Ammonothermal synthesis of rhombohedral boron nitride

We exposed hexagonal boron nitride (h-BN) with Ba3N2 and MgB2 to supercritical ammonia at 750 °C and 120 MPa. Analysis by X-ray diffraction indicates that the product is a mixture of h-BN and rhombohedral boron nitride (r-BN), and triangular-plate crystals appear in the product. Hexagonal BN and r-BN are also obtained when Ba3N2 and MgB2 are exposed to supercritical ammonia. When h-BN is exposed to supercritical ammonia without Ba3N2 and MgB2, the crystal structure and morphology of h-BN does not change. The results thus reveal that r-BN forms upon ammonothermal treatment with Ba3N2 and MgB2.