Point defects stabilise cubic Mo-N and Ta-N

We employab 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 Mo , V N ) as long as their concentration is below ?15at.\% . The overall lowest energy of formation were obtained for Ta 0.78 N and Mo 0.91 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, Mo 0.84 N , Mo 0.91 N , MoN 0.69 and 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 Ta 0.75 N and Ta 0.78 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.

Keywords

Mo-N, Ta-N, point defects, vacancies, stability, DFT

Citation

Journal of Physics D: Applied Physics. 2016, vol. 49, issue 37, p. 1-8.
http://dx.doi.org/10.1088/0022-3727/49/37/375303

Document type

Peer-reviewed

Document version

Published version

Language of document

en