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N. Koutná, D. Holec, R Hahn, M. Friak, P.H. Mayrhofer:
"Vacancies-driven energetics and elasticity of MoN/TaN superlattices";
Poster: ViCoM, Vienna; 2018-04-04 - 2018-04-06.

Excellent mechanical properties of nitride-based coatings are closely related to their microstructure. To meet the ever more and more demanding industrial requirements coatings are nanoengineered as single or multilayer multicomponent systems. Especially superlattices with thickness
of only a few nm represent a powerful concept for tuning electronic, mechanical, optical or magnetic
properties. Within the framework of the Density Functional Theory, we investigate thermodynamic
stability and elastic properties of (001) oriented MoN/TaN superlattices based on the cubic rocksalt
structure. Our calculations reveal that the bi-axial coherency stresses in MoN/TaN break the cubic symmetry beyond simple tetragonal distortions, leading to a new tetragonal ζ-phase (P4/nmm,
#129). Both ζ-MoN and ζ-TaN have lower formation energy than their cubic counterparts. Unlike the cubic TaN, the ζ-TaN is also dynamically stable. Furthermore, MoN/TaN superlattices containing vacancies on the MoN sublattice are energetically strongly favoured over the defect-free configuration. Dynamical stability as well as elastic properties of defected superlattices are influencd also by a specific distribution of vacancies (e.g. their distance from the interface) as well as by the bi-layer period. Importantly, an alternation of ∼2nm-layers of MoN and TaN with ordered N vacancies on the MoN sublattice leads to high elastic moduli and improved ductility, as compared to the film forming ζ-phases.