigonal (P3121 or P3221) space groups. This transition, driven by the condensation of a zone-centre Γ3 phonon mode, reverses the screw-axis type given by the space-group symbol while leaving the sign of the optical activity unchanged. By following the evolution of the structure and the optical response along the transition pathway, we clarify the microscopic origin of this behaviour. We demonstrate that the sense of optical rotation is determined not by the nominal handedness of the screw axis given in the space-group symbol but by the atomic-scale handedness of the most polarizable atoms of the structure.