Lattice distortion and the influence on mechanical and thermodynamic properties of high entropy (HfZrTaNbTi)X (X=C, N, NC) by Ab initio investigation

Novel high-entropy ceramics have been rapidly developing and exhibiting excellent properties. In this paper, the structural distortion and the influence on mechanical and thermodynamic properties of (HfZrTaNbTi)C, (HfZrTaNbTi)N and (HfZrTaNbTi)(NC) are systematically studied by employing density functional theory in conjunction with special quasi-random structure (SQS). Our results show that lattice distortion enhances the structural stability of these high-entropy ceramics by lowering the formation and mixing enthalpy. The local lattice distortion in considered ceramics is further quantified in terms of constituent atoms displacement and bond length distribution, suggesting that (HfZrTaNbTi)C and (HfZrTaNbTi)N possess the smallest and largest distortion, respectively. While the distortion in anion and cation mixed (HfZrTaNbTi)(NC) is intermediate. The elastic properties illustrate that the distortion improved the ductility of high-entropy ceramics at the slight expense of strength and stiffness. The electronic structures show that (HfZrTaNbTi)C has the strongest covalent bonds and (HfNbTaTiZr)N possesses a stronger ionic character. After distortion, the DOS at the Fermi level for the three ceramics is reduced and the charge density is significantly enhanced, revealing the underlying mechanism for stability improvement. Combining the Debye-Grüneisen model, the thermodynamic properties and the influence of lattice distortion are further studied. It is found that lattice distortion improves thermal expansion and heat storage capacity. Simultaneously, the distortion also increases the entropy of ceramics, thereby enhancing the stability in high-temperature environments. This paper provides a valuable reference for understanding crucial properties and designing promising multi-cation and anion mixed high-entropy ceramics by revealing the vital role of lattice distortion.