Theoretical prediction on electronic structure, mechanical properties and lattice dynamics of YB4 for ultrahigh temperature applications

The electronic structure, mechanical properties and lattice dynamics of YB4 are investigated using first-principles calculations. The Y-B bonding is ionic-covalent, the Y-Y bonding is metallic, the B-B bonding within B6 octahedron is weak τ-type covalent and that connecting the octahedra is strong σ-type covalent. The chemical bonding anisotropy has also been confirmed by lattice dynamic properties, and is reflected by the anisotropic elastic stiffness. The maximum Young's modulus (427 GPa) is 1.5 times larger than its minimum (285 GPa). The lowest shear moduli are associated with the [1 1 0](1 0 0) and [1 1 0](1 1 0) systems. The linear compressibility of a (1.70 × 10−3 GPa) is larger than that of c (1.47 × 10−3 GPa). Based on the low c44, c66 and Pugh's ratio G/B, YB4 is predicted as a damage to lerant ceramic. YB4 also has a relatively low Young's modulus (339 GPa), which ensures it good thermal shock resistance.