High-Pressure Synthesis of Novel Boron Oxynitride B6N4O3 with Sphalerite Type Structure

A novel crystalline boron oxynitride (BON) phase has been synthesized under static pressures exceeding 15 GPa and temperatures above 1900 °C, from molar mixtures of B2O3 and h-BN. The structure and composition of the synthesized product were studied using high-resolution transmission electron microscopy, electron diffraction, automated diffraction tomography, energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy (EELS). BON shows a hexagonal cell (R3m, Z = 3) with lattice parameters a = 2.55(5) Å and c = 6.37(13) Å, and a crystal structure closely related to the cubic sphalerite type. The EELS quantification yielded 42 at % B, 35 at % N, and 23 at % O (B:N:O ≈ 6:4:3). Electronic structure calculations in the framework of Density Functional Theory have been performed to assess the stabilities and properties of selected models with the composition B6N4O3. These models contain ordered structural vacancies and are superstructures of the sphalerite structure. The calculated bulk moduli of the structure models with the lowest formation enthalpies are around 300 GPa, higher than for any other known oxynitride