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

Abstract

The MAX phase boride ${\mathrm{Nb}}_{2}\mathrm{SB}$ and the solid solutions $\mathrm{N}{\mathrm{b}}_{2}\mathrm{S}{\mathrm{B}}_{x}{\mathrm{C}}_{1\ensuremath{-}x}(x=0--1)$ were synthesized via solid-state methods and characterized by x-ray powder diffraction. All phases crystallize in the hexagonal ${\mathrm{Cr}}_{2}\mathrm{AlC}$ type with the space group $P{6}_{3}/mmc$. The lattice parameters increase with the boron content $[a=3.278(1)\ensuremath{-}3.334(1)\phantom{\rule{0.16em}{0ex}}\AA{}\phantom{\rule{0.16em}{0ex}}(+1.7%),\phantom{\rule{0.28em}{0ex}}c=11.49(1)\ensuremath{-}11.54(1)\phantom{\rule{0.16em}{0ex}}\AA{}\phantom{\rule{0.16em}{0ex}}(+0.5%)]$ and the distortions of the $\mathrm{N}{\mathrm{b}}_{6}(\mathrm{B},\mathrm{C})$ octahedra slightly decrease. Magnetic susceptibility and dc resistivity measurements confirm that ${\mathrm{Nb}}_{2}\mathrm{SC}$ is a superconductor while ${\mathrm{Nb}}_{2}\mathrm{SB}$ shows no superconducting transition above 1.9 K. The solid solutions $\mathrm{N}{\mathrm{b}}_{2}\mathrm{S}{\mathrm{B}}_{x}{\mathrm{C}}_{1\ensuremath{-}x}$ are metals and superconductors for $x=0--0.6$ with critical temperatures of ${T}_{\mathrm{c}}=4.8--2.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, 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 ${\mathrm{Nb}}_{2}\mathrm{SB}$ are similar to ${\mathrm{Nb}}_{2}\mathrm{SC}$ and are probably not the reason for the absence of superconductivity in the boride. We therefore suggest that the lower $N({\ensuremath{\varepsilon}}_{\mathrm{F}})$ of the boride reduces the interaction strength and thus the superconducting critical temperature.