Niobium monoxide

The hearth cavity is filled with high-purity NbjOj powder and Nb metal rod or powder in appropriate proportions. The entire assembly is flushed with argon gettered over Ti chips or foil at 900° to remove any Oj, Nj, or hydrocarbons. A continuous argon flow of approximately 2 L/min is maintained during operation to carry the volatile impurities out of the chamber. The arc is struck by touching the electrode momentarily to the hearth. The electrode is then raised and swiveled until the arc plasma bathes the mixed powders. As the metal melts and reacts with the oxide powder, the material sinks into the crucible and the piston is continuously raised to keep the melt in view. If a simple cup-shaped hearth is employed in place of the piston hearth, the electrode is lowered in the course of the reaction. 

After the reaction is complete, the power is raised to melt the entire button and then gradually reduced to ensure slow cooling. After the arc is extinguished, it is desirable to flip the button over with the electrode and then to 

Strike a new arc and remelt the button this procedure ensures greater homogeneity. 

The stoichiometry of the sample is determined readily by heating an aliquot portion in air for 16 h at 800° to form Nb20s. The oxygen/metal ratio is calculated from 

A typical analysis using the spark-source mass spectrograph shows the following impurities N = 400, C = 300, Ta = 200, Fe = 90, Mo = 40, V = 10 atomic ppm. All other impurities are below the 10-ppm limit it is interesting that only 1 ppm each of Cu and W have been detected, even though these are the major construction materials of the furnace used. 

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Niobium Oxides. The solubihty of oxygen in niobium obeys Henry s law to the solubiHty limit of the first oxide phase of 850—1300°C (123). The amount of oxygen in solution in niobium is 1.3 at. % at 850°C and nearly 2 at. % at 1000°C (124). Only three clearly defined anhydrous oxides of niobium have been obtained in bulk, ie, NbO, Nb02, and Nb20. Niobium monoxide, NbO, is obtained by hydrogen reduction of the pentoxide at... 

In the case of vanadium, the suboxide, vanadium monoxide, would be more volatile than carbon monoxide except at very high carbon concentrations in the metal. The removal of the residual oxygen from this metal by carbon deoxidation is, therefore, difficult. In the case of niobium and tantalum, the partial pressure of carbon monoxide is higher than that of niobium monoxide or tantalum monoxide, even when the residual carbon concentration in the metal is as low as 200 ppm. It may therefore be expected that practically all the oxygen would be removed by evaporation of carbon monoxide without any metal loss from niobium and tantalum metals containing both oxygen and carbon. 

The monoxide has been variously described, according to the method of preparation, as a black powder, or glistening crystals, or beautiful black cubes. Its density varies from 6 3 to 6-7, and it is a good conductor of electricity. On being heated in air it oxidises readily to the pentoxide with considerable evolution of heat. It dissolves in hydrofluoric acid and hydrochloric acid with evolution of hydrogen, and a pentavalent niobium salt is formed in solution. It is also dissolved by boiling potassium hydroxide to form potassium niobate. This behaviour of niobium monoxide towards acids and alkali indicates that divalent salts of niobium are too unstable to exist. A divalent chloride of tantalum has, however, recently been isolated (see p. 192). 

E. R. Pollard, Electronic Properties of Niobium Monoxide, Ph.D. thesis, Dept, of Material Sciences, M.l.T. (1968). 

The unit cell of niobium monoxide NbO can be derived from the cubic unit cell of NaCl, by deleting the body-central Na+ (coordinate and the vertex... 

LiNbOj is prepared by the reaction of LijNb04 and NbO at 1050° as in Kumada et al. Unless otherwise specified, the reactants are heated to the reaction temperature in 1-2 h. The lithium niobium(V) oxide, Li3Nb04, is made as a white powder by heating a 3 1 molar ratio of Li2C03 (6.236 g, 84.41 mmol, Cerac, 99.8%) and Nb205 (7.478 g, 28.13 mmol, Cerac, 99.95%) in an alumina crucible at 900° for 3 days. Niobium monoxide, NbO, is obtained by heating a 3 1 molar ratio of Nb powder (5.118 g, 55.09 mmol, Cerac, 99.8%) and NbjO, (4.882 g, 18.37 mmol) in a fused-silica tube, sealed under vacuum, at 1100° for 3 days. 

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