Stability and elasticity of metastable solid solutions and superlattices in the MoN-TaN system: a first-principles study

Abstract

Employing ab initio calculations, we discuss chemical, mechanical, and dynamical stability of MoN-TaN solid solutions together with cubic-like MoN/TaN superlattices, as another materials design concept. Hexagonal-type structures based on low-energy modifications of MoN and TaN are the most stable ones over the whole composition range. Despite being metastable, disordered cubic polymorphs are energetically significantly preferred over their ordered counterparts. An in-depth analysis of atomic environments in terms of bond lengths and angles reveals that the chemical disorder results in (partially) broken symmetry, i.e., the disordered cubic structure relaxes towards a hexagonal NiAs-type phase, the ground state of MoN. Surprisingly, also the superlattice architecture is clearly favored over the ordered cubic solid solution. We show that the bi-axial coherency stresses in superlattices break the cubic symmetry beyond simple tetragonal distortions and lead to a new tetragonal $\zeta$-phase (space group P4/nmm), which exhibits a more negative formation energy than the symmetry-stabilized cubic structures of MoN and TaN. Unlike cubic TaN, the $\zeta \text{-TaN}$ is elastically and vibrationally stable, while $\zeta$-MoN is stabilized only by the superlattice structure. To map compositional trends in elasticity, we establish mechanical stability of various Mo${}_{1-x}$Ta${}_x$N systems and find the closest high-symmetry approximants of the corresponding elastic tensors. According to the estimated polycrystalline moduli, the hexagonal polymorphs are predicted to be extremely hard, however, less ductile than the cubic phases and superlattices. The trends in stability based on energetics and elasticity are corroborated by density of electronic states.

References in corpus (10)

• Necessary and Sufficient Elastic Stability Conditions in Various Crystal Systems
• Intrinsic Correlation between Hardness and Elasticity in Polycrystalline Materials and Bulk Metallic Glasses
• The closest elastic tensor of arbitrary symmetry to an elasticity tensor of lower symmetry
• Trends in the elastic response of binary early transition metal nitrides
• Ab-initio elastic tensor of cubic Ti${}_{0.5}$Al${}_{0.5}$N alloy: the dependence of the elastic constants on the size and shape of the supercell model
• Point defects stabilise cubic Mo-N and Ta-N
• Effect of the nitrogen-argon gas mixtures on the superconductivity properties of reactively sputtered molybdenum nitride thin films
• Superconductor to weak-insulator transitions in disordered Tantalum Nitride films
• Superconducting Energy Scales and Anomalous Dissipative Conductivity in Thin Films of Molybdenum Nitride
• Vacancy-driven extended stability of cubic metastable Ta-Al-N and Nb-Al-N phases

Cited by in corpus (1)

• Point-defect engineering of MoN/TaN superlattice films: A first-principles and experimental study