Temperature- and pressure-induced structural transformations in NbN: A first-principles study

Structural transformations at high temperatures and pressures, elastic moduli, hardness, fracture toughness, Debye temperature, stress-shear strain relations, electronic structure, and lattice dynamics in the experimentaly observed NaCl-NbN (d, Fm-3m), anti-TiP-NbN (e, P6(3)/mmc), anti-NiAs-NbN (d', P6(3)/mmc), WC-NbN (?, P-6m2), and TiP-NbN (e', P6(3)/mmc) phases and hypothetical new tP4-129 (P4/nmm), hP6-189 (P-62m), oP8-25 (Pmm2) and cP2-221 (Pm-3m) structures are studied by using first-principles calculations and molecular dynamics simulations. The possible mechanisms of the phase transitions between these structures based on the condensation of a certain phonon mode with subsequent spontaneous strains are suggested. The e, ? and d', and cP2-221 structures are brittle materials and exhibit highest shear moduli (200.5-216.8 GPa), Young moduli (492.4-528.8 GPa), Vickers hardness (23.9-27.1 GPa), fracture toughness (4.54-4.72 MPa m1/2), and Debye temperatures (730.5-767.0 K). It is found that the main slip systems should be (0001)<10-10> for e and ?, and both (0001)< 10-10> and (0001)<-12-10> for d'.