Finding the Stable Structures of WxN1-x with an ab-initio High-Throughput Approach
Abstract
Using density functional theory calculations, many researchers have predicted that various tungsten-nitride compounds WNx(x > 1) will be "ultra-incompressible" or "superhard", i.e. as hard as or harder than diamond. These compounds are predicted to have large bulk and shear moduli, (> 200 GPa) and to be elastically and vibrationally stable. Compounds with such desirable properties must be energetically stable against decomposition into other compounds. This stability can only be found after the determination of the convex hull for WxN1-x lines which connect the lowest enthalpy structures as a function of composition. The phase diagram of the W-N structure is uncertain, both experimentally and computationally. Complex van der Waals forces play a significant role in determining the structure of solid N2. Here we use high-throughput calculations to map out the convex hull and other low energy structures for the W-N system. We find that the ground state of the system is the NbO structure, and that the WN2 structures found by Wang et al. are also stable when van der Waals forces are neglected. Other proposed structures are above the convex hull of the W-N system. We show how the choice of density functional influences the shape of the curve and the structures that form the hull. In nitrogen-rich compounds, the choice of functional can dramatically change the structural parameters and mechanical behavior. Using any of the functionals, the bulk and shear moduli of the NbO phase are comparable to the WNx compounds that have been claimed to be ultra-incompressible or superhard.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 13, 2014
- Accession Number
- ADA600591
Entities
People
- Christian Dane
- Daniel Finkenstadt
- Gus L. Hart
- Michael J. Mehl
- Stefano Curtarolo
Organizations
- United States Naval Research Laboratory