Stability and elasticity of metastable solid solutions and superlattices in the MoN–TaN system: First-principles calculations

Publication date: 15 April 2018
Source:Materials & Design, Volume 144
Author(s): Nikola Koutná, David Holec, Martin Friák, Paul H. Mayrhofer, Mojmír Šob
In order to develop design rules for novel nitride-based coatings, we investigate trends in thermodynamic, structural, elastic, and electronic properties of Mo1−xTaxN single-phase alloys together with (MoN)1−x/(TaN)x superlattices. Our calculations predict that hexagonal Mo1−xTaxN are the overall most stable ones, followed by the disordered cubic solid solutions and superlattices. The disordered cubic systems are energetically clearly favoured over their ordered counterparts. To explain this unexpected phenomenon, we perform an in-depth structural analysis of bond-lengths and angles, revealing that the disordered phase is structurally between the NaCl-type and the hexagonal NiAs-type modifications. Similarly, 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. The hexagonal alloys are predicted to be extremely hard, though, much less ductile than the cubic polymorphs and superlattices.

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