Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

Abstract

Chromium tetraboride (CrB4), a recently proposed candidate for superhard materials, has been synthesized at high pressure and temperature by a solid-state reaction. As a byproduct, chromium diboride (CrB2) also forms and co-exists with CrB4 in the final product. The comparative studies of crystal structure, elastic property, and hardness of both phases have been conducted at the same sample environment conditions. The crystal structure of CrB4 has been refined with an orthorhombic symmetry of Immm(space group no. 71) or Pnnm (space group no. 58) using X-ray diffraction data. Further simulations indicate that the structural distinction between Immm and Pnnm can be resolved by neutron diffraction, due to the high scattering cross-section of boron (11B) by neutrons. Although CrB2 and CrB4 have close bulk modulus at about 230 GPa, the measured asymptotic Vickers hardness yields 16 GPa for CrB2 but 30 GPa for CrB4, which is nearly two times that of CrB2. The dramatic enhancement in hardness in CrB4 is attributed to the strong three-dimensional Cr-B network, in contrast to the layered lattice structure of hexagonal CrB2.