Emergent chirality and enantiomeric selectivity in layered NbOX$_2$ crystals

TL;DR

This study explores how layered NbOX2 crystals can spontaneously develop chirality, with external factors like pressure and electric fields enabling control over enantiomeric states, advancing materials design.

Contribution

It reveals the structural pathways and energy landscapes that facilitate chirality emergence and control in layered NbOX2 compounds through first-principles calculations.

Findings

Identification of an intermediate achiral C2/m phase bridging high- and low-symmetry phases.

External electric fields can bias the system toward a specific enantiomer by lifting degeneracy.

Pressure and thermal effects can stabilize chiral phases by influencing energy minima.

Abstract

The spontaneous emergence of chirality in crystalline solids has profound implications for electronic, optical, and topological properties, making the control of chiral phases a central challenge in materials design. Here, we investigate the structural and electronic properties of a new family of layered compounds, $\mathrm{NbOX_2}$, and explore the connection between their achiral $I m m m$ phase and chiral $C 2$. Through first-principles calculations, we identify an intermediate achiral $C 2/m$ phase that bridges the high- and low-symmetry phases within a three-dimensional order parameter space. By analyzing the Born-Oppenheimer energy surfaces, we find that the shallow energy minima of the $C2/m$ phase suggest it may be stabilized either by external factors such as pressure, as demonstrated here, or by ionic quantum or thermal fluctuations and the resulting lattice anharmonicity. Additionally, we show how an external electric field, by breaking the necessary symmetries, biases the system toward a preferred chirality by lifting the energy degeneracy between the two enantiomers. This, combined with the small energy barrier between the enantiomers in the $C 2$ phase, enables handedness control and allows us to propose a mechanism for selective handedness stabilization by leveraging electric fields and pressure or temperature-dependent anharmonic effects. Our findings establish a framework for understanding chirality emergence in layered materials and offer a pathway for designing systems with tunable enantiomeric populations.