In situ observation of the reaction of tantalum with nitrogen in a laser heated diamond anvil cell

Tantalum nitrides were formed by reaction of the elements at pressures between 9(1) and 12.7(5) GPa and temperatures >1600-2000K in the laser-heated diamond anvil cell. The incorporation of small amount of nitrogen in the tantalum structure was identified as the first reaction product on weak laser irradiation. Subsequent laser heating led to the formation of hexagonal β-Ta2Nand orthorhombic η-Ta2N3, whichwas the stable phase at pressures up to 27 GPa and high temperatures. No evidence was found for the presence of ε-TaN, ∂-TaN, δ-TaN, Ta₃N₅-I or Ta ₃N₅-II, which was predicted to be the stable phase at P > 17 GPa and T = 2800 K, at the P, T-conditions of this experiment. The bulk modulus of η-Ta₂N₃ was determined to be B₀ = 319(6) GPa from a 2nd order Birch-Murnaghan equation of state fit to the experimental data, while quantum mechanical calculations using the density functional theory gave a bulk modulus of B₀ = 348.0(9) GPa for a 2nd-order fit or B₀ = 339(1) GPa and B′ = 4.67(9) for a 3rd-order fit. The values show the large incompressibility of this high-pressure phase. From the DFT data the structural compression mechanism could be determined.