Abstract
Applying the evolutionary algorithm for crystal structure prediction and first-principles calculations, a newly orthorhombic Cmcm structure of HfB4 is predicted theoretically. The result shows that this new phase is much more superior to the early proposed YB4-, ReP4-, FeB4-, CrB4-, and MnB4-type structures. The calculations of the phonon dispersion curve and elastic constants verify that the predicted Cmcm phase is dynamically and mechanically stable. Moreover, the predicted large shear modulus (240 GPa) and high hardness (45.7 GPa) imply that the Cmcm-HfB4 phase is a potential superhard material. At the same time, the dependences of the Young's modulus, bulk modulus, and shear modulus for HfB4 on the crystal orientation are also systematically studied. Further analyses of the electronic structure and chemical bonding properties indicate that the strong BB and BHf covalent bonds make great contributions to its high hardness and stability.
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