Fluorine-Assisted Stabilization of Cubic Phase Boron Nitride Polycrystalline Solids

The synthesis of cubic boron nitride (cBN) solids by consolidation of cBN powders at high pressure and high temperature (HPHT) is challenging, as it leads to the hexagonal BN (hBN) phase. Here we report an HPHT synthesis of polycrystalline fluorinated cBN (F-cBN) solids that show twinned nanodomains, the presence of F at the grain boundaries, and excellent mechanical hardness and second-order nonlinear susceptibility. In contrast to pure diamagnetic cBN, we found that F-cBN is paramagnetic at room temperature, as confirmed by electron paramagnetic resonance spectroscopy, suggesting the role of defects in tuning the magnetic properties of cBN. Density functional theory calculations reveal that F stabilizes the cBN phase by passivating the dangling bonds and, in addition, forms substitutional defects (F-N) leading to F-N-V-B defect complexes that generate local magnetic moments. These properties of F-cBN might lead to new applications of cBN and also open a pathway to synthesize new metastable phase materials with emergent properties.