Stability and physical properties of yttrium-based new MAX phases Y2AX (A = Al, Si, Ga, and Ge; X = C and N): a first-principles prediction

In this article, for the first time, the structural, thermodynamic, mechanical, dynamical stability, and electronic properties of yttrium-based new MAX phases Y2AX (A = Al, Si, Ga, and Ge; X = C and N) have been investigated in the hexagonal structure. The structural stability results showed that Nitride-based MAX phases Y2AN are more energetically stable than Carbon-based MAX phases Y2AC. Cohesive energy and formation enthalpy ΔHcp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Delta H}_{\mathrm{cp}}$$\end{document} analysis indicated that Nitride-based MAX phases are more stable than Carbon-based MAX phases, and specifically, Y2GeN and Y2AlC have shown the highest and lowest stability, respectively. From the calculation of the elastic constants cij\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{ij}$$\end{document}, it can be concluded that the new MAX phases of Y2AX in the hexagonal structure have satisfied the conditions of mechanical stability at ambient pressure. Also, the bulk modulus B0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document} calculated in the structural and mechanical properties shows that Nitride-based MAX phases are harder than Carbon-based MAX phases, and Y2SiN has exhibited the highest hardness of the Y2AX family. The phonon scattering results show that Y2AX MAX phases are dynamically stable and can be synthesized experimentally. According to the total density of states results, it can be concluded that the Y2AX MAX phases have a metallic behavior and have a pseudogap close to the Fermi level and at the left side, which justifies the structural stability of the studied compounds. Electron density distribution indicates that the bonding in Y2AX (A = Al, Si, Ga, and Ge; X = C and N) MAX phases are a mixture of metallic, covalent, and ionic types.