MXynes: From MXenes to carbon-extended two-dimensional carbide frameworks

We propose MXynes, a new class of carbon-extended MXene analogues obtained by expanding the carbide sublattice with allene-like C 3 units. O-terminated M 2 C 3 O 2 systems (M = Ti, Nb, Mo) were investigated in 1T- and 2H-like configurations using first-principles calculations. The structural screening shows that the most stable phases are Mo 2 C 3 O 2 -1T (HCP), Nb 2 C 3 O 2 -2H (C-top), and Ti 2 C 3 O 2 -1T (FCC). Cohesive- and formation-energy analyses indicate that the proposed MXynes are energetically well-defined phases, although the positive reaction energies for the formal transformation from parent MXenes and graphitic carbon reveal that direct MXene-to-MXyne formation is endothermic and would likely require non-equilibrium or precursor-driven routes. Phonon calculations and AIMD simulations support the dynamical and short-time thermal stability of the selected structures. A high-frequency vibrational mode near ∼ 55 THz is identified as a fingerprint of the stiff C–C stretching vibrations within the allene-like/cumulenic C 3 segments. Mechanically, the selected MXynes satisfy the Born–Huang stability criteria, with Young’s modulus reaching 344.61 N/m for Mo 2 C 3 O 2 -1T (HCP). Electronic-structure calculations show dispersive bands with significant metal- d and carbon- p contributions. Mo 2 C 3 O 2 -1T (HCP) is semiconducting, with a direct PBE gap of 0.20 eV and a quasi-direct HSE06 gap of 0.71 eV. Nb 2 C 3 O 2 -2H (C-top) remains metallic, whereas Ti 2 C 3 O 2 -1T (FCC) changes from metallic/nearly metallic at the PBE level to a narrow-gap semiconductor at the HSE06 level, with an indirect gap of 0.40 eV. The optical spectra reveal an essentially isotropic in-plane response, with absorption extending from the infrared/visible region to the near-UV. These results establish MXynes as chemically meaningful, mechanically robust, and electronically versatile carbon-extended MXene analogues.