211-MAX borides: The stable boron-substituted 211-MAX compounds by first-principles

Abstract

Applying the first-principles density functional pseudopotential plane-wave calculations, the energetic, mechanical, and dynamic stabilities of boron-substituted M2AlB (M=Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, Os, Ir, Pt, Ag, and Au) compounds as the members of 211-MAX borides with Cr2AlC-type crystal structure were investigated. After well-converged geometry optimizations, the cohesive energy, formation enthalpy, single-crystal elastic constants, and phonon dispersion curves were calculated to explain the stabilities of the compounds. Except for M=Y, Ag, and Au compounds, the others are thermodynamically stable due to the negative formation enthalpies. M=Sc, Y, Zr, Hf, Os, Ir, and Pt compounds are dynamically unstable because of the negative phonon frequencies, and M=Ag and Au compounds are mechanically unstable. Beyond the detailed stability analysis, the structural, elastic, and mechanical properties, anisotropy factors, exfoliation states, and specifically hardness nature of the stable compounds were examined. A new modified hardness method, which includes the metallicity, is proposed to characterize the softness nature of MAX-phases. The relevant quantities, such as bond electronegativity, ionicity, and metallicity, were debated. The presented data were compared with available theoretical and experimental literature.