Figure 2. Formation energy versus the number of metal layers within the unit cell for (a) WB₃ and (b) MoB₃ polytypes.

The formation energy, defined as ΔE = E(TMB₃) − E(TM) − 3E(B), is obtained from the first-principles calculations, referring to W and Mo in body-centered cubic phases and to B in α-B phase as their respective ground states. The red and black dashed lines indicate the lowest and highest energy values, with an energy distribution width as small as 37 meV (27 meV) per WB₃ (MoB₃) formula unit (f.u.), amongst all the calculated polytypes.