Research master thesis: First-principles search for phonon-driven achiral-to-chiral transitions

Abstract

Chirality in crystalline solids can emerge not only from static atomic arrangements, but also through lattice instabilities that drive an achiral parent phase into a chiral daughter structure. In this thesis, a first-principles workflow was developed to search for phonon-driven achiral-to-chiral transitions in inorganic materials. The approach combines database screening, crystallographic symmetry analysis, density functional theory calculations, density-functional perturbation theory phonon calculations, irreducible-representation analysis, and mode-condensation searches. Candidate materials were first selected from phonon databases by identifying achiral parent space groups compatible with enantiomorphic chiral subgroups and by checking for finite-wave-vector phonon instabilities at relevant high-symmetry q-points. Four materials were then investigated in detail: TeBr₂, SnBr₂, PtO, and ZrSi. Electronic band-structure calculations showed that TeBr₂ and SnBr₂ are semiconducting, while PtO and ZrSi are metallic. Phonon and symmetry analyses identified relevant finite-q instabilities capable of generating enantiomorphic chiral phases in TeBr₂, SnBr₂, and PtO. Subsequent mode-condensation searches showed that these systems can relax into chiral structures lower in energy than their achiral parent phases. However, lower-energy competing distortions were also found, indicating that the predicted chiral structures should be interpreted as metastable candidates rather than confirmed ground states. ZrSi remains dynamically unstable and symmetry-relevant, but the targeted enantiomorphic pair was not recovered in the present mode-condensation results. Overall, this work demonstrates a practical computational strategy for identifying materials in which chirality can emerge dynamically from soft phonon modes.