First-principles thermal equation of state of tungsten carbide

We report a first-principles investigation of the thermal equation of state of hexagonal tungsten monocarbide (WC) within the quasiharmonic approximation. The equilibrium c/a ratio of WC at a given unit cell volume and temperature is obtained through an explicit minimization of the Helmholtz free energy. The predicted isotherms, compressibility, thermal expansivity, shock Hugonoit, heat capacity, Gruneisen parameter, and Debye temperature of WC show excellent agreements with existing experimental measurements. Similar to ReB2, WC simultaneously exhibits both anisotropic linear compressibility and isotropic thermal expansivity. Such an unusual property is found to be the consequence of a complete cancelation of the effects due to free energy anisotropy and lattice anharmonicity. We demonstrate that the Mie-Gruneisen-Debye formalism, widely used for interpreting experimental pressure-volume-temperature data, may not be of sufficient accuracy for WC. By analyzing the electronic density of states and inter-atomic force constants, the nature of chemical bonding in WC is further elucidated in our study. (C) 2012 Elsevier B.V. All rights reserved.