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The crystal structure of tantalum nitrides is examined by X-ray and electron diffraction techniques and the following seven phases are identified: TaN∼0.05 (cubic, β-phase), Ta2N (hexagonal, γ-phase), δ-TaN (hexagonal), ε-TaN (hexagonal), Ta5N6 (hexagonal), Ta4N5 (tetragonal) and Ta3N5 (tetragonal or monoclinic). Thin films of Ta nitrides for electron diffraction study were prepared by nitriding thin evaporated films of tantalum in ammonia and nitrogen. By heating thin films of Ta3N5 in vacuum, the phase transformations occur successively as follows: Ta3N5→Ta4N5→Ta5N6→ε-TaN→Ta2N. The stucture of each nitride form is discussed.

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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