First-principle calculations of the bulk properties of 4d transition metal carbides and nitrides in the rocksalt, zincblende and wurtzite structures

Bulk properties and stability of the entire series of group 4d transition metal carbides and nitrides are reported in this work. The theoretical calculations were carried out within Local Density Approximation and Generalized Gradient Approximation using the Perdew, Burke and Ernzerhof exchange correlation functional. The generalized gradient approximation predictions were found to be closer to experimental values than the local density approximation predictions. In particular, LDA predictions were found to overestimate bulk moduli properties by as much as 5.6–11.5% while equilibrium lattice constants were found to be underestimated by as much as 0.2–5% compared to experimental values. On the other hand, GGA calculations were found to overestimate the lattice parameters by 0.2–6.9%, while underestimating the bulk moduli by as much as 0.07–5%. Out of the carbides considered, TcC and RuC were found to have the highest values of bulk moduli while YC and CdC had the lowest. Similarly, out of the nitrides, MoN and TcN were found to exhibit the largest bulk moduli, indicating that they were the hardest, while CdN had the lowest value and hence relatively softer. Overall, the nitrides presented higher values of bulk moduli than the carbides, an observation that is well supported by their correspondingly shorter bondlengths. The cohesive and structural properties of the 4d transition metal carbides and nitrides are also reported.

Research Highlights
► We calculated the optimized structures and stability of the 4d transition metal carbides and nitrides in the rocksalt, zincblende and wurtzite structure using both LDA-PAW and GGA exchange correlation functionals.
► For each structure we calculated optimized lattice constants, bond lengths, bulk moduli and cohesive energies. We also compared the trends of these properties across the whole series of group 4d transition metals.
► From these we compared the relative hardness of group 4d transition metal carbides and nitrides with other theoretical predictions as well as experimental results where available.