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

Abstract

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$\overline{3}$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.