Electron–phonon interaction and superconductivity in hexagonal ternary carbides Nb2 AC (A: Al, S, Ge, As and Sn)

The superconducting transition temperatures T c of hexagonal Nb 2 A C ( A : Al, S, Ge, As and Sn) are investigated using density functional perturbation theory to model the electron–phonon interaction. A critical assessment of the calculated electronic structure and density of states revealed that the electronic states near to the Fermi level are mostly composed of the Nb 4d states, which are responsible for the electrical conductivity. The theoretical T c data from electron–phonon calculations are in excellent agreement with the Fröhlich model, and this model was used as a computationally efficient screening method to identify promising Nb–C M 2 AX phase materials. For Nb 2 A C ( A : Zn, Cd, Al, Ga, In, Tl, Si, Pb and P), the model indicated that Nb 2 AlC should have the highest T c of this set, a little lower than Nb 2 GeC and comparable to Nb 2 SC and Nb 2 SnC. Superconductivity in Nb 2 AlC has not been studied experimentally, but this result was confirmed by full electron–phonon calculations, which also revealed that the mechanism for superconductivity is the interactions of Nb 4d-state electrons with low-frequency phonons (in particular, acoustic phonon and low-frequency optical phonons dominated by Nb and the A element). The average electron–phonon coupling parameter was found to be λ ∼ 0.646, 0.739, 0.685, 0.440 and 0.614 for Nb 2 A C ( A : Al, S, Ge, As and Sn), respectively, with a corresponding superconducting critical temperature T c ∼ 6.7 K, 7.7 K, 9.8 K, 2.1 K and 6.3 K, respectively.