The electronic structures, phonon dispersions, and electron−phonon coupling of Nb 2 CT 2 (T = O, S, Se, or Te) MXenes were investigated via first-principles calculations. Different models of Nb 2 CT 2 were constructed, and the results show that the low-energy models of Nb 2 CT 2 are intrinsic phonon-mediated superconductors. Of the four Nb 2 CT 2 MXenes, Nb 2 CO 2 MXene exhibits the largest superconducting critical temperature (T c ) of 14.43 K. The existence of soft modes induced by Kohn anomalies and the contribution of Nb atoms to the Fermi level lead to strong electron−phonon coupling (λ = 0.92) in Nb 2 CO 2 MXene. The T c of Nb 2 CO 2 is further enhanced by biaxial tensile strain and reaches up to 18.28 K under 4% tensile strain. The predicted T c of Nb 2 CS 2 is 4.5 K, which is comparable with experimental data. These findings will further stimulate the search for superconducting MXenes.

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The electronic, mechanical and thermodynamic properties of layered ThB2C are investigated using the first-principles calculations with generalized gradient and local density approximations. The equilibrium geometry and elastic stiffness constants of ThB2C are studied, and various elastic moduli, Poisson’s ratios and velocities are estimated from the elastic stiffness constants. ThB2C exhibits brittle characteristics. The phonon dispersion relationship verifies the thermodynamic stability of ThB2C. Considering the effect of phonon vibration on the thermodynamic properties of ThB2C, the variation of Gibbs free energy, bulk modulus and heat capacity at constant pressure with temperature are calculated using quasi-harmonic approximation in the temperature range of [Formula: see text][Formula: see text]K.