Ab initio study of phase stability, elastic anisotropy and minimum thermal conductivity of MnB₂ in different crystal structures

The phase stability, elastic anisotropy, and minimum thermal conductivity of MnB₂ in different crystal structures have been investigated by first-principles calculations based on density functional theory. The results found that P6₃/mmc (hP6-MnB₂), P6/mmm (hP3-MnB₂), Pmmn (oP6-MnB₂), R 3 ¯ m (hR3-MnB₂), Pnma (oP12-MnB₂), and Immm (oI18-MnB₂) all exhibit mechanical and dynamic stability under environmental conditions, and the sequence of phase stability was hP6 > hR3 > oP6 > oI18 > oP12 > hP3. In addition, Vickers hardness calculations indicated that hP6, hR3, oP6, and oI18 of MnB₂ have potential as hard materials, while hP3 and oP12 are not suitable as hard materials. Moreover, the elastic anisotropy of different MnB₂ phases were also comprehensively investigated. It is found that the anisotropic order of bulk modulus is oP12 > hP3 > hP6 > hR3 > oI18 > oP6, while that of Young’s modulus is oP12 > hR3 > hP6 > oP6 > hP3 > oI18. Furthermore, the minimum thermal conductivity of different MnB₂ phases was evaluated by means of Clarke’s and Cahill’s models. The results suggested that these MnB₂ diborides are all not suitable as thermal barrier coating materials.