First synthesized by William Henry Balmain in 1842, hexagonal boron nitride ('white graphite') stacks honeycomb B-N sheets in an eclipsed registry with a wide 5.9–6.4 eV insulating band gap.
Atomically flat h-BN monolayers serve as the premier dielectric substrate and encapsulating barrier for graphene and 2D electronic devices, while bulk h-BN is stable to 1,000 °C in air and 2,800 °C in inert gas.
Its high-pressure cubic polymorph (c-BN), first synthesized by Robert H. Wentorf Jr. at General Electric in 1957 (Borazon), rivals diamond in hardness (45 GPa Knoop) and machines ferrous steels without dissolving.