YB6: A ‘Ductile’ and Soft Ceramic with Strong Heterogeneous Chemical Bonding for Ultrahigh-Temperature Applications

Transition metal borides have contributed to the success of ultrahigh-temperature materials (UHTM) development. Current ZrB2- and HfB2-based UHTMs exhibit high-temperature environmental stability, high strength and modulus but inevitable brittleness due to strong covalent bonding. In contrast but interestingly, YB6 is predicted as a 'ductile' and soft ceramic for ultrahigh-temperature applications, through density functional theory investigations on its electronic structure and bonding properties. The 'ductility' is underpinned by chemical bonding anisotropy, that is, a strong sigma bond connecting the B-6 octahedra and a weak banana bond formed by overlapping the lobes of two perpendicular p orbitals (tau bond).