First principles investigation on the ultra-incompressible and hard TaN.

The structural, electronic, and mechanical properties of TaN were investigated by use of the density functional theory (DFT). Eight structures were considered, i.e., hexagonal WC, TaN, NiAs, wurtzite, and CoSn structures, cubic NaCl, zinc-blende and CsCl structures. The results indicate that TaN in TaN-type structure is the most stable at ambient conditions among the considered structures. Above 5 GPa, TaN in WC-type structure becomes energetically the most stable phase. They are also stable both thermodynamically and mechanically. TaN in WC-type has the largest shear modulus 243 GPa and large bulk modulus 337 GPa among the considered structures. The volume compressibility is slightly larger than diamond, but smaller than c-BN at pressures from 0 to 100 GPa. The compressibility along the c axis is smaller than the linear compressibility of both diamond and c-BN. The estimated hardness is 34 GPa. Thus, TaN in WC-type structure is a potential candidate to be ultra-incompressible and hard. The unique mechanical properties of TaN in WC-type structure would make it suitable for applications under extreme conditions.