  Motivated by frequently reported deviations from stoichiometry in cubic
transition metal nitride (TMNx) thin films, the effect of N-vacancy
concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and
MoNx (0.72<x<1.00) is systematically studied by density functional theory (DFT)
calculations. The predictions are validated experimentally for VNx
(0.77<x<0.97). The DFT results indicate that the elastic behavior of the TMNx
depends on both the N-vacancy concentration and the valence electron
concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit
vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase.
These trends can be rationalized by considering vacancy-induced changes in
elastic anisotropy and bonding. While introduction of N-vacancies in TiNx
results in a significant reduction of elastic modulus along all directions and
a lower average bond strength of Ti-N, the vacancy-induced reduction in [001]
direction of VNx is overcompensated by the higher stiffness along [011] and
[111] directions, resulting in a higher average bond strength of V-N. To
validate the predicted vacancy-induced changes in elasticity experimentally,
close-to-single-crystal VNx (0.77<x<0.97) are grown on MgO(001) substrates. As
the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray
diffraction, decreases from 4.128 A to 4.096 A. This reduction in lattice
parameter is accompanied by an anomalous 11% increase in elastic modulus, as
determined by nanoindentation. As the experimental data agree with the
predictions, the elasticity enhancement in VNx upon N-vacancy formation can be
understood based on the concomitant changes in elastic anisotropy and bonding.
