An ab initio investigation of the temperature-dependent energetic barriers towards CrAlB and (Mo,Cr)AlB formation in a metastable synthesis scenario

The orthorhombic CrAlB MAB phase has not been synthesized so far and was shown to be energetically unstable vs. the competing Cr2AlB2 phase in previous theoretical reports, which, however, did not explicitly investigate the magnitude of the energetic barrier towards CrAlB formation as a function of temperature. Temperature-dependent Gibbs energies of formation, obtained from density-functional-theory-based lattice dynamics simulations performed in this study, reveal that this barrier is very small (around 10 kJ mol(-1) approximate to 0.008 eV per atom, on average) and may readily be overcome during high-energy synthesis scenarios, likely resulting in metastable phase formation. Furthermore, the electronic structures of MoAlB, a phase synthesized experimentally both in bulk and thin film form, and CrAlB are shown to be similar in direct comparison, with MoAlB exhibiting a higher electronic stability due to a local DOS minimum in proximity to the Fermi level, and quaternary compositions lying between the ternaries. Likewise, bonding characteristics are qualitatively very similar between both phases, with the transition metal-boron bonds being the dominant interaction in the entire unit cell, even though individual B-B bonds are stronger; quantitatively, all interactions are again stronger in MoAlB compared to CrAlB. It is reasonable to assume that, considering the successful synthesis of phase-pure MoAlB and known formation of metastable phases during physical vapor deposition, direct synthesis of metastable CrAlB thin films is possible due to the aforementioned small energy barrier. Furthermore, stability is enhanced upon alloying with Mo as this lowers the energy of formation, with a Mo/Cr ratio of approx. 0.33 sufficient to stabilize the Cr-rich (Mo,Cr)AlB solid solution vs. the primary competing phases, allowing for deposition of Mo-concentration-dependent stable and metastable (Mo,Cr)AlB solid solution phases.

Automated summary

This study reveals that the orthorhombic CrAlB MAB phase can be synthesized as a metastable phase with a low energy barrier. The electronic structures of MoAlB and CrAlB are similar, but MoAlB is more electronically stable. Both phases exhibit similar bonding characteristics, with the transition metal-metal bonds being the dominant interaction. It is possible to directly synthesize metastable CrAlB thin films, and the stability can be enhanced by alloying with Mo.