Synthesis of ΝbN faceted grains from oxides and boron nitride with sodium

Agglomerates of ε-NbN hexagonal prismatic faceted grains with a size between 5 and 50 μm were formed by heating Nb2O5 or NbFeO4 powder and Na lump in a BN crucible at 900–1100 °C under an Ar atmosphere. Wavelength-dispersive X-ray analysis demonstrated that the ε-NbN faceted grains have a nearly stoichiometric composition. In addition to the ε-NbN grains, powders of niobium nitrides containing δ-NbN and Nb2N, and water-soluble Na3BO3 were included in the sample prepared from Nb2O5 at 1100 °C, while powders of Fe and Na3BO3, and a trace of δ-NbN were included in the sample from NbFeO4. These reactions were attributed to metathesis reactions in which the oxygen in the oxides and the nitrogen in the solid nitrogen source, BN, were replaced by the formation of Na3BO3. An extra amount of nitrogen supplied from the starting material enhances the formation of ε-NbN and suppressed the formation of δ-NbN and Nb2N.