Phase stability, mechanical properties and thermal conductivity of technetium diborides in different crystal structures

The possible structures of technetium diboride under ambient conditions are predicted by the structure prediction algorithm CALYPSO with density functional theory calculations, and their phase stability, mechanical properties and thermal conductivity are further investigated. The results showed that P 6 3 / mmc ( hP 6-TcB 2 ), P 6/ mmm ( hP 3-TcB 2 ), Pmmn ( oP 6-TcB 2 ), R 3 ¯ m ( hR 9-TcB 2 ), Cmcm ( oC 12-TcB 2 ) are all mechanically and dynamically stable. Moreover, the Vickers hardness of the five structures is evaluated by empirical model. In which, hP 6-TcB 2 has the highest hardness (38.4 GPa), and hP 6-TcB 2 , oP 6-TcB 2 , and hR 9-TcB 2 are both the potential hard materials. In addition, the elastic-dependent anisotropy properties of these structures are further characterized by three-dimensional graphical representations. The anisotropic order of bulk modulus B is hR 9 > hP 6 > oP 6 > hP 3 > oC 12, while the anisotropy ranking of Young’s modulus E and shear modulus G should be hP 3 > oP 6 > hP 6 > hR 9 > oC 12. Finally, the minimum thermal conductivity of TcB 2 in different structures are further investigated by using Clarke’s and Cahill’s models. The results showed that these technetium diborides may also be potential high-temperature thermal barrier coating materials.