Abstract
The structure change of tantalum on heating in air and oxygen was followed by X-ray diffraction. Numerous forms of Ta oxides have been identified. Ta-(O) (solid solution), TaO y , TaO z , δ-form and β-Ta2O5 are formed in air at either atmospheric or reduced pressure. The nitrogen in the air is important for high temperature oxidation of tantalum since nitrides such as Ta2N and δ-TaN were observed. Heating of Ta powder in oxygen causes violent ignition and the α-form which has hitherto been considered to be a high-temperature form of Ta2O5 is obtained. The transformation α→β occurs on heating in oxygen and the inverse transformation β→α occurs on heating in vacuum at a temperature higher than 1300°C. The helium method of density measurement showed that the α-form is an oxide lower than Ta2O5. In the products of thermite reactions of (Ta+CuO) system, the dioxide TaO2 and a new cubic form of Ta oxide were observed.

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