(color online) (color online) (a) Elastic wave velocities (V_P and V_S) of polycrystalline hexagonal ε-NbN at high pressure; a representative acoustic echo of the ultrasonic measurements at the highest pressure, ∼12 GPa, is shown in the inset. (b) Elastic bulk and shear moduli (B_S and G) of polycrystalline hexagonal ε-NbN at high pressure. Red circles denote velocities upon compression, and blue solid circles those during decompression; red solid lines are from third-order finite strain fits. Figure adapted with permission from Ref. [24].