A solution and solid state study of niobium complexes University of ...

Chapter 2
The carbide is then remixed with a stoichiometric amount of oxide and then reheated
to 2000 °C under reduced pressure:
5 NbC + Nb2O5 → 7Nb + 5 CO (2.2)
The product is then hydrated, crushed and rehydrated to a powder. All of the carbon
and most of the oxygen is removed by high temperature sintering. The powder is
then compacted by electron-beam or arc melting. 17
The powders of niobium can also be produced by the reduction of niobium oxide with
magnesium, or by the reduction of potassium niobium heptafluoride with sodium. 18
The latter reaction is carried out in an iron vessel filled with alternating layers of
K2NbF7 and oxygen free sodium:
K2NbF7 + 5 Na → Nb + 2 KF + 5 NaF (2.3)
The reaction is performed under an argon or helium atmosphere and the sodium (in
excess) drives the reaction to completion. The excess sodium is leached with alcohol
and the potassium and sodium fluorides are extracted with water. The metal powder
is leached with hydrochloric acid to remove iron contamination from the vessel.
2.4 Chemistry of Niobium
Niobium reacts with most non-metals to give products which are frequently interstitial
and nonstoichiometric. Most of niobium’s chemistry is confined to the +5 oxidation
state, as this is the most studied oxidation state.
2.4.1 Oxides
Niobium pentoxide (Nb2O5) is relatively stable and difficult to reduce. Concentrated
HF is the only acid that can attack Nb2O5 and it also dissolves in fused alkali
hydrogen sulphate, carbonate and hydroxide. Extensive polymorphism is displayed
by Nb2O5, based on octahedrally coordinated niobium atoms. High temperature
reduction of Nb2O5 with hydrogen gives NbO2 and further reduction produces NbO.
NbO2 has a distorted rutile structure and NbO has a cubic structure. 2
17 H. Stuart, O. de Souza Paraiso, R. de Fuccio, Iron Steel, 11, 1980
18 F. Perfect, Trans. Metall. Soc., AIME 239, 1282, 1967.
13