Density functional theory predictions of the Hf-HfC-HfN ternary: Phase stability and properties

In this work, we investigate the phase stability and mechanical, thermal, and electronic properties of the close -packed structures in the Hf-HfC-HfN ternary system using electronic structure density functional theory. The phase stability results, as determined from enthalpy of formation, indicate that vacancies on the non-metal sublattice enhances the HCP metal atom stacking sequence while carbon stabilizes the FCC metal atom stacking sequence. Thus, phases with mixtures of FCC and HCP metal atom stacking sequences form readily on the Hf-N side of the phase diagram and only one such phase forms in the interior of the phase diagram: zeta-Hf8CN4. The remaining phases in the interior of the phase diagram have an FCC metal atom stacking sequence and all interior phases have high non-metal atom concentrations. The material properties are also evaluated, including the elastic stiffness, Debye temperatures, and the electronic density of states (DOS). In general, the stiffness increases with non-metal concentration and, more specifically, with the carbon concentration. The Debye temperature follows the same trend. The DOS for all compounds are continuous through the Fermi level, indicating that they are all conductors, while the electronic entropy generally is higher for compounds with higher hafnium content and higher nitrogen content.