Syntheses and physical properties of the MAX phase boride Nb2SB and the solid solutions Nb2SBxC1-x(x=0-1)

The MAX phase boride Nb2SB and the solid solutions Nb2SBxC1-x(x = 0-1) were synthesized via solid-state methods and characterized by x-ray powder diffraction. All phases crystallize in the hexagonal Cr2AlC type with the space group P6(3)/mmc. The lattice parameters increase with the boron content [a = 3.278(1) - 3.334(1) angstrom (+1.7%), c = 11.49(1) - 11.54(1) angstrom (+0.5%)] and the distortions of the Nb-6(B, C) octahedra slightly decrease. Magnetic susceptibility and dc resistivity measurements confirm that Nb2SC is a superconductor while Nb2SB shows no superconducting transition above 1.9 K. The solid solutions Nb2SBxC1-x are metals and superconductors for x = 0-0.6 with critical temperatures of T-c = 4.8-2.6 K, which decrease with increasing boron content. First-principles density-functional theory calculations confirm the metallic state and a lower electronic density of states at the Fermi energy in the boride. The calculated elastic constants, phonon density of states, and Debye temperatures of Nb2SB are similar to Nb2SC and are probably not the reason for the absence of superconductivity in the boride. We therefore suggest that the lower N(epsilon(F)) of the boride reduces the interaction strength and thus the superconducting critical temperature.