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

TL;DR

This study uses first-principles calculations to explore the energetics and stability of structural transformations in transition-metal diborides via plane sliding, revealing trends in barriers, stability, and mechanical properties.

Contribution

It introduces a displacive transformation model for TMB2 structures and predicts energetics, stability, and mechanical trends based on plane sliding simulations.

Findings

MnB₂ and MoB₂ have the lowest sliding barriers.

Predicted trends in stability and mechanical properties across TMB₂.

Identified chemical and structural factors influencing transformation energetics.

Abstract

Transition metal diborides crystallise in the $\alpha$, $\gamma$, or $\omega$ 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 $\alpha$, $\gamma$, and $\omega$ structure. Chemistry-related trends are predicted in terms of energetic and structural changes along a transformation pathway, together with the mechanical and dynamical stability of the different stackings. Our results suggest that MnB$_2$ and MoB$_2$ possess the overall lowest sliding barriers among the investigated TMB$_2$s. Furthermore, we discuss trends in strength and ductility indicators, including Young's modulus or Cauchy pressure, derived from elastic constants.