Point defects stabilise cubic Mo-N and Ta-N

Abstract

We employ \textit{ab initio} calculations to investigate energetics of point defects in metastable rocksalt cubic Ta-N and Mo-N. Our results reveal a strong tendency to off-stoichiometry, i.e. defected structures are predicted to be more stable than perfect ones with 1:1 metal-to-nitrogen stoichiometry, in agreement with previous literature reports. While Ta-N significantly favours metal vacancies, Mo-N exhibits similar energies of formation regardless of the vacancy type ($V_{\text{Mo}}$, $V_{\text{N}}$) as long as their concentration is below $\approx 15\text{at.\%}$. The overall lowest energy of formation were obtained for ${\text{Ta}}_{0.78}\text{N}$ and ${\text{Mo}}_{0.91}\text{N}$, which are hence predicted to be the most stable compositions. To account for various experimental condition during synthesis, we further evaluated the phase stability as a function of chemical potential of individual species. The proposed phase diagrams reveal four stable compositions, ${\text{Mo}}_{0.84}\text{N}$, ${\text{Mo}}_{0.91}\text{N}$, ${\text{MoN}}_{0.69}$ and ${\text{MoN}}_{0.44}$, in the case of Mo-N and nine stable compositions in the case of Ta-N indicating the crucial role of metal under-stoichiometry, since ${\text{Ta}}_{0.75}\text{N}$ and ${\text{Ta}}_{0.78}\text{N}$ significantly dominate the diagram. These results are important for understanding and designing experiments using non-equilibrium deposition techniques. Finally, we discuss a role of defects ordering and estimate a cubic lattice parameter as a function of a defect contents.

9 pages, 6 figures, 2 tables

Cited by in corpus (4)

• Stability and elasticity of metastable solid solutions and superlattices in the MoN-TaN system: a first-principles study
• Vacancy-driven extended stability of cubic metastable Ta-Al-N and Nb-Al-N phases
• Point-defect engineering of MoN/TaN superlattice films: A first-principles and experimental study
• Interface-Controlled Defect Engineering in TiN/TaN Superlattices for Enhanced Hardness and Fracture Toughness