Syntheses and physical properties of the MAX phase boride <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Nb</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>SB</mml:mi></mml:mrow></mml:math> and the solid solutions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">b</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mi>x</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn><mml:mo>–</mml:mo><mml:mn>1</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>

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.

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