Unexpected structural softening of interstitial boron solid solution WB3+x

Abstract

Using first-principles calculations, we reveal an unexpected structural softening in a recently proposed WB3+x structural model that tries to explain the X-ray diffraction, high resolution TEM, pressure dependence of the normalized lattice c/a ratio, and hardness experimental results of the synthesized tungsten boride compounds with a nominal composition WB4. We show that the interstitial boron in WB3+x, which was proposed to strengthen the covalent bonding network, unexpectedly weakens the atomic bonding, resulting in a large reduction of its indentation strength to well below that of WB3. This is in direct contradiction to the experimental results showing that synthesized WB4 is harder than WB3. The unusual structural softening is attributed to the unique three-center covalent bonding formed by the interstitial boron atoms that can easily deform under indentation. Our results show that the proposed interstitial boron solid solution WB3+x structure is incompatible with experimental results, which calls for further investigations to determine the crystal structure of the synthesized WB4.