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