Ab initio study of elastic anisotropies and thermal conductivities of rhenium diborides in different crystal structures

Abstract

The phase stabilities, elastic anisotropies, and thermal conductivities of ReB₂ diborides under ambient conditions have been investigated by using density functional theory calculations. It was found that P6₃/mmc (hP6-ReB₂), Pmmn (oP6-ReB₂), Rm (hR3-ReB₂), Rm (hR6-ReB₂), and C2/m (mC12-ReB₂) of ReB₂ are both mechanically and dynamically stable, and the order of phase stability is hP6 > oP6 > hR3 > hR6 > mC12. Moreover, the calculated Vickers hardness showed that hP6-ReB₂, oP6-ReB₂, hR3-ReB₂, and mC12-ReB₂ were potential hard materials, while hR6-ReB₂ could not be used as a candidate hard material. In addition, the elastic-dependent anisotropy properties of ReB₂ in different crystal structures were also investigated. The results show that the anisotropic order of the Young's modulus and shear modulus of ReB₂ is hR6 > mC12 > oP6 > hP6 > hR3, while that of the bulk modulus is mC12 > hR3 > hP6 > oP6 > hR6. Finally, by means of Clarke's and Cahill's models, the minimum thermal conductivities of ReB₂ in different crystal structures were further evaluated, and the order of them is hR3 > hP6 > mC12 > oP6 > hR6. Moreover, the results show that all these ReB₂ diborides exhibit relatively low thermal conductivities and are suitable for thermal insulation materials.