Electronic and Lattice Distortions Induce Elastic Softening in Refractory Multicomponent Borides

Borides are extensively employed in applications demanding exceptionally high hardness, which arises from the unique and strong crystallographic arrangement of boron atoms therein. Addition of multiprincipal elements in borides is expected to enhance their structural properties due to lattice distortion and high configurational entropy. In contrast, we unravel a phenomenon of elastic softening in refractory multicomponent borides from first-principle predictions, which concur with experimentally determined metrics in their single-phase multiprincipal element counterparts. The reductions in the bulk and Young's modulus of these compounds are attributed to the lengthening and distortion of the boron-boron bonds and angles, but more critically to the perturbation in the charge densities arising from the different cations and the consequential increase in statistical weights of the d⁵ configuration states of the transition metals present in the boride..