Ab Initio Predictions of Hexagonal Zr(B,C,N) Polymorphs for Coherent Interface Design

Density functional theory calculations are used herein to explore the hexagonal (HX) NiAs-like polymorphs of Zr(B,C,N) and compare them with the corresponding Zr(B,C,N) Hagg-like face-centered-cubic rocksalt (B1) phases. Although all predicted compounds are mechanically stable according to the Born-Huang criteria, only HX Zr(C,N) are dynamically stable according to ab initio molecular dynamics simulations and lattice dynamics calculations. HX ZrN emerges as a candidate structure with a ground-state energy, elastic constants, and extrinsic mechanical parameters comparable with those of B1 ZrN. Ab initio band structure and semiclassical Boltzmann transport calculations predict a metallic character and a monotonic increase in electrical conductivity with the number of valence electrons. Electronic structure calculations indicate that the HX phases gain their stability and mechanical attributes through Zr d-nonmetal p hybridization and broadening of the Zr d bands. Furthermore, it is shown that the HX ZrN phase provides a low-energy coherent interface model for connecting B1 ZrN domains, with significant energetic advantage over an atomistic interface model derived from high-resolution transmission electron microscopy (HRTEM) images. The ab initio characterizations provided herein should aid the experimental identification of non-Hagg-like hard phases. The results can also enrich the variety of crystalline phases potentially available for designing coherent interfaces in superhard nanostructured materials and in materials with multilayer characteristics.