Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides

Tantalum nitride (TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and high-temperature (HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch-Murnaghan equation of state fitted to the x-ray diffraction pressure-volume (P-V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B₀=369(2) GPa with pressure derivatives of B₀'=4 for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result (B₀=351 GPa), which is close to the recent theoretical calculation result (B₀=378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta-N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.