Unusual suppression of tungsten 5d electron depletion in superhard tungsten tetraboride solid solution with chromium under compression

Abstract

The lattice compressibility and deformation in superhard tungsten tetraboride (WB₄) solid solution with chromium (Cr) are investigated by high-pressure x-ray diffraction and x-ray absorption fine structure (XAFS) spectroscopy up to 54 GPa. In contrast to pure WB₄, the c-axis softening is effectively suppressed in W_0.9Cr_0.1B₄, and less compressibility is shown for the a- and c-axes in the entire pressure range. Meanwhile, the white-line peak of W L₃-edge XAFS in W_0.9Cr_0.1B₄ shows an absence of the sudden intensity drop as previously observed in WB₄ at ∼21 GPa, suggesting a strong inhibition of W 5d electron depletion. This phenomenon is followed by an initial increase and then decrease for the W–B bond disorder, with the magnitude greatly lower than that of WB₄. Besides the apparent atomic size mismatch effect, these results imply that addition of Cr, which has the same number of valence electrons as W, can introduce an unexpected electronic structure change to strengthen the W-B bond via a modification of W vacancies and B trimers distribution in WB₄ lattice. Our findings point out the great significance to precise manipulation of the intrinsic W vacancies and B trimers through different solute atoms to rational optimization of WB₄ hardness.