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.