Lattice stress states of superhard tungsten tetraboride from radial x-ray diffraction under nonhydrostatic compression

Abstract

In this work, we examine the lattice behavior of the economically interesting superhard material, tungsten tetraboride (${\mathrm{WB}}_{4}$), in a diamond anvil cell under nonhydrostatic compression up to 48.5 GPa. From the measurements of lattice-supported differential stress, significant strength anisotropy is observed in ${\mathrm{WB}}_{4}$. The (002) planes are found to support the highest differential stress of 19.7 GPa within the applied pressure range. This result is in contrast to ${\mathrm{ReB}}_{2}$, one of the hardest transition metal borides known to date, where the same planes support the least differential stress. A discontinuous change in the slope of c/a ratio is seen at 15 GPa, suggesting a structural phase transition that has also been observed under hydrostatic compression. Speculations on the possible relationship between the observed structural changes, the strength anisotropy, and the orientation of boron-boron bonds along the $c$ direction within the ${\mathrm{WB}}_{4}$ structure are included.