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'.

Automated summary

The structural transformations, mechanical properties, electronic structure, and lattice dynamics of several phases and hypothetical structures of NaCl-NbN, anti-TiP-NbN, anti-NiAs-NbN, WC-NbN, and TiP-NbN were studied using first-principles calculations and molecular dynamics simulations. The possible mechanisms of phase transitions between these structures were proposed based on the condensation of specific phonon modes and subsequent spontaneous strains. The e, ?, d', and cP2-221 structures were found to be brittle materials with high shear moduli, Young moduli, Vickers hardness, fracture toughness, and Debye temperatures.