Pressure-induced structural transition and thermodynamic properties of NbN and effect of metallic bonding on its hardness

¹ Institute of Atomic and Molecular Physics, Sichuan University - Chengdu 610065, China
² International Centre for Materials Physics, Academia Sinice - Shenyang 110016, China
³ Physics department, Sichuan normal university - Chengdu 610068, China

^a scu_kxy@163.com

Received: 10 October 2010
Accepted: 19 November 2010

Abstract

Using first-principles calculations, the elastic constants, thermodynamic properties and structural phase transition of NbN under high pressure are investigated by means of the pseudopotential plane-waves method, in addition to the effect of metallic bonding on its hardness. Three candidate structures are chosen to investigate NbN, namely, rocksalt (NaCl), NiAs and WC types. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure P_t (P_t = 200.64 GPa) between the WC phase and the NaCl phase of NbN is predicted for the first time. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Poisson's ratio of NbN are derived. The calculated results are found to be in good agreement with the available experimental data and theoretical values. According to the quasi-harmonic Debye model, the Debye temperature under high pressure is derived from the average sound velocity. Moreover, the effect of metallic bonding on the hardness of NbN is investigated and the hardness shows a gradual decrease rather than increase under compression. This is a quantitative investigation on the structural and thermodynamic properties of NbN, and it still awaits experimental confirmation.