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..