Structural Chirality from Short-Range Order in Heteroanionic Materials

Established routes to structural chirality in inorganic crystals depend on symmetry-lowering atomic displacements of an achiral parent structure. We show that chirality can instead be forced by the ordering of two anion species over the sites of an achiral parent, independent of atomic displacements. ReO$_3$-type oxyfluorides with a 2:1 anion stoichiometry, such as NbO$_2$F, exhibit two ordering patterns: cis octahedral coordination and period-three anion-chain ordering, both rooted in the off-centring of $d^0$ cations. By direct enumeration, we prove that every configuration combining both orderings on the smallest commensurate cell is chiral, belonging to a Sohncke space group. For the specific case of NbO$_2$F, density-functional theory calculations predict that the ground state is the maximally symmetric chiral configuration. Wang-Landau Monte Carlo simulations of a DFT-trained cluster-expansion model predict an equilibrium chiral phase up to a first-order transition at 494 K. These results establish configurational ordering as a chemically realisable route to structural chirality, and mark heteroanionic materials with analogous ordering chemistry as candidates for a new class of chiral functional materials.