First-principles calculations on the elastic and thermodynamic properties of NbN

The elastic and thermodynamic properties of NbN at high pressures and high temperatures are investigated by the plane-wave pseudopotential density functional theory (DFT). The generalized gradient approximation (GGA) with the Perdew—Burke—Ernzerhof (PBE) method is used to describe the exchange—correlation energy in the present work. The calculated equilibrium lattice constant a0, bulk modulus B0, and the pressure derivative of bulk modulus B'0 of NbN with rocksalt structure are in good agreement with numerous experimental and theoretical data. The elastic properties over a range of pressures from 0 to 80.4 GPa are obtained. Isotropic wave velocities and anisotropic elasticity of NbN are studied in detail. It is indicated that NbN is highly anisotropic in both longitudinal and shear-wave velocities. According to the quasi-harmonic Debye model, in which the phononic effect is considered, the relations of (V − V0)/V0 to the temperature and the pressure, and the relations of the heat capacity Cv and the thermal expansion coefficient α to temperature are discussed in a pressure range from 0 to 80.4 GPa and a temperature range from 0 to 2500 K. At low temperature, Cv is proportional to T3 and tends to the Dulong—Petit limit at higher temperature. We predict that the thermal expansion coefficient α of NbN is about 4.20 × 10−6/K at 300 K and 0 GPa. (condensed matter: electronic structure, electrical, magnetic, and optical properties)