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

Abstract

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 (δ, Fm-3m), anti-TiP-NbN (ε, P63/mmc), anti-NiAs-NbN (δ′, P63/mmc), WC-NbN (η, P-6m2), and TiP–NbN (ε′, P63/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 ε, η and δ′, 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 ε and η, and both (0001)<10-10> and (0001)<-12-10> for δ’.