Niobium Compounds

Niobium is very similar to tantalum and zirconium in many ways. It reacts with fluorine at room temperature, with chlorine and hydrogen at 200°C, and with nitrogen at 400°C, with the products generally being interstitial non-integer ratio compounds. Niobium metal oxidizes in air at 200°C and is resistant to molten bases and a variety of acids, including aqua regia, hydrochloric, sulfuric, nitric, and phosphoric acids. However, hydrofluoric acid and mixtures of hydrofluoric and nitric acids can attack niobium.

1. Oxides and sulfides

The oxides of niobium can be in the following oxidation states: +5 (Nb2O5), +4 (NbO2) and +3 (Nb2O3), and more rarely in the +2 state (NbO). Niobium pentoxide is the most common niobium oxide from which niobium metal and all niobium compounds are prepared. To make niobates, niobium pentoxide can be dissolved in alkaline hydroxide solutions or melted in alkali metal oxides.

2. Halides

Niobium can form halides possessing +5 and +4 oxidation states, as well as various substoichiometric compounds.

Negative halide ions of niobium are also present, due to the fact that all five halides of niobium are Lewis acids. The most important one is [NbF7], which is an intermediate compound in the mineral separation of niobium and tantalum. It is more readily converted to oxygen pentahalides than the corresponding tantalum compounds.

3. Nitrides and carbides
Niobium nitride (NbN) turns into a superconductor at low temperatures and is used in infrared detectors. The predominant niobium carbide is NbC, which is so hard that it is a fire-resistant ceramic material and can be used as a cutting tool tip material.