Thermodynamic and mechanical stabilities of tantalum nitride.

We perform first-principles density functional calculations on a newly discovered tantalum nitride with an orthorhombic U2S3 structure to assess its thermodynamic and mechanical stabilities. Our random search unveils a tetragonal Ta2N3 structure that is energetically more favorable than an orthorhombic Ta2N3 at zero pressure. We predict that the tetragonal Ta2N3 transforms into the orthorhombic phase above a relatively low pressure of 7.7 GPa. Single-crystal elastic constant calculations reveal that orthorhombic Ta2N3 is mechanically unstable because of a negative c66. Our calculations suggest that minor oxygen substitution for nitrogen plays an important role in stabilizing the orthorhombic Ta2N3 structure.