On energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing

Abstract

Transition metal diborides crystallise in the α, γ, or ω type structure, in which pure transition metal layers alternate with pure boron layers stacked along the hexagonal [0001] axis. Here we view the prototypes as different stackings of the transition metal planes and suppose they can transform from one into another by a displacive transformation. Employing first-principles calculations, we simulate sliding of individual planes in the group IV-VII transition metal diborides along a transformation pathway connecting the α, γ, and ω structure. Chemistry-related trends are predicted in terms of energetic and structural changes along a transformation pathway, together with mechanical and dynamical stability of the different stackings. Our results suggest that MnB2 and MoB2 possess the overall lowest sliding barriers among the investigated TMB2s. Furthermore, we discuss trends in strength and ductility indicators, including the Young's modulus or Cauchy pressure, derived from elastic constants.