Chemical Design Rule for Ultraviolet-Emitting Color Centers: The Role of Host Ionicity.

Solid-state ultraviolet (UV) single-photon emitters (SPEs) are highly desirable for solar-blind free-space quantum communication, ion-compatible quantum networking, and label-free spectroscopy, yet robust and well-understood UV color centers remain scarce even in wide-gap hosts such as diamond. Here we show that the energy of zero-phonon lines (ZPLs) is not controlled by the bandgap alone, but is governed by the host bonding character (covalent versus ionic) through the bonding-antibonding splitting of lattice divacancy-derived defect states. In covalent diamond, the divacancy yields only a 0.79 eV separation within the optical two-level manifold, so that most established centers exhibit ZPLs predominantly in the near-infrared or visible. By contrast, in ionic cubic boron nitride (c-BN), the corresponding divacancy splitting can reach 5.61 eV, naturally pushing the analogous transitions into the UV band. Building on this basic understanding, we establish a unified rule, combining impurity-associated atomic potential, lattice strain, and orbital hybridization, to predict ZPL trends of vacancy-impurity-vacancy (XV) centers in c-BN. Guided by this framework, we identify a chemically rich set of UV-emitting XV centers and reveal a roughly, though not strictly, monotonic redshift tendency of the ZPL for heavier impurities, with local deviations arising from competing strain, orbital hybridization, the Jahn-Teller effect, and spin-orbit coupling. These results provide a chemically transparent design rule for engineering UV SPEs in wide-gap hosts and help rationalize the scarcity of UV centers across materials, highlighting c-BN as a particularly fertile platform for solid-state quantum photonics.